Marc Andrew Gibson

Glenn Research Center, Cleveland, Ohio

Thermosyphon Flooding in 

Reduced Gravity Environments

NASA/TM—2013-216536

September 2013
	
	
	








	



	

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Marc Andrew Gibson

Glenn Research Center, Cleveland, Ohio

Thermosyphon Flooding in 

Reduced Gravity Environments

NASA/TM—2013-216536

September 2013


	



	


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NASA/TM—2013-216536 iii 

Contents 

Abstract ......................................................................................................................................................... 1

1.0 Introduction ............................................................................................................................................ 1

1.1 Heat Rejection of Nuclear Power Systems for Planetary Surface Applications ........................... 1

1.2 Thermosyphons, Heat Pipes, and the Effects of Gravity .............................................................. 4

1.3 Thermosyphon Limits ................................................................................................................... 5

1.3.1 The Flooding Limit Literature Survey .............................................................................. 5

1.3.2 One Dimensional Model of the Flooding Limit ................................................................ 9

1.3.3 Other Thermosyphon Heat Transfer Limitations ............................................................ 10

2.0 Experiment Design and Hardware ........................................................................................................ 12

2.1 Reduced Gravity Testing and Vehicles ....................................................................................... 12

2.2 Experiment Design ...................................................................................................................... 12

2.3 Instrumentation ........................................................................................................................... 16

3.0 Ground Testing in Earth’s Gravity ....................................................................................................... 18

3.1 Test Methods and Procedures ..................................................................................................... 18

3.2 Data Analysis .............................................................................................................................. 19

4.0 Reduced Gravity Testing ...................................................................................................................... 23

4.1 The 2011 and 2012 Flight Campaigns ........................................................................................ 23

4.2 Reduced Gravity Data Analysis .................................................................................................. 25

5.0 Data Correlation ................................................................................................................................... 28

5.1 Non-Dimensional Analysis and Data Correlation....................................................................... 28

5.2 Conclusion .................................................................................................................................. 32

Appendix A.—Symbols and Acronyms...................................................................................................... 33

Appendix B.—One Dimensional Analysis ................................................................................................. 35

Appendix C.—Error Analysis ..................................................................................................................... 39

Appendix D.—Test Equipment Data Package ............................................................................................ 45

Bibliography ............................................................................................................................................... 90

List of Tables 

Table 2.1.—Instrumentation List ................................................................................................................ 16

Table 2.2.—Thermocouple ID and location ............................................................................................... 16

Table C.1.—Lunar parabolic flight data and statistical analysis................................................................. 43

Table C.2.—1g Laboratory data and statistical analysis ............................................................................. 43

List of Figures 

Figure 1.1.—Notional 40 kWe Fission Surface Power System .................................................................... 2

Figure 1.2.—Photograph and infrared image of a 1.7- by 2.7-m full scale fission surface power radiator. 

The IR image shows 16 thermosyphons isothermally spreading heat to the radiator facesheet. ........... 3

Figure 1.3.—Illustration of a fully wicked heat pipe .................................................................................... 4

Figure 1.4.—Illustration of condenser flooding. ........................................................................................... 6

Figure 1.5.—Faghri and Tien and Chung correlations of the thermosyphon flooding limit for Earth and 

lunar gravity levels. ................................................................................................................................ 8

Figure 1.6.—Expected thermosyphon performance of a Fission Power System on the lunar surface versus 

current thermosyphon flooding models from Faghri and Tien and Chung. ........................................... 9

Figure 1.7.—One dimensional diagram of a thermosyphon. ...................................................................... 10

Figure 1.8.—Thermosyphon heat transfer limits using experiment thermosyphon geometry. ................... 11

Figure 2.1—Thermosyphon components. ................................................................................................... 13
NASA/TM—2013-216536 iv 

Figure 2.2.—Top: National Instruments PXI Data System and electrical controls; Bottom: Screen shot of 

flight software showing critical information and controls for all 12 thermosyphons. .......................... 14

Figure 2.3.—Thermosyphon flooding experiment payload. ....................................................................... 15

Figure 2.4.—Thermocouple location diagram. ........................................................................................... 17

Figure 3.1.—Graphical representation of two different methods used to approach the flooding limit. ..... 18

Figure 3.2.—Typical 1-g flooding event using the constant power method.. ............................................. 19

Figure 3.3.—Initial 1g test data showing scattered results caused by fluid charge differences. ................. 20

Figure 3.4.—Initial individual thermosyphon flooding limits showing dissimilar results during fluid 

charge investigation. ............................................................................................................................. 21

Figure 3.5.—Effect of fluid charge on thermosyphon flooding limit. ........................................................ 21

Figure 3.6.—Flooding limit of correctly filled thermosyphons using 2.0 g of water. ................................ 22

Figure 4.1.—G-Force One aircraft and 2011 research teams. .................................................................... 23

Figure 4.2.—Pre-flight test readiness review at the reduced gravity office, Ellington Field. ..................... 24

Figure 4.3.—Thermosyphon flooding experiment. .................................................................................... 25

Figure 4.4.—Flooding event data of thermosyphon number 8 taken during Martian and lunar gravity 

parabolas in September 2011................................................................................................................ 26

Figure 4.5.—“First parabola flood” depicting 3 key events. 1) Lunar gravity forces decrease mass flow, 

heater temperature increases, upper evaporator temperature decreases. 2) Flooding occurs and heat 

transfer is stalled, slope change shows increase in heater temperature rate, no sign of recovery. 3) 

Hyper-g forces push fluid back to evaporator and cools heater until next parabola. ........................... 27

Figure 4.6.—Parabolic flooding data during lunar gravity compared to predictive models. ...................... 27

Figure 5.1.—Non-dimensional parameter q from the original Faghri model versus non-dimensional q test 

data at 1g, top, and lunar gravity, bottom. ............................................................................................ 29

Figure 5.2.—New non-dimensional flooding correlation “q” showing gravity independence. .................. 30

Figure 5.3.—New thermosyphon flooding correlation versus parabolic lunar test data. ............................ 30

Figure 5.4.—New thermosyphon flooding correlation versus 1g test data. ................................................ 31

Figure 5.5.—New thermosyphon flooding model, expected performance of an FPS thermosyphon on the 

lunar surface, and existing flooding models ......................................................................................... 32

Figure B.1.—One dimensional diagram of the thermosyphon ................................................................... 35

Figure B.2.—Top: One dimensional analysis of the thermosyphon axial power limits versus several 

ratios. Bottom: Non-dimensional power parameter “q” from the one dimensional analysis as 

compared to different values of  and the latest flooding model. ....................................................... 38

Figure C.1.—Standard deviation of the 1g and lunar test data compared to the new flooding model. ...... 40

Figure C.2.—Three axis accelerometer data during typical lunar gravity parabolas. ................................. 41

Figure C.3—The 95 percent confidence interval for the 1g and lunar model ............................................ 42
NASA/TM—2013-216536 1 

Thermosyphon Flooding in Reduced Gravity Environments 

Marc Andrew Gibson 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

An innovative experiment to study the thermosyphon flooding limits was designed and flown on a 

parabolic flight campaign to achieve the Reduced Gravity Environments (RGE) needed to obtain 

empirical data for analysis. Current correlation models of Faghri and Tien and Chung do not agree with 

the data. A new model is presented that predicts the flooding limits for thermosyphons in Earth’s gravity 

and lunar gravity with a 95 percent confidence level of ± 5 W. 

1.0 Introduction 

1.1 Heat Rejection of Nuclear Power Systems for Planetary Surface Applications 

Fission Power Systems (FPS) have long been recognized as potential multi-kilowatt power solutions 

for lunar, Martian, and extended planetary surface missions. Current heat rejection technology associated 

with fission surface power systems has focused on titanium water thermosyphons embedded in carbon 

composite radiator panels. The thermosyphons, or wickless heat pipes, are used as a redundant and 

efficient way to spread the waste heat from the power conversion unit(s) over the radiator surface area 

where it can be rejected to space. It is well known that thermosyphon performance is reliant on 

gravitational forces to keep the evaporator wetted with the working fluid. One of the performance limits 

that can be encountered, if not understood, is the phenomenon of condenser flooding. This occurs when 

the gravity forces acting on the condensed fluid cannot overcome the shear forces created by the vapor 

escaping the evaporator throat. When this occurs, the heat transfer process is stalled and may not restabilize to effective levels without corrective control actions. The flooding limit in Earth’s gravity 

environment has been studied as experimentation is readily accessible, but when the environment and 

gravity change relative to other planetary bodies, experimentation becomes difficult. 

Fission power can provide decades of uninterrupted power, day or night, making them especially 

attractive where solar intensity is limited or non-existent. Typically, 30 to 40 percent of the reactor heat 

gets converted to electricity and the remaining 60 to 70 percent gets rejected to space through large 

surface area heat rejection radiators. Figure 1.1 provides a graphical representation of a potential 40-kWe, 

186-kWt, Moon-based Fission Surface Power (FSP) system (1) that has a total heat rejection surface area 

of 184 m2

 with over 300 thermosyphons. Unique to surface power systems, when compared to in-space 

power systems, is the presence of gravity. Gravitational forces are a significant variable in fluid system 

design and directly impact the amount of power produced and rejected in a FSP system. A pumped water 

loop is used to transfer the waste heat from the cold side of the Stirling power conversion system to a total 

quantity of 20 (1.7- by 2.7-m) radiators. The radiators are equipped with heat exchangers to transfer the 

heat from the pumped water to the individual thermosyphon evaporators. The closed two phase 

thermosyphons are separate, cylindrical pressure vessels with the sole purpose of transferring the heat 

from the evaporator to the condenser where an attached fin can radiate the heat to the space environment. 

This heat transfer is accomplished using the saturated vapor, generated in the evaporator, to create the 

needed pressure difference to force the vapor up the tube where it can lose its latent heat to the cooler 

condenser wall. The heat transport capability of the saturated vapor can travel several meters from the 

evaporator to the condenser making the thermal conductance of thermosyphons several thousand times 

better than the most conductive materials. This high thermal conductance provides a near isothermal 

temperature throughout the condenser and allows the heat to be spread over several square meters of 
NASA/TM—2013-216536 2 

Figure 1.1.—Notional 40 kWe Fission Surface Power System 

radiator area. Figure 1.2 shows an Infrared image and photograph of a full scale FSP radiator with 16 

embedded thermosyphons isothermally spreading heat to the radiator facesheet. 

In the FSP concept, the heat rejection system must reject approximately 120-kWt using the energy 

balance between the convective pumped loop and the radiation heat transfer according to the following 

generalized convective and radiation equations where the pumped loop convection is Qin and the radiation 

rejection is Qout. The thermosyphons are denoted as Qtransport and are required to transport the heat from 

the pumped loop to the radiator surface. It is important to note that the pumped loop and thermosyphons 

are two separate pressure systems using the same working fluid, de-ionized water, and   refers to the 

mass flow of the Pumped Loop and   refers to the mass flow within the Thermosyphon. With these 

pressure systems being separate, different working fluids could be used for different applications. 

	
 =  
(
  ) (1.1) 

	
 =   (1.2) 

	 = (

  

) (1.3) 
NASA/TM—2013-216536 3 

The thermosyphons must be able to transfer the required amount of thermal power axially down their 

length without encountering any heat transfer limitations of the two phase system. This research was 

aimed at studying the heat transfer limitations of thermosyphons in Earth’s gravity, as well as RGE, with 

the end goal of providing a model that will accurately predict where the maximum limitations occur. 

Because these limits have never been verified in RGE, potential inaccuracies of the current models could 

lead to problems in transporting the required thermal power through the thermosyphons when located on 

other planets. Should the heat transfer from the pumped loop to the radiative surface become decreased or 

stalled, the power system may not be able to deliver its required power output. 

Figure 1.2.—Photograph and infrared image of a 1.7- by 2.7-m full scale fission 

surface power radiator. The IR image shows 16 thermosyphons isothermally 

spreading heat to the radiator facesheet. 
NASA/TM—2013-216536 4 

1.2 Thermosyphons, Heat Pipes, and the Effects of Gravity 

Thermosyphons are categorized as wickless heat pipes and rely on gravitational forces to pull the 

condensed fluid back to the evaporator, which is always positioned below the condenser. A fully wicked 

heat pipe uses the capillary pressure generated in the wick to pump the fluid back to the evaporator, in any 

position, with or without the help of gravity. Figure 1.3 illustrates the functional aspects of a fully wicked 

heat pipe. A thermosyphon is identical, other than there is no wick present to provide capillary forces, 

thus leaving gravity as the sole restoring force. When the fluid condenses to the thermosyphon wall it 

must overcome the same pressure force that originally pushed the vapor up the tube. In addition, the 

viscous forces between the two phases begins to retard the motion of the fluid returning to the evaporator 

as the liquid layer grows and the vapor area is reduced. In order for the fluid to overcome these forces and 

descend back to the evaporator to start the cycle over, gravitational forces must be present in a 

thermosyphon. The overall heat transport capability of heat pipes and thermosyphons is constrained by 

the simple expression below where Q is the overall power throughput. Later sections will show just how 

difficult it is to predict the mass flow rate amongst the many governing variables, one of which is gravity. 

	 =   (1.4) 

As gravity is reduced, the mass flow rate of the fluid returning to the evaporator decreases and ultimately 

reduces the heat transport capability of the device. As the mass flow decreases, the evaporator 

temperature increases while the condenser temperature decreases, causing a larger temperature difference 

across the device and thus a lower thermal conductance. The heat input at the evaporator must re-balance 

with the heat output at the condenser to stabilize the heat transfer. The thermal conductance of the device 

can be evaluated using the following equation. 

!" = #

($%&)

 (1.5) 

Figure 1.3.—Illustration of a fully wicked heat pipe 
NASA/TM—2013-216536 5 

The thermosyphon working fluid is specifically chosen, based on its thermophysical properties, to 

operate in the desired temperature range, typically determined through trade studies of the total system 

specific power (W/kg) of the system. When gravity forces are present, such as on the surface of the Moon 

or Mars, thermosyphons can trade better than heat pipes because they do not have a wick structure in the 

condenser and may ultimately have a lower mass. This trade must be taken into careful consideration as 

the additional mass of the heat pipe wick can be balanced by the increased power the wick gives the heat 

pipe in reduced gravity. 

1.3 Thermosyphon Limits 

There are a few main heat transfer limits that will influence the amount of thermal power that can be 

transferred by thermosyphon. The boiling limit, viscous limit, sonic limit, and flooding limit are the most 

important recognized limitations that have been extensively studied and have been included for a full 

evaluation. It is important to evaluate each heat transfer limit to fully understand the results and determine 

what limit the thermosyphons reach during the experiment. The flooding limit will be extensively 

discussed in Sections 1.3.1 and 1.3.2 as it is the main focus of this research. A generalized discussion and 

analysis of the other limitations will also be included in Section 1.3.3 to give the reader adequate 

background information that may influence other thermosyphon designs and applications. 

1.3.1 The Flooding Limit Literature Survey 

The flooding limit is heavily influenced by gravity forces and can stall the heat transfer process of a 

thermosyphon during operation. Flooding can typically occur throughout the majority of the 

thermosyphon temperature range making it the most often encountered heat transfer limit. The flooding 

limit occurs when the shear forces from the counter current flow between the vapor and liquid overpowers 

the gravity forces required to keep the evaporator wetted with the working fluid. This will occur near the 

top of the evaporator were the liquid layer is at its thickest point and the vapor is traveling through the 

local minimized cross sectional area at its peak velocity (Figure 1.4). This creates a liquid orifice that will 

eventually be broken by the vapor shear force interacting with the liquid boundary. When this occurs, the 

fluid can form wave instabilities which can then become torn away from the liquid layer and be 

transported up to the condenser. This phenomenon has been called flooding because it floods the 

condenser with the working fluid. When wicks are present, a similar event occurs, identified as 

entrainment, that differs only in the fact that the capillary effect of the wick can influence the wave 

instability and make it harder for the vapor to rip the liquid from the wick structure. In some cases reports 

have indicated that this entrained fluid can be heard as the fluid particles hit the end cap. With small 

diameter thermosyphons, it may also be possible that the instabilities of the liquid layer near the flooding 

limit may cause the fluid to close off the vapor space. This would cause all the fluid in the adiabatic and 

condenser sections to flood the condenser and be held there until the pressure in the evaporator subsides, 

allowing the fluid to return to the condenser. In either case, flooding will cause a stall in the mass flow 

rate allowing the evaporator to overheat and dryout. When this happens the evaporator temperatures can 

rapidly increase and cause high pressure conditions inside the thermosyphon which may eventually lead 

to a rupture in the container wall. During experimentation, careful detection methods must be used to 

keep the evaporator temperature limits within manageable levels. 
NASA/TM—2013-216536 6 

 

Figure 1.4.—Illustration of condenser flooding. 

A literature review concerning thermosyphon flooding was examined to determine if there were 

existing formulas and correlations that would adequately predict the phenomenon. It was important to 

find a predictive model that did not require test results to form the correlation coefficients as predictions 

of the axial thermal power and the associated thermosyphon diameter would need to be determined 

beforehand. It was also important that the models be verified experimentally with water as the working 

fluid, and if possible with smaller diameter thermosyphons. Using these constraints, the initial research 

led to two models developed by Faghri et al. (2) and Tien and Chung (3). 

Tien and Chung 	

' = *

+,[-(.  ./)]

0

1 2./

%0

1 + .

%0

1 4

%+

 (1.6) 

* = 53.2tanh (0.5;<0

1 ) (1.7) 

Faghri: 	

' = !,[-(.  ./)]

0

1 2./

%0

1 + .

%0

1 4

%+

 (1.8) 

! = 2

?@

?A

4

B.0

tanh+;D

0

1 = E tanh+;D

0

1 (1.9) 

; = GH

I(J@KJA)

L M

N

O (1.10) 

The variable 	

' denotes the flooding limit in Watts and depicts the maximum axial heat transfer that 

can be transported down the length of the thermosyphon. Both sets of equations originate from the work 

of Wallis (4) and Kutateladze (5) with slight variations between Faghri’s variable K and Tien’s variable 

Ck. The non-dimensional forms of these equations are analyzed in Section 5.0. 
NASA/TM—2013-216536 7 

The Wallis correlation used a balance between inertia and hydrostatic forces of open two-phase 

systems to determine the available axial thermal power in countercurrent flow. The empirically 

dimensionless constants m and Cw are functions of the fluid properties with reported values typically 

between 0.7 and 1.0. The values PQ and P, are the liquid and vapor volumetric flow rates divided by the 

vapor area of the thermosyphon. 

 (1.11) 

 (1.12) 

 (1.13) 

Wallis determined that flooding would occur when PQ = 1 and P, = 0. In the case of a closed thermosyphon 

system, Q =  , = R

 1 where q is the heat flux in S/+and Av is the cross sectional area of the 

vapor passage. The Wallis correlation for flooding is: 

 Wallis 	 = VXYZ

O\^I_`(?b%?X)?X

c0fj?X ?b 1 k

N m1

o

O (1.14) 

The Kutateladze number !
 is a balance between the dynamic head, surface tension, and gravitational 

force. The Kutateladze correlation assumes: 

(!,)

0

+1 + (!Q)

0

+1 = * (1.15) 

 (1.16) 

Tien and Chung combined the Wallis and Kutateladze correlations assuming that PQ = 0 and Ck = 53.2

based on letting D equal the critical wavelength of the Taylor Instability. The hyperbolic tangent in Tien 

and Chung’s equation comes from the experimental results of Wallis and Makkenchery (6) in which the 

Kutateladze number decreases to the dimensionless diameter, or Bond number, giving Cw as seen in 

Equation (1.11). The Tien and Chung correlation was shown to be accurate within 15 percent based on 

water as the working fluid but was not as accurate for other fluids. 

Faghri took note of the fact that the flooding limit with other fluids needed a new dimensionless 

group as a ratio of the density of the fluid over the density of the vapor. Faghri recombined the 

correlations to predict the flooding phenomenon per Equation (1.8). 

Both Faghri’s and Tien and Chung’s correlations used the Bond number which is believed to be 

important as the diameter decreases below 0.5 in. (12.7 mm). The Bond number takes into account the 

density of the liquid and vapor as well as the surface tension, acceleration of gravity, and the diameter of 

the thermosyphon. Other sources were investigated but many had limited or no correlations with water, 

had limited or no test results using small diameter thermosyphons, or had equation variables that could 

only be determined after testing. Figure 1.5 shows the flooding limit using the Faghri and Tien 

correlations for both Earth’s gravity and lunar gravity. The graph shows just how different each 

correlation is when predicting the flooding limit of a thermosyphon. It is also evident that lunar gravity 

will have a major effect on the total axial thermal power that the thermosyphon can carry. The geometry 

inputs for Figure 1.5 are based on using the actual dimensions of the experimental thermosyphons as 

described in Section 2.0. 
NASA/TM—2013-216536 8 

Figure 1.5.—Faghri and Tien and Chung correlations of the thermosyphon flooding 

limit for Earth and lunar gravity levels. 

Figure 1.6 reinforces the necessity for this research by showing the expected performance of a fission 

surface power thermosyphon for a lunar application when compared to the Faghri and Tien and Chung 

models. The thermosyphon will flood if the required axial thermal power lies above the flooding curve. 

According to the Faghri model, the thermosyphons will not flood on the surface of the Moon seeing as the 

expected performance points (green triangles) lay below the red curve. According to the Tien and Chung 

model, the thermosyphon would flood well before reaching its expected thermal performance, as the 

points lie above the purple curve. The fact that these models don’t agree after numerous 1g test 

experiments and that no experiments have been performed in RGE makes the level of uncertainty, of 

when a thermosyphon reaches its heat transfer limit, extremely high. Which model would a thermal 

engineer choose to effectively analyze the thermal performance of a thermosyphon radiator for a lunar 

power system, and most importantly, keep the thermosyphons from reaching any heat transfer 

limitations? 

10

60

110

160

210

260

310

360

410

460

0 50 100 150 200 250 300

Thermosyphon Flooding Limit (W) 

Adiabatic Temperature (C)

Faghri 1g

Chung 1g

Faghri Lunar

Chung Lunar
NASA/TM—2013-216536 9 

Figure 1.6.—Expected thermosyphon performance of a Fission Power System on the lunar surface 

versus current thermosyphon flooding models from Faghri and Tien and Chung. 

1.3.2 One Dimensional Model of the Flooding Limit 

A one dimensional, laminar, incompressible model was evaluated to understand the fluid dynamics of 

the system. Figure 1.7 illustrates the velocity profiles, dimensional variables, and the associated Cartesian 

coordinate systems for the liquid and gas inside a thermosyphon. The fluid flow is a balance of the 

pressures associated with the liquid layer. As always, the impeding viscous forces of the liquid produce a 

retarding pressure that hinders the flow of the liquid layer from returning to the evaporator. Likewise, the 

pressure that drives the vapor up the tube will also inhibit the liquid from returning to the evaporator. The 

gravity component is the only pressure assisting the liquid back to the evaporator. A detailed look at the 

resulting equations used in this section can be found in Appendix B. 

The analysis provided an expression relating the maximum axial heat transfer in a thermosyphon as a 

function of the liquid layer thickness compared to the thermosyphon radius j

p

Q

k. 

	' = +q\^I?^Qv

wx^

j

p

Q

k

w

y1 + w

+

xI

 x^

j

p

Q

k

+

j1  p

wQk j1  pQ

k

%w{

%0

 (1.17) 

The equation was also put into the dimensionless power parameter “q” which is used in Section 5.0. Both 

dimensional and non-dimensional equations are graphically analyzed and discussed in Appendix C. 

R = #|}~

\^I?IVX_` = Q_`?^

wx^?I

j

p

Q

k

w

y1 + w

+

xI

 x^

j

p

Q

k

+

j1  p

wQk j1  pQ

k

%w{

%0

 (1.18) 

0

200

400

600

800

1000

1200

1400

1600

1800

0 50 100 150 200 250 300 350

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Faghri Lunar Model

Tien and Chung Lunar Model

Expected Performance of FSP Lunar 

Thermosyphon
NASA/TM—2013-216536 10 

Figure 1.7.—One dimensional diagram of a thermosyphon. 

1.3.3 Other Thermosyphon Heat Transfer Limitations 

Boiling Limit.—The boiling limit can be reached with thermosyphons that have large fill volumes 

and high radial heat fluxes. As with any boiling limitations, the high flux heat transfer leads to nucleate 

boiling followed by film boiling which overcomes the ability of the fluid to reach the heated surface. This 

causes the evaporator to overheat, producing a positive feedback mechanism with no chance of recovery 

without dramatically decreasing the evaporator heat flux. As discussed in the previous section, the 

reduction in gravity decreases the mass flow in the thermosyphon which ultimately reduces the amount of 

cooling that can take place over the evaporator. This, in turn, makes the boiling limitation reliant on the 

gravity forces. Equation (1.19) correlates the maximum power that can be transferred radially from the 

thermosyphon evaporator (7). 

Vapor 

Liquid



 









l

gravity 

Evaporator

Condenser 

L
NASA/TM—2013-216536 11 

	' = 0.16 ,_.,[-(.Q  .,)]B.+ y1  exp j

%

$

k j?b

?X

k

B.0w{ (1.19) 

Sonic Limit.—The sonic limit becomes important during the early stages of heat up when large 

pressure differences can be encountered between the evaporator and condenser sections that can cause 

choked flow at the exit of the evaporator. When the vapor Mach number is increased beyond 0.3 the 

vapor is compressed enough that the fluid dynamics must be evaluated using compressible flow models. 

The sonic limitation does not stall the heat transfer process but only limits it until the pressures 

differences can be decreased as the vapor temperatures increase and become less dense. The Following 

equation is a generalized correlation for establishing the sonic limit of a thermosyphon or heat pipe (8). 

	 = 0.474 ,(.,,)

N

O (1.20) 

Viscous Limit.—This limit is dominated by the viscous forces of the vapor during the lower 

temperature startup of a thermosyphon. The limit is set based on the fact that the condenser pressure 

cannot be lower than 0 and therefore sets the amount of pressure drop available at the condenser end. At 

low temperatures the vapor is just slightly above the vapor pressure and cannot overcome the viscous 

forces associated with driving the vapor to the condenser end. As the temperatures and pressures build, 

the flow will no longer be limited by the viscous flow as the inertial forces begin to dominate the fluid 

dynamics. This transition from viscous limiting flow to sonic limiting flow becomes less critical as the 

temperatures rise and other limits become more prevalent. Equation (1.21) can be used to evaluate the 

viscous limits of thermosyphons (9). 

	 = X

O\^IVX?XX

0XQ$^^

 (1.21) 

A graphic representation of the combined limits is shown in Figure 1.8. The viscous and sonic limits only 

come into play during low temperature operation and quickly increase as the temperature of the vapor 

increases and the density decreases. The flooding limit and boiling limit are both dependent on gravity 

forces and are shown for both Earth and lunar gravity levels. It should be noted that the boiling limit is 

typically determined through experimentation as many manufacturing variables make it hard to predict 

using analytical models. This is due to the surface conditions affecting the nucleation sites and wetting 

characteristics of the fluid with the inner wall of the container. 

Figure 1.8.—Thermosyphon heat transfer limits using experiment thermosyphon 

geometry. 
NASA/TM—2013-216536 12 

2.0 Experiment Design and Hardware 

2.1 Reduced Gravity Testing and Vehicles 

NASA has established numerous methods to study Reduced Gravity Environments (RGE) in an effort 

to provide research opportunities for developing gravity influenced technologies at an affordable cost. 

The current Flight Opportunities Program, or FOP, is run through NASA’s Office of the Chief 

Technologist (OCT) under the Game Changing Development (GCD) program. The FOP sends out 

solicitations for proposals and awards flights to the technologies that align with the current OCT 

technology roadmap. The FOP has established a number of Suborbital Re-useable Launch Vehicles 

(SrLV) which are chosen depending on the research payload and RGE requirements. This fleet of vehicles 

currently includes conventional aircraft, sounding rockets, and other suborbital launch vehicles to provide 

parabolic and suborbital flight platforms that can provide a wide range of RGE for various levels of 

research. 

Initially, research flight requirements start with the desired time at the specific gravity level needed to 

perform the research. The FOP has a diverse set of flight vehicles to allow several options when 

considering the total research objectives. After careful examination, the FOP determines which vehicle 

best suits the payload objectives and can award the flight. Using this approach, FOP provided a parabolic 

flight campaign to conduct this research with the Zero G Corporation using a Boeing 727 named G-Force 

One, based on the need for lunar and Martian gravity and the size of the experiment. 

2.2 Experiment Design 

In order to fly the experiment on G-Force One, a number of requirements had to be met to ensure safe 

operations while in flight. The experiment would be strictly investigated during internal and external Test 

Readiness Reviews (TRR) for safe operations of both mechanical and electrical systems before being 

allowed to fly. The Test Equipment Data Package listed in Appendix D gives specific details of the 

experiment under the FOP guidelines and operational procedures. These safety requirements typically 

drive the engineering constraints for the experiment and would ultimately limit the size and power of the 

experiment. Within these constraints, the experiment would need to successfully determine the heat 

transfer limits of the thermosyphons and would require innovative techniques to design the experiment 

and acquire the needed data. 

The NASA Glenn Research Center has a long history of parabolic flights and microgravity research 

which helped acquire a flight chassis to house the experiment. The chassis had already been structurally 

proven through other parabolic flights and would become the geometric constraints of the system with 

dimensions 24- by 24- by 42-in. 

Next, the electrical system and thermosyphon design would have to be specified to meet the aircraft’s 

strict electrical requirements. It was imperative that the experiment be designed to meet the requirements 

of the 1g laboratory testing as well as the parabolic flight testing, to minimize any error associated with 

the hardware. For the flight, a baseline of 2 kW at 115 V was used as the maximum electrical constraint 

for the experiment based on the aircraft’s available power. For laboratory 1g testing, the experiment 

would have to provide almost three times that power to simultaneously achieve the heat transfer limits of 

all twelve thermosyphons. 

The next consideration was to determine the number and size of thermosyphons to meet the electrical 

constraints while still being able to reach the desired flooding limit. Knowing that the flooding limit was 

going to be extremely difficult to obtain in parabolic flight, it was determined that 12 thermosyphons, if 

possible, would be a good balance between getting multiple chances at capturing the flooding event while 

keeping in mind the electrical constraints. Using 12 thermosyphons, each heater would have a total of 

165 W of supply power while in reduced gravity. 

Faghri’s Equation (1.8) was used for early predictions as it represented the more conservative 

approach to staying within the aircraft electrical budget. The diameter of the thermosyphon would be 
NASA/TM—2013-216536 13 

estimated by graphically evaluating the flooding limit of 165 W and a lunar gravity value of 1.622 m/s2. 

Both lunar and Martian gravity environments were analyzed but the lesser lunar gravity was chosen as the 

desired target because it would allow a wider range of data for the intended correlation, as well as require 

less heater power. The decision to test mostly in lunar gravity, as opposed to half lunar and half Martian, 

was due to the fact that during the parabolic flights only a limited number of parabolas are dedicated to 

reduced gravity and the experiment needed as much time as possible to pass through a flooding event. 

After careful examination, the final decision was to design the experiment with a total of 12 

thermosyphons made from 0.25 by 0.035 in. (6.35 by 0.889 mm) wall titanium tube using water as the 

working fluid. The total thermosyphon length of 24 in. (60 cm) was built with a 2.5 in. (6.35 cm) 

evaporator, a 2.5 in. (6.35 cm) adiabatic section, and a 19 in. (45.7 cm) condenser, providing a length to 

diameter (L/D) ratio of 130, similar to the thermosyphons designed for the fission surface power system 

in Figure 1.1. Two wraps of 100-mesh titanium screen were used in the evaporator section to increase 

fluid flow during nucleation and prevent dryout. The condenser was designed to be air cooled, using a 

finned aluminum tube that would enhance heat transfer and allow the internal fluid temperature to be 

altered via a variable speed fan. Combination of these components would allow testing over a wide range 

of expected heat transfer limitations that would provide new data for the research community. 

Extreme caution was used at the higher temperatures due to the pressure increase of the saturated 

vapor within each thermosyphon. Each thermosyphon would require proof pressure testing as well as 

extended temperature testing to adequately prove they would not rupture during any portion of the 

experiment. Photos of the thermosyphon components are shown in Figure 2.1. 

When reaching any of the heat transfer limits, it is known that the heater temperatures will rapidly 

increase based on the associated power level. This drove the most important control system decision 

which required that the heater power interlock have double redundancy to protect the heater from running 

away during the experiment. The control system relied on several thermocouples to provide the necessary 

information to appropriately shut down the heaters should the operator miss the runaway event. 

 

Figure 2.1—Thermosyphon components. 
NASA/TM—2013-216536 14 

Figure 2.2.—Top: National Instruments PXI Data System and electrical controls; Bottom: Screen shot of flight 

software showing critical information and controls for all 12 thermosyphons. 

The data acquisition and control (DAC) system was accomplished using a National Instruments PXI 

chassis and real-time controller with customized Labview programming to provide the system logic and 

user interface functions. The end product used a laptop computer that was linked to the PXI controller 

inside the flight rack and let the operator view and collect data signals, control the heater power, view 

alarms, and record notes. Several revisions of the DAC system were implemented to establish a 

streamline process that allowed quick communication and control needed during parabolic maneuvers. 

Pictures of the experiment hardware can be seen in Figure 2.2 and Figure 2.3. More detail relating to each 

individual component can be seen in the TEDP in Appendix D. 
NASA/TM—2013-216536 15 

 

Figure 2.3.—Thermosyphon flooding experiment payload. 
NASA/TM—2013-216536 16 

2.3 Instrumentation 

Table 2.1 and Table 2.2 and Figure 2.4 provide all the needed information for the experiment 

instrumentation. The data was collected at 1 Hz and provided the necessary information to analyze the 

flooding limit of the thermosyphons. Thermocouples one and two, located at the lower evaporator, were 

used as a flooding event marker. The screen wick extended from the top of the evaporator at z= 3.5 down 

to z=0 and provided the capillary pressure necessary to bring the fluid stored in the lower evaporator up to 

the heated section during increased thermal power periods. This one inch section of tube acted as a fluid 

reservoir and allowed water to be inserted into the flow when additional mass was needed. With this 

design, the two lower evaporator thermocouples could gauge the amount fluid in the lower evaporator. 

When there was water present in the lower evaporator the temperatures were cooler. When the 

thermosyphon needed additional mass flow, the water in the lower evaporator would be pumped into the 

evaporator to begin the two phase thermodynamic process which could be seen as an increase in 

temperature at the lower evaporator temperatures. Prior to evaporator dryout, the temperatures in both 

lower evaporator thermocouples would increase, providing a trigger for the expected heater dryout. These 

thermocouples provided a good indication of what was happening in the evaporator and could be used to 

identify the differences between dryout and flooding. 

The upper evaporator and condenser thermocouples were used in conjunction with the evaporator 

thermocouples to help identify the heat transfer limits. As the thermosyphon approached a limit, the 

difference in evaporator and condenser thermocouples would increase as the fluid mass flow was 

degraded. This difference caused a significant decrease in the thermal conductance and signaled a stalling 

event in the heat transfer process. All of these triggering events were seen as slope changes on the DAC 

time history graphs. All 12 thermosyphons could be monitored at once with the DAC software and helped 

the test operators identify the flooding events as they unfolded. 

TABLE 2.1.—INSTRUMENTATION LIST 

Instrumentation Sample Rate: 1 Hz Quantity 

Type T Thermocouples ......................................60 

Voltage Meter ............................................................... 13 

Current Meter ................................................................ 15 

3 Axis Accelerometer ..................................................... 1 

TABLE 2.2.—THERMOCOUPLE ID AND LOCATION 

TC 

no. 

TC 

name 

TC location, 

z, in. 

1 Lower Evaporator_1 0.375 

2 Lower Evaporator_2 0.625 

3 Heater Block 2.25 

4 Upper Evaporator 4.5 

5 Condenser 23 

 
NASA/TM—2013-216536 17 

Figure 2.4.—Thermocouple location diagram. 

Insulation 

Z=0 

Heater 

Block 

Upper 

Evaporator 

Condenser 

Fins 

Condenser 

Top 

Evaporator 

with wick 

Lower 

Evaporator 

5 

4 

2 

1 

3 
NASA/TM—2013-216536 18 

3.0 Ground Testing in Earth’s Gravity 

3.1 Test Methods and Procedures 

Testing was conducted in the laboratory well before any flight testing took place to address the 

functionality of the experiment and determine how the sensors would be used to detect the flooding limit. 

The thermosyphons were taken to their flooding limit using several procedures, which were eventually 

down selected into the most appropriate for parabolic flight. Figure 3.1 provides a graphical 

representation of the two selected methods for determining the flooding limit during experimentation. 

These separate methods were used to approach the flooding limit to determine if either method provided 

different results. During the constant temperature procedure, the heaters were taken up in temperature 

using a voltage ramping function that would vary the heater voltage at 1 V/min. Using the variable speed 

fans, the operator adjusted the airflow across the finned condenser to control the adiabatic temperature of 

the thermosyphon. Using this strategy, the power increases as the adiabatic temperature stays constant, 

and eventually the thermosyphon passes through the flooding limit. This can be seen graphically by the 

“Constant Temperature” arrow and crossover point 1 in Figure 3.1. Another method that was incorporated 

into the test procedures was to keep the power constant and increase the airflow, thus cooling the 

adiabatic temperature and ultimately passing through the flood limit from a different angle. As the 

adiabatic temperature decreases, the flooding limit will eventually be crossed at point 2 following the 

“Constant Power” arrow in Figure 3.1. Both methods produced similar results and would be used for 

parabolic flight. 

Figure 3.1.—Graphical representation of two different methods used to approach the 

flooding limit. 
NASA/TM—2013-216536 19 

3.2 Data Analysis 

When flooding occurs, the heat transfer process is stalled causing an increase in the heater 

temperature and a decrease in the condenser temperature. This can be seen in the thermocouple data as a 

change in slope and is easily visible during testing. An example of a 1-g flooding event can be seen in 

Figure 3.2 and depicts the change in slope of both the heater and condenser temperatures. During the 

laboratory and parabolic testing, 12 thermosyphons would be monitored visually to detect if a flooding 

event had occurred and would initiate shutdown of the individual heaters. Using this philosophy, all 

thermosyphons were set and ramped at exactly the same settings, providing a total of 12 flooding data 

points during an ideal test run. This procedure was used to gather multiple flooding points over a large 

temperature range providing the needed data to compare to Equations (1.6) and (1.8). 

Figure 3.2.—Typical 1-g flooding event using the constant power method. 
NASA/TM—2013-216536 20 

Figure 3.3.—Initial 1g test data showing scattered results caused by fluid 

charge differences. 

Results from Initial 1g laboratory testing are compared to the models from Faghri and Tien and 

Chung in Figure 3.3. The combined flooding limits of all 12 thermosyphons in numerous test runs show 

significant data scatter. This made it difficult to determine, with any accuracy, exactly where the flooding 

limit was and how to best define it. This triggered an investigation to try and understand exactly what was 

causing the data dispersion. One idea that had surfaced throughout initial 1 g testing was the exact amount 

of working fluid in each thermosyphon and how it affected the test results. Of the initial 12 

thermosyphons, three were tested individually to their flooding limits and are reported in Figure 3.4. It 

was thought that these differences were due to the amount of working fluid, but to determine this, the 

thermosyphons would have to be cut open. The investigation would have to wait as preparations were 

already underway for the 2011 flight campaign. After the September 2011 flight campaign, the 

thermosyphons were all weighed and cut open so that the water could be completely evaporated through 

the open end. After completing the dryout process the assemblies were reweighed to determine the fill 

charge. The thermosyphons had fluid charges ranging from 0.76 to 2.5 grams and could be related to the 

performance of the individual units with number 6 having 0.76 grams of fluid, 8 with 2.5 grams, and 12 

with 2.1 grams. The fluid charge differences in these initial 12 units were attested to filling procedures 

that worked well with larger diameter thermosyphons but would prove difficult using the 0.125 in. 

(3.2 mm) fill tube associated with the 0.25 in. (6.35 mm) thermosyphons. 

Before the thermosyphons were re-welded, a test was completed to determine, for this specific 

thermosyphon geometry, what the best fluid charge is, and how the amount of fluid charge affects the 

heat transfer limit. Figure 3.5 reports the results as the charge was changes from 0.3 grams up to 5.0 

grams. The results are clear that fluid charge has a significant impact on the heat transfer limit of the 

thermosyphon. When using too little fluid, the evaporator dries out before ever getting to the flooding 

limit and when too much fluid was used, the heat pipe would not work at all. Although not reported in the 

figure, the maximum amount of fluid that could be used was around 3.0 grams. At 3.0 grams and above, 

the thermosyphons could not be started. Also worth mentioning is the fact that 2.0 grams of fluid took up 

5 in. (12.7 cm) of the 24 in. (60 cm) total length. This volume of fluid was needed to achieve the 

maximum flooding limit, but may not be practical in some design applications. It was determined that 

2.0 grams of fluid would be used as the new fluid charge. The 1g laboratory testing was redone using the 

newly filled thermosyphons with 2.0 grams of working fluid. The new results are shown in Figure 3.6 

showing a much tighter grouping of the data. 

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250 300 350

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Faghri 1g

Tien and Chung 1g

Initial 1g Test Data
NASA/TM—2013-216536 21 

Figure 3.4.—Initial individual thermosyphon flooding limits showing dissimilar results 

during fluid charge investigation. 

Figure 3.5.—Effect of fluid charge on thermosyphon flooding limit. 

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250 300

Thermosyphon Flooding Limit (W) 

Adiabatic Temperature (C)

Faghri 1g

Tien and Chung 1g

Thermosyphon #6

Thermosyphon #8

Thermosyphon #12

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250 300 350

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Eq (1) Prediction

2 grams

1.0 grams

0.3 grams

Tien and Chung 1g
NASA/TM—2013-216536 22 

Figure 3.6.—Flooding limit of correctly filled thermosyphons using 2.0 g of water. 

Notice in Figure 3.5 that the slope and curvature of the data directly relates to the fluid charge. The 

results suggest that the differences between the predictive models and the test data may very well be 

related to fluid charge and whether or not the flooding limit is actually reached, or if the evaporator is 

drying out before flooding occurs. Typically, after a flooding event starts, it is quickly followed by 

evaporator dryout, but knowing that the fluid charge determines which event happens first makes the 

analysis much more difficult. Through careful temperature measurement, both below, inside, and above 

the evaporator, it was possible with this experiment to determine whether dryout had occurred before or 

after flooding. With the use of a wicked evaporator, the dryout limit could be detected in the data when 

the lower evaporator temperature changed slope and started increasing before condenser temperatures 

started falling. The lower evaporator can be best described as a 1-in. adiabatic section just below the 

heater, which served as a fluid reservoir. As the fluid left the reservoir to increase the mass flow needed to 

transfer the increased heat output, the thermocouple in that section would show an increase in 

temperature. This signified the start of dryout and depending on the fluid charge, may or may not be close 

to the flooding limit. After some time, the evaporator section directly under the heater block would also 

dry out and start the familiar slope increase of the heater block temperature, which would ultimately limit 

the heat transfer. Conversely, when flooding occurred as shown in Figure 3.2, the lower evaporator 

temperatures would initially not show signs of dryout, but the stalled heat transfer due to flooding would 

suddenly increase the heater block temperature. The timing of these events can be used to help determine 

whether or not flooding is actually occurring or if the thermosyphon is running out of working fluid as in 

the dryout case. Understanding this difference is key to finding the maximum heat transfer limit of the 

thermosyphons in all gravity fields. 

The proposed theory that correlations between different sources might be explained by fluid charge 

and test methods, will be hard to prove without a large test program covering numerous thermosyphon 

geometries and working fluids, which is not under the scope of this project. As with many heat transfer 

and fluids experiments, it is important to update existing models to improve the understanding of the 

engineering and physics associated with the process and hardware. Section 5.0 will construct a new 

predictive model using the 1g data in this section and reduced gravity lunar data in the following section. 

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250 300 350

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Laboratory 1g Results

Faghri 1g

Tien and Chung 1g
NASA/TM—2013-216536 23 

4.0 Reduced Gravity Testing 

4.1 The 2011 and 2012 Flight Campaigns 

The FOP awarded the proposal to conduct this research in the fall of 2010 and provided a week of 

parabolic flight in September 2011 and again in May 2012 under their recycle program. The Zero Gravity 

Corporation is the flight provider and uses a modified Boeing 727 named G-Force One (Figure 4.1) to 

conduct the parabolic research. The NASA sponsored flights are run through the Reduced Gravity Office 

(RGO) located at the Ellington Field Joint Reserve Base in Houston, Texas, near the NASA Johnson 

Space Center (JSC). Typically, each flight campaign is a weeklong effort with a total of four flights in 

4 days, 40 parabolas per day, for a total of 160 parabolas. The number of parabolas at various gravity 

levels depends on the research experiments flying that particular week and is negotiated between RGO 

and the research groups prior to conducting flight operations. During the September 2011 flight week 

RGO agreed to fly 12 lunar gravity parabolas, three Martian parabolas, and 25 zero gravity parabolas. 

These 40 parabolas were repeated for every flight during that flight campaign. For the May 2012 week, a 

decision was made to cancel the Martian parabolas and instead get as many lunar parabolas as possible. 

This decision was made because three Martian parabolas did not give the thermosyphons enough time to 

reach their flooding limit. It was more important to maximize the number of lunar parabolas to try and 

collect more flooding data. The larger flooding data range obtained from the lunar correlations would 

incorporate the Martian gravity levels and predictions could be interpolated. Each flight of the May 2012 

campaign consisted of 15 lunar gravity parabolas and 25 zero gravity parabolas. For reference, the Mars 

gravity field is 3.711 m/s2

 or 0.376 g and the lunar gravity field is approximately 1.622 m/s2 or 0.1654 g. 

Figure 4.1.—G-Force One aircraft and 2011 research teams. 
NASA/TM—2013-216536 24 

Most of the work in the first three chapters was completed in an effort to provide a successful 

experiment for the first flight in September 2011. Leading up to each flight were several iterations of the 

Test Equipment Data Package (TEDP, Appendix D) that are reviewed by the FOP and the Reduced 

Gravity Office (RGO) to ensure that the experiment is safe for both personnel and aircraft. When arriving 

at Ellington Field, members from each team prepare their experiment for the Test Readiness Review 

(TRR) where a safety committee from JSC examines each experiment package and asks questions related 

to the research, experiment functionality, and most importantly, the safety features. After completion of 

the TRR, the teams load their experiment payloads onto the aircraft and prepare for the next day’s flight. 

On the flight day, approximately 1 h before takeoff, the RGO flight crew briefs the day’s flight 

operations and begins administering flight meds. Prior to takeoff, team members can perform ground 

operations to prepare experiments for flight. From a few minutes before takeoff, up to 10,000 ft, the cabin 

power is shut off and all researchers are seated at the rear of the aircraft. The powered down time period 

during takeoff is about 10 to 15 min. depending on taxi time and climb rate. Once at 10,000 ft, the 

researchers are able to leave their seats and move to the experiments to begin preparing for parabolic 

operations. The time period from 10,000 ft to parabolic maneuvers is approximately 10 min. 

Figure 4.2.—Pre-flight test readiness review at the Reduced Gravity Office, Ellington Field. 
NASA/TM—2013-216536 25 

Figure 4.3.—Thermosyphon flooding experiment. 

The thermosyphons were operated close to their highest operating temperature prior to takeoff in an 

effort to keep the vapor flowing while the heaters were powerless and continually cooling. During the 

first flight week, it had become evident that insulating the thermosyphons during takeoff would help keep 

them operational and decrease the heat losses to the cabin environment. This higher starting temperature 

would allow a shorter warm-up time once at 10,000 ft and give the thermosyphons time to reach steady 

state. This modification was made for the May 2012 flight week and provided enough additional time to 

let the thermosyphons reach a near steady state temperature before parabolic operations. Pictures of the 

thermosyphon flooding experiment payload can be seen in Figure 4.3. 

4.2 Reduced Gravity Data Analysis 

Initiating the flooding phenomenon during parabolic flights was more complicated than originally 

thought. Each parabola lasts approximately 80 s; 50 to 60 s of hyper-g at 1.8 g and 17 to 25 s at the 

reduced gravity target of zero, lunar, or Martian. Figure 4.4 shows a snapshot of data for thermosyphon 

no. 8 taken during parabolic flight during the 2011 flight week. The left axis of the graph gives the power 

level in Watts and temperature in degrees Celsius. The horizontal axis gives the time elapsed in seconds 

and the right hand vertical axis provides the g level produced by the three axis accelerometer mounted on 

the experiment rack. Three distinct Martian parabolas can be seen in the first few minutes of the data 

followed by the first six of 12 lunar parabolas. The gravity levels cannot be held exactly at the ideal levels 

due to the fact that the aircraft constantly has to adjust to the surrounding air. Small variations around the 

desired gravity levels can be seen in both reduced and hyper-g portions of the accelerometer data. 

The flooding event can be seen taking place on the third lunar parabola using the constant power 

method as described in Section 3.0. With the power held constant, the variable speed fan was increased to 
NASA/TM—2013-216536 26 

provide a cooler adiabatic temperature which forced the thermosyphon to cross the flooding limit. This 

can easily be depicted in the data as a sharp rise in the heater temperature and a sharp decrease in the 

condenser temperature. This is the telltale sign that the heat transfer has been stalled and the 

thermosyphon is not moving vapor up to the condenser section. 

Interestingly, the hyper-g pull of the aircraft forces the fluid from the condenser back to the 

evaporator to restart the heat transfer. This hyper-g push allows the thermosyphon to recover only to go 

into the next lunar gravity portion of the parabola where it stalls once again. This enabled the data analyst 

to make sure that the thermosyphon did in fact cross over into flooding by verifying multiple occurrences. 

Due to the fact that the thermosyphons could not work in zero gravity, all the flooding events needed to 

be captured during the lunar parabolas giving a total test time of approximately 15 to 20 min. 

One of the most difficult tasks involved in parabolic testing was determining the correct power level and 

fan speed required to get close to the flooding limit without exceeding it. The 1g laboratory testing as well 

as the parabolic testing confirmed that the slower the approach to the flooding limit, the higher the power 

level that could be obtained before flooding occurred. When the power level was set above the flooding 

limit, the thermosyphon would flood on the first parabola. This “First Parabola Flood” (FPF) would provide 

an upper bound to the heat transfer limit but not the precise flooding point. Figure 4.5 shows a good 

example of a FPF that almost made it through the first parabola but flooded half way through at a power 

level of 55 W. Had the power been set at 50 W this thermosyphon may have continued to transfer heat into 

the second or third parabola giving an exact flooding power level. Having twelve thermosyphons in the 

array provided more opportunities to collect numerous flooding events throughout the flight. Even so, only 

24 flooding events out of 96 attempts were recorded in the 2011 and 2012 flight weeks. 

After compiling all the data and performing the required analysis the flight data was compared to the 

predictive models to see if there were discrepancies. As with the 1g testing, the test data did not confirm 

agreement with the model predictions. During the cooler temperatures the data seemed to agree well with 

Tien and Chung’s model but as vapor temperatures increased it approached the Faghri model. 

Figure 4.4.—Flooding event data of thermosyphon number 8 taken during Martian and lunar gravity 

parabolas in September 2011. 
NASA/TM—2013-216536 27 

Figure 4.5.—“First parabola flood” depicting three key events. 1) Lunar gravity forces 

decrease mass flow, heater temperature increases, upper evaporator temperature 

decreases. 2) Flooding occurs and heat transfer is stalled, slope change shows increase 

in heater temperature rate, no sign of recovery. 3) Hyper-g forces push fluid back to 

evaporator and cools heater until next parabola. 

Figure 4.6.—Parabolic flooding data during lunar gravity compared to predictive models. 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

-50

0

50

100

150

200

600 650 700 750 800 850 900

g Level

Power (W), Temperature (C)

Time (s)

hp5_htr_block

hp5_upper_evap

Power (W)

Gz

1

2

3

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250 300 350

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Parabolic Lunar Test Data

Faghri Lunar

Tien and Chung Lunar
NASA/TM—2013-216536 28 

5.0 Data Correlation 

5.1 Non-Dimensional Analysis and Data Correlation 

In an effort to find a better correlation concerning the new flooding data, Faghri’s equation was put 

into dimensionless form with the hopes that a new semi-empirical equation could be formed. The 

dimensionless form of Equation (1.8) is shown as: 

R = KB%0

1 (1  )0+1 ( )%0+1 (1 + 01 )%+ (5.1) 

R = #|}~

?X\^IVX_` (5.2) 

B = `O?b

 (1) (5.3) 

 = ?X

?b

 (5.4) 

K = 0

.Nm tanh+ j;0

1 k (5.5) 

The right hand side of Equation (5.1) is obtained directly from the fluid properties and the diameter of 

the thermosyphon and can be plotted as a function of temperature. The right hand side of Equation (5.2) 

is plotted using the flooding limit from the test data as Qmax. Plots of the two equations are shown in 

Figure 5.1. 

Determining which non-dimensional group holds more prominence with the test data can be found by 

assigning and varying exponents to each group. During this process it was determined that setting the 

exponents for the Bond number B, and modified Kutateladze number K to zero made an accurate fit to the 

data. With these groups set to unity, the nondimensional flooding equation “q” became much simpler and 

relied solely on the ratio of the vapor density to the liquid density. The equation simplifies to: 

Gibson R = 50%

5 (0fN m1 )O

 (5.6) 

Plotting this function against both Earth and lunar gravity data shows the uniqueness of the correlation 

and its independence from gravity levels (Figure 5.2). 

Taking if back to the dimensional form and solving for Qmax brings gravity, diameter, density, and 

enthalpy back into play as such: 

Gibson 	' = ?XVX\^I_`(0%)

5 (0fN m1 )O (5.7) 
NASA/TM—2013-216536 29 

Figure 5.1.—Non-dimensional parameter q from the original Faghri model versus 

non-dimensional q test data at 1g, top, and lunar gravity, bottom. 

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200

q (dimensionless)

Adiabatic Temperature (C)

q Faghri (1g)

q modified Faghri

q (1g test data)

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200

q (dimensionless)

Adiabatic Temperature (C)

q Faghri (Lunar)

q modified Faghri (Lunar)

q (Lunar test data)
NASA/TM—2013-216536 30 

Figure 5.2.—New non-dimensional flooding correlation “q” showing gravity independence. 

Figure 5.3.—New thermosyphon flooding correlation versus parabolic lunar test data. 

0

20

40

60

80

100

120

140

160

180

200

0 50 100 150 200

q (dimensionless)

Adiabatic Temperature (C)

q Gibson, all gravity levels

q (Lunar test data)

q (1g test data)

0

20

40

60

80

100

120

140

160

180

0 50 100 150 200 250 300 350

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Parabolic Lunar Test Data

Gibson Lunar Model
NASA/TM—2013-216536 31 

Figure 5.4.—New thermosyphon flooding correlation versus 1g test data. 

The dimensional form also shows good agreement with the test data in both Earth and lunar gravity 

fields (Figure 5.3 and Figure 5.4). The data from the parabolic flights is a bit more sporadic which is 

understandable considering the nature of the gravity levels and the movement of the aircraft. During the 

data analysis it was evident that individual thermosyphons had better performance in some flights versus 

others. The presence of gravity disturbances in addition to the existing test variables could be seen with 

individual test units experiencing wider bands of flooding events. A statistical analysis of the errors 

associated with the experiment is included in Appendix B. 

In the 1g data of Figure 5.1 and Figure 5.3 is it observed that under 50 °C the data is less repetitive 

and does not follow any of the models presented in this paper. This could possibly be a separate heat 

transfer limit, most likely the viscous limit, which can come into play at the lower operating heat fluxes 

and temperatures. The fact that the dimensionless parameter “q” shows a lot of variation at these lower 

temperatures as compared to the power Qmax is a good indicator that fluid properties, such as vapor 

density, may be dominating the results. The error analysis and additional explanation associated with the 

lower temperature data is included in Appendix C. 

It is also important to note that the surface tension component from the Bond number has been left 

out of the Equations (5.6) and (5.7) which directly disagrees with leading correlations. Most literature 

claims that the surface tension is associated with wave instability at the evaporator throat and keeps the 

vapor shear forces from ripping fluid from the surface. One of the differences between this research and 

past work is the small diameter thermosyphons. There may be a point at small diameters where the fluid 

dynamics behave differently during flooding scenarios. 

0

50

100

150

200

250

300

350

400

0 50 100 150 200 250 300 350

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Laboratory 1g Results

Gibson 1g
NASA/TM—2013-216536 32 

5.2 Conclusion 

The thermosyphon flooding limits are tested for the first time in reduced gravity. After two parabolic 

flights and several laboratory tests, the flooding heat transfer limit for thermosyphons in Earth’s gravity 

and lunar gravity is determined with a 95 percent confidence level of ± 5 W. The non-dimensional limit 

“q” in Equation (5.6) is a function of the density ratio “r” alone for both 1 g and lunar g. The dimensional 

form in Equation (5.7) provides the flooding heat transfer limit in Watts using gravity as one of the 

function variables. The parabolic testing provides data in simulated lunar gravity and aided development 

of the new correlation with the density ratio. 

The question of what flooding limit model is valid for a thermosyphon deployed in a reduced gravity 

environment such as the Moon or Mars is addressed. Figure 1.6 shows that the Faghri and Tien and 

Chung models differ. The new flooding correlation from Equation (5.7) shown in Figure 5.5 gives 

confidence that thermosyphons proposed in the current lunar FPS design will not flood on the lunar 

surface. 

Additional testing using different thermosyphon diameters and fluids will need to be completed to 

determine how well the new model holds up. History has shown that numerous thermosyphon flooding 

tests over several decades, from different sources, have been in disagreement. This reality leaves the 

opinion that each individual design should undergo testing, for the purpose of finding the heat transfer 

limits, in the laboratory, as well as in the relevant gravity environment, using the flight opportunities 

program and vehicle platforms. 

Figure 5.5.—New thermosyphon flooding model, expected performance of an FPS 

thermosyphon on the lunar surface, and existing flooding models 

0

200

400

600

800

1000

1200

1400

1600

1800

0 50 100 150 200 250 300 350

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Faghri Lunar Model

Tien and Chung Lunar Model

Gibson Lunar Model

Expected Performance of 

Lunar FPS Thermosyphon
NASA/TM—2013-216536 33 

Appendix A.—Symbols and Acronyms 

g acceleration of gravity (m/s2), grams 

 heat of vaporization 

i subscript used to denote individual variables for certain nomenclature 

l thermosyphon inner radius 

P
 Wallis velocity parameter (m/s) 

 mass flow rate (kg/s) 

q dimensionless power 

r density ratio, .,.Q


 velocity (m/s) 

z statistical parameter 


 area 

, vapor area 

; Bond number 

* dimensionless constant, Tien and Chung 

 dimensionless constant, Wallis 

D thermosyphon inner diameter 

DAC Data Acquisition and Control 

FOP Flight Opportunities Program 

FPF First Parabola Flood 

FPS Fission Power System 

FSP Fission Surface Power 

GCD Game Changing Development 

GRC NASA Glenn Research Center 

JSC NASA Johnson Space Center 

!" thermal conductance (W/K) 

K dimensionless constant, Kutateladze and Faghri 

L thermosyphon length 

N statistical sample size 

OCT Office of the Chief Technologist 

P pressure 

Q thermal power (W) 

RGE Reduced Gravity Environment 

RGO Reduced Gravity Office 

SrLV Suborbital re-useable Launch Vehicle 

T temperature 

TC thermocouple 

TEDP Test Equipment Data Package 

TRL Technology Readiness Level 

TRR Test Readiness Review 
NASA/TM—2013-216536 34 

W thermosyphon width 

 confidence parameter 

liquid thickness (m) 

 emissivity 


 dynamic viscosity (Pa s) 


 kinematic viscosity (m2

/s) 

. density of the liquid 

./ density of the vapor 

 surface tension (N/m) 

 standard deviation 

 Stefan Boltzmann constant (5.67×10–8 W/m2

K4) 


 shear stress (Pa) 

() cumulative normal distribution function 
NASA/TM—2013-216536 35 

Appendix B.—One Dimensional Analysis 

The following work gives details of the one dimensional analysis discussed in Section 1.0. The 

analysis starts with the differential equation for the fluid flowing down along the tube walls. For 

reference, Figure B.1 gives the graphical representation of the geometry and coordinate systems. 

Figure B.1.—One dimensional diagram of the thermosyphon Vapor Liquid



 









l

gravity 

Evaporator 

Condenser 

L 
NASA/TM—2013-216536 36 

  

O^

¡^

O¢  

'^

+ .-=0 (B.1) 

 ;. 1  = 0 @ ¤ = 0 (B.2) 

;. 2 

^

¡^

@ =  = 

¥

 () (B.3) 

Integrating twice using the given boundary conditions provides the desired fluid velocity profile as: 

 = ¡^

+^

2.-¦2 ¤§  ¥

 ¦2  ¤§4 (B.4) 

p = p

+^

2.-()  ¥

 (2  )4 (B.5) 

In order to obtain the mass flow rate of the fluid, the velocity is integrated over the fluid thickness 

between zero and delta and multiplied by the fluid density, giving the mass flow rate per unit width as: 



 = Qv

x^

j

p

Q

k

+

 ̈

?^p

wQ  ©

+ j1  pwQka (B.6) 

Similar equations are developed for the vapor traveling up the center of the tube with the fluid as its 

boundary. The differential equation gives the following relationships: 

  

OI

¡I

O¢ + ©



= 0 (B.7) 

;. 1  =  @ ¤ = 0 (B.8) 

;. 2 I

¡I

= 0 @ ¤ =   (B.9) 

Integrating to find the velocity profile of the gas gives: 

 = ©



¡I

+I

H2()  ¤M + p

+^

2

©

 (2  )  .-4 (B.10) 

Integrating the vapor velocity between and () and multiplying by the vapor density gives the mass 

flow rate per width as:



 = ©



Qv

xI

y

0

w j1  pQ

k

w

+ I

^

p

Q j1  pQ

k j1  p

+Qk  ?IQv

+x^

j

p

Q

k

+

j1  p

Q

k{ (B.11) 

The I

^

 term drops out as it is << 1 giving: 



 = ©



Qv

xI

y

0

w j1  pQ

k

w

 ?IQv

+x^

j

p

Q

k

+

j1  p

Q

k{ (B.12) 

Continuity ensures that the mass flows must be equal between the fluid and vapor. Setting the mass flow 

>
>

	

	P/L gives: 

©

 =

N

v

?^j«

b

k

v

¬^

¬I

N

v

j0%«

b

k

v

fN

O

j

«

b

k

O

j0%«

vbk

 (B.13) 
NASA/TM—2013-216536 37 

Equation (1.04) can be applied to the following one dimensional analysis as follows: 

   =   = #

+q\^I

(B.14) 

Inserting ©

 back into the fluid mass flow Equation (B.6), and plugging into (B.14) gives an expression 

relating Q as a function of 

 1 as: 

#|}~x^

+q\^I?^Qv = 0

w j

p

Q

k

w

y1 + w

+

xI

 x^

j

p

Q

k

+

j1  p

wQk j1  pQ

k

%w{

%0

 (B.15) 

Assuming negligible differences between the rectangular and cylindrical dimensions of the tube wall, a 

change is made to try and match the non-dimensional power parameter “q” as described in Section 5.0. In 

order to achieve this, the following assumptions are made: 

2­ = 2®; Area of the vapor = , = ®+

R = #|}~

\^I?IVX_` = Q_`?^

wx^?I

j

p

Q

k

w

y1 + w

+

xI

 x^

j

p

Q

k

+

j1  p

wQk j1  pQ

k

%w{

%0

 (B.16) 

The following one dimensional work was left out of the data analysis sections of the paper because it 

was not found to be entirely accurate when compared to the data, but it did give insight into the fluid 

dynamics associated with the liquid vapor shear in thermosyphons. A graph of the thermosyphon axial 

thermal power, Qmax in Equation (B.15), versus several values of 

 1 for the 1g laboratory case is shown 

in Figure B.2. Values were chosen that accurately scaled the curves with the latest model produced from 

this research. The curve that best represents the 1g model has a 

 1 value of 0.04 which corresponds to a 

water fluid charge of approximately 0.8 grams, assuming all the fluid is in motion. As described in the 

paper, a fluid charge of 2.0 grams was found to give the best results. The equation gives a good 

representation of the flooding limit but determination of the correct 

 1 ratio that would allow the model 

to be used as a predictive tool would be difficult. 

The non-dimensional form from Equation (B.16) was also compared to the latest model using 

representative values of p

Q

. The results are shown in figure B.2 and give a good approximation to the shape 

and scale of the latest model representing the test data. 

 
NASA/TM—2013-216536 38 

Figure B.2.—Top: One dimensional analysis of the thermosyphon axial 

power limits versus several p

Q

 ratios. Bottom: Non-dimensional power 

parameter “q” from the one dimensional analysis as compared to 

different values of p

Q

 and the latest flooding model. 

0

100

200

300

400

500

600

0 100 200 300 400

Thermosypon Axial Power (W)

Adiabatic Temperature (C)

0.015

0.025

0.035

0.04

0.045

Gibson 1g Model

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200

q (dimensionless)

Adiabatic Temperature (C)

0.015

0.025

0.035

0.04

0.045

q Gibson Model








NASA/TM—2013-216536 39 

Appendix C.—Error Analysis 

The error associated with the test data was analyzed to compare it to the new flooding model that was 

formulated through this research. The goal was to quantify the amount of error, identify the source of the 

error, and determine the confidence level of the new flooding model. 

The following standard deviation equation was used to quantify the amount of error between the new 

predictive flooding model and the test data. 

 = 3 ́ ¦¡μ%¡¶§ · O

μ

 ̧%0 ;¤
 = model prediction (C.1) 

Data from both 1g and lunar test results were graphically analyzed using error bars as can be seen in 

Figure C.1. As mentioned in Section 5.0, the lower temperature data from the 1g testing did not match the 

model and it was assumed that the heat transfer limit seen in this temperature range was not a flooding 

event. The 1 g data was split into two groups to help identify the error difference between the 1g data 

greater than and less than 60 °C. The error analysis enforces the theory that the data falling below 60 °C is 

most likely not the flooding limit with a one sigma value of ± 25 W which is more than 50 percent error at 

that power level. In order to start the thermosyphons from room temperature, an evaporator temperature 

of 30 to 60 °C was needed and was influenced by the amount of noncondensable gas (NCG), or lack of 

vacuum, in the thermosyphon. It is believed that the amount of NCG in the thermosyphons affected the 

heat transfer limit at the low end of the operating range and provided data associated with the viscous 

limit and not the flooding limit. This shows that the new flooding model should not be used below the 

temperature of 60 °C when water is the working fluid. The lunar testing did not result in any data below 

57 °C and thus this error was not seen. 

The 1g data above 60 °C had a standard deviation of 2.5 with the new flooding model The lunar 

gravity data from the parabolic flight had a deviation of 12.59 W from the model and the 1g laboratory 

data had a deviation of 12.58 W. Although numerically similar, the percentage of these values compared 

to the vertical axis flooding limit for the 1g and lunar data was much different as can be seed in Figure 

C.1. In effort was made to try and understand the differences between the higher percentage error 

associated with the parabolic flight. Figure C.2 shows a typical three axis acceleration plot during 

parabolic maneuvers and depicts the deviation of the lunar gravity levels from the 0.1654 g target. The 

deviation of the gravity level from the target was then used to determine its impact on the total measured 

error and was found to be 0.6 W of the 12.6 total, approximately 5 percent. Although not proven, it is 

believed that these error differences between the laboratory and parabolic flights are probably related to 

the vibration and pitch of the aircraft which can affect the fluid dynamics of the flooding phenomenon. 

A confidence interval using the tabulated data was also calculated using Equations (C.2) and (C.3) to 

gain assurance that the new flooding model would accurately predict a new data set. A confidence level of 

95 percent was used for the calculations and resulted in values of ± 5.4 and 4.3 W for the 1g and lunar test 

data respectively. 

confidence interval = ¤
 ±  

5 ̧ (C.2) 

=%0¦(z)§; (z) =1 1⁄2

+ ;  = 1  (confidence level) (C.3) 

Graphs of the data are shown in Figure C.3. The test data and statistical analysis are shown in Table 

C.1 and Table C.2 for both lunar and 1g testing. 
NASA/TM—2013-216536 40 

Figure C.1.—Standard deviation of the 1g and lunar test data compared to the 

new flooding model. 

 

0

50

100

150

200

250

300

350

400

450

0 100 200 300 400

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Gibson Model

1g Test Data

1g Test Data < 60 C

Model Range

0

20

40

60

80

100

120

140

160

180

200

0 100 200 300 400

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Gibson lunar 

Model

Lunar Test 

Data

Model Range
NASA/TM—2013-216536 41 

Figure C.2.—Three axis accelerometer data during typical lunar gravity parabolas. 

 

-0.5

0

0.5

1

1.5

2

500 700 900 1100 1300 1500 1700 1900

Gx

Gy

Gz
NASA/TM—2013-216536 42 

Figure C.3.—The 95 percent confidence interval for the 1g and lunar model. 

0

50

100

150

200

250

300

350

400

450

0 100 200 300 400

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Gibson 1g 

Model

Model Range

0

20

40

60

80

100

120

140

160

180

0 100 200 300 400

Thermosyphon Flooding Limit (W)

Adiabatic Temperature (C)

Gibson lunar 

Model

Model Range
NASA/TM—2013-216536 43 

TABLE C.1.—LUNAR PARABOLIC FLIGHT DATA AND STATISTICAL ANALYSIS 

TABLE C.2.—1g LABORATORY DATA AND STATISTICAL ANALYSIS 
NASA/TM—2013-216536 44 
NASA/TM—2013-216536 45 

Appendix D.—Test Equipment Data Package 
Microgravity Heat Pipe Flooding Experiment 

Test Equipment Data Package 

NASA/TM—2013-216536 46 

Thermosyphon Flooding in Reduced Gravity 

Environments 

Test Equipment Data Package 

April 3, 2012 

Space Power and Propulsion Division 

NASA Glenn Research Center 

21000 Brookpark Rd. 

Cleveland, OH 44135 

Marc Gibson PI marc.a.gibson@nasa.gov 

James Sanzi HTLE james.l.sanzi@nasa.gov 

Damir Ljubanovic ETE Damir.ljubanovic-1@nasa.gov 

 

Team Contact: Marc Gibson; 216-433-5562 

 marc.a.gibson@nasa.gov 
Microgravity Heat Pipe Flooding Experiment 

Test Equipment Data Package 

NASA/TM—2013-216536 47 

REVISION HISTORY 

Revision Description Date 

TEDP-1.0 Initial Release 08/08/11 

TEDP-1.1 Back up flyer, Structural Verification Calculations & Casters 

Added, EP30HT-LO Data Sheets Removed 

08/24/11 

TEDP-1.2 Structural Verification Calculations Updated, Vertical 

Equipment Rack, Laptop Computer Storage, Caster Installation 

and Removal 

09/01/11 

TEDP-1.3 Structural Verification Section Updated, Hazards Analysis 

Updated, Flight Manifest Updated 

09/14/11 

TEDP-1.4 Thermosyphon inner rack attachment details, Figure 6b 11/04/11 

TEDP-1.5 Recycle 2012 changes, basic word changes to reflect fully 

wicked heat pipes that will be flown in 1/6 g and zero gravity; 

changes to max. Amperage with real time monitoring and 

adjustable limits based on flight; change of PI and flight 

manifest. 

1/20/2012 

TEDP-1.6 Section 5a was added for hinged access to heat pipes 3/17/2012 

TEDP-1.7 Electrical Questionnaire attached to end of document 3/28/2012 

TEDP-1.8 Removed Don Jaworske from Flight Manifest on page 5 4/3/2012 

 
Microgravity Heat Pipe Flooding Experiment 

Test Equipment Data Package 

NASA/TM—2013-216536 48 

QUICK REFERENCE DATA SHEET 

Principal Investigator: Marc Gibson 

Contact Information: marc.a.gibson@nasa.gov, (216) 433-5562 

Experiment Title: Thermosyphon Flooding in Reduced Gravity Environments 

Work Breakdown Structure (WBS): 887359.04.01.05.03

Flight Date(s): 

Overall Assembly Weight: 275 [lb]

Assembly Dimensions (L, W, H): 24.0, 24.0, 42.19 [in] 

Equipment Orientation Requests: The zero-g flight approved Vertical Equipment Rack which 

holds the experiment equipment will be bolted to the floor of the aircraft. Heat Pipe side of 

experiment will face toward cabin wall. 

Proposed Floor Mounting Strategy (Bolts/Studs or Straps): Bolts 

Gas Cylinder Requests (Type & Quantity): No

Overboard Vent Request (Yes or No): No

Power Requirement (Voltage and Current Required): 115 VAC, 20 amps (Max amperage is 

monitored and adjustable for each flight to accommodate RGO requirements. In past flights 

RGO gave us limits that we verified before each flight. 

Free Float Experiment (Yes or No): No 

Flyer Names for Each Proposed Flight Day: Marc Gibson, Damir Ljubanovic, Jim Sanzi as 

follows: Flt. Day 1: MG, DL, JS; Flt Day 2: MG, DL, JS, Flt Day 3: MG, DL,JS; Flt. Day 4: 

MG, DL,JS 

Camera Pole and/or Video Support: No 

 
Microgravity Heat Pipe Flooding Experiment 

Test Equipment Data Package 

NASA/TM—2013-216536 49 

Table of Contents 

1. ............................................................................................................................... FLIGHT MANIFEST

....................................................................................................................................................... 50

2. EXPERIMENT BACKGROUND ......................................................................................... 50

3. EXPERIMENT DESCRIPTION ........................................................................................... 51

4. EQUIPMENT DESCRIPTION ............................................................................................. 52

5. STRUCTURAL VERIFICATION ........................................................................................ 65

6. ELECTRICAL ANALYSIS/VERIFICATION ..................................................................... 71

7. PRESSURE VESSEL/SYSTEM ........................................................................................... 79

8. PARABOLA DETAILS AND GROUND CREW ASSITANCE ......................................... 79

9. HAZARD ANALYSIS .......................................................................................................... 80

10. TOOL REQUIREMENTS ................................................................................................. 80

11. AIRCRAFT LOADING..................................................................................................... 81

12. GROUND SUPPORT REQUIREMENTS ........................................................................ 82

13. MATERIAL SAFETEY DATA SHEETS......................................................................... 83

14. EQUIPMENT PROCEDURE DOCUMENTATION ........................................................ 86

15. ELECTRICAL QUESTIONNAIRE .................................................................................. 88
Microgravity Heat Pipe Flooding Experiment

Test Equipment Data Package

NASA/TM—2013-216536 50 

1. FLIGHT MANIFEST 

Flight One: Marc Gibson, Damir Ljubanovic, Jim Sanzi 

Flight Two: Marc Gibson, Damir Ljubanovic, Jim Sanzi 

Flight Three: Marc Gibson, Damir Ljubanovic, Jim Sanzi 

Flight Four: Marc Gibson, Damir Ljubanovic, Jim Sanzi 

Backup Flyer, on call: N/A 

2. EXPERIMENT BACKGROUND 

 Normal Operation Flooding 

Figure 1 [1] Figure 2 [1]

The fundamental equations governing heat pipe technology have been developed over the years. 

These relatively simple devices can vary in size from, but are not limited to, pen-size to several 

meters in length. Heat pipes can be used in a variety of applications, ranging from thermal 

conductors in a CPU heat sink to environmental control, as seen with the Trans-Alaskan 

Pipeline. Though the fundamental equations governing the operation of heat pipes have been 

developed, validating the models in 1/6 g and zero g has never been done. Flooding correlations 

are being developed under this research for both wicked and wickless heat pipes. The wickless 

heat pipes (often referred to as thermosyphons) will not work in zero gravity and therefore be 

flown in lunar gravity parabolas. The fully wicked heat pipes will function in zero gravity where 
Microgravity Heat Pipe Flooding Experiment 

Test Equipment Data Package 

NASA/TM—2013-216536 51 

their heat transport limits will be studied. There will be 8 thermosyphons and 4 heat pipes flown 

under this research flight campaign. The thermosyphons will be turned off for the zero gravity 

parabolas. 

Heat pipes, while simple, are constrained by heat transfer limits associated with the flow of the 

working fluid through its thermodynamic cycle. One of the more common limits for 

thermosyphons in reduced gravity is the flooding limit. The limit that occurs most often with 

wicked heat pipes in microgravity is the capillary limit. This research effort will continue the 

thermosyphon flooding limits performed in lunar gravity during the 2011flight campaign as well 

as address the capillary limits of heat pipes in zero gravity. This will be accomplished by flying 

both thermosyphons and heat pipes, with the only difference being the wick structure in the heat 

pipe specimens. The capillary limit occurs in microgravity when the amount of power applied to 

the heat pipe exceeds the pumping capacity of the heat pipe wick structure to return liquid from 

the condenser, in essence starving the evaporator. The capillary limit is influenced not just by 

the design of the heat pipe but also by gravitational acceleration. For instance, a heat pipe with 

specific dimensions and 100 Watts of power flowing through it might not hit the limit here on 

Earth; however, if it were placed in a microgravity environment the same heat pipe could exceed 

its capillary limit. The Low-g flight experiment will allow us to compare a numerical model that 

predicts heat pipe limits with real life results in order to validate the model. 

3. EXPERIMENT DESCRIPTION 

Purpose: The purpose of this experiment is to study the effect that gravitational acceleration has 

heat transfer limits of two-phase heat pipes. 

Objective: The recorded data will be used to determine whether or not our numerical model is 

accurate in predicting heat pipe flooding. 

Layout: The experiment consists of twelve, nineteen inch long heat pipes, mounted to an 

internal rack that fits into a Zero-G flight approved vertical equipment rack, which will be bolted 

to the floor. Also in this rack, there are the 12 power supplies, which are bolted to a 1⁄4 in. 

aluminum plate located at the lowest region on the rack. 

Experiment: A range of power will be applied over the twelve heat pipes. For instance, if 100 

W were applied to the first heat pipe and 144 W applied to the twelfth, the step, or interval, 

between each heat pipe would be 12 W. This will, in a sense, create a spectrum which can be 

adjusted by increasing or decreasing the power, along with adjusting the change in power 

between the heat pipes. The maximum power going through a heat pipe will not exceed 200 W. 

Heat is removed from the system via forced air cooling and a range of cooling can be applied by 

incrementing fan speed. 

Thermocouples, strategically placed on all 12 heat pipes, will relay information to a data 

acquisition system. Temperature and acceleration data will be gathered each second along the 

parabolic trajectory of the Zero-G aircraft and will be sent to a laptop computer in order to 

observe the effect that gravity has on heat pipe flooding. Under normal operation, the heat pipes 
Microgravity Heat Pipe Flooding Experiment 

Test Equipment Data Package 

NASA/TM—2013-216536 52 

are isothermal; however, at the onset of flooding, the thermocouple in the heater block begins to 

increase indicating that that region is becoming liquid-starved. 

Our correlation model will be used to determine the initial conditions that will be applied to the 

heat pipes during flight. Adjustments will be made to the power and cooling settings in order to 

gather data and to compare the experimental data with predicted results. 

4. EQUIPMENT DESCRIPTION 

Equipment Dimensions (L; W; H) [in.] Weight [lb] Type of Hardware 

Equipment Rack 24.0; 24.0; 42.19 60 Experimental 

Power Supplies (x12) 1.7; 6.4; 9.4 50 Experimental 

Data Acquisition System (DAS) 16; 17; 7 45 Experimental 

Heat Pipes (x12) N/A

15 

Experimental 

Heater Blocks 1.0; 1.0; 2.6 Experimental 

Cartridge Heaters (x48) N/A Experimental 

Thermocouples N/A 

5 

Experimental 

Plugs N/A Experimental 

Circuit Breakers (x13) N/A Experimental 

Emergency Cutoff N/A Experimental 

Internal Racks N/A 25 Experimental 

Cooling Fan System N/A 15 Experimental 

Laptop 12.0; 12.0; 1.5 10 Experimental 

Other Electronics/Cords N/A 45 Experimental 

***Note: Also see the table in the structural analysis section 

EQUIPMENT RACK 

Figure 3 (units: inches) Figure 4
Microgravity Heat Pipe Flooding Experiment 

Test Equipment Data Package 

NASA/TM—2013-216536 53 

Figure 5 Full Equipment Rack 

The Aluminum 6061-T6 Flight Rack was designed for low-g flight and is the structural support 

for our experiment. The inner rack that holds the heat pipes in place, along with the DAS, power 

supplies, and other equipment is attached to this frame. 

HEAT PIPES 

Figure 6 Outline of Heat Pipe Assembly 

The twelve, 19 in. long Titanium CP-3 heat pipes are attached to an inner rack as seen in Figure 

6 and 6b. This inner rack is connected to the inside of the equipment rack for support. The heat 

pipes have an outer diameter of 0.247 in, along with a wall thickness of 0.032 in. During the 

experiment, the temperature of the heat pipes could reach 400 K. The working fluid for all heat 

pipes is distilled water. Though not shown in Figure 7, a guard is to be placed over the heat 

pipes to prevent accidentally touching of the hot surfaces. 
Microgravity Heat Pipe Flooding Experiment 

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Figure 6b 

Figure 7 Photo of Heat Pipe Array
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CARTRIDGE HEATERS 

 Cartridge Heater Drawing Cartridge Heater Photo 

 Figure 8 Figure 9 

The 200 W, 0.25” O.D., 2” long cartridge heaters fit into the cartridge heater blocks and provide 

thermal power to the heat pipes. Since the thermosyphons will not function in a zero-g 

environment, precautions will be taken to make sure the cartridge heaters are not functioning 

during a zero-g parabola. Their maximum operating temperature, however, is 1141 K, so 

overheating should not be a problem. 

CARTRIDGE HEATER BLOCKS 

 Tech Sketch of Heater Block Photo of Heater Blocks 

 

 Figure 10 Figure 11 

The Aluminum cartridge heater blocks transfer the thermal energy generated by the cartridge 

heaters to the heat pipes. Four cartridge heaters fit into one block, and each heat pipe has one 

heater block. The heat from the heater blocks is dissipated by the heat pipes and their radial fin 

heat sinks. 
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THERMOCOUPLES 

Simple Thermocouple Circuit Actual Size of Selected Thermocouple 

 

 Figure 12 Figure 13 

Insulated Thermocouples

Figure 14 

Type T glass braid thermocouples with an operating temperature range of 23 K to 623 K are 

bonded to each heat pipe by high temperature aluminum tape at specific locations along each 

heat pipe. These will allow us to monitor the temperatures of the heat pipes in order to detect 

whether or not a heat pipe is flooding (evaporator dry out). 

ELCIs 

TRC 30 mA trip 120 V/20 A ELCI’s (SKU: TRC-26140-010) were tested successfully on the 

flight power supplies and did not trip when plugging in four of the power supplies through one 

power cord as originally wired. NASA RGO uses these ELCIs on occasion and these ELCIs will 

be utilized here. The TRC ELCIs will be wired with amphenol connectors to plug in to the 

Aircraft Power Panel (120VAC) supply. 
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TRC® ELCI 120/20A & 240V/20A In-Line Attachable

SKU: TRC-26140-010 

AC Current Transducers 

Four A/CTE-50 EnergyControl.com Part Number: ACI A/CTE-50 AC current transducers were 

installed in order to measure AC current for each of the four circuits. The output of all four 

transducers was utilized to monitor the AC amperage being drawn by each of the four circuits. 

 
Microgravity Heat Pipe Flooding Experiment 

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POWER SUPPLIES 

Power Supply Drawing Power Supply Photos

Figure 15 Figure 16

There are a total of 12 power supplies included in our experiment. Each power supply supplies 

power to the cartridge heaters in one cartridge heater block (4 active heaters per block), which in 

turn heats the heat pipe. The heat pipes are cooled by radial finned extended surface tubes, by 

natural convection. 
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POWER SUPPLY SPECIFICATIONS 

Input Voltage: 90-265 VAC, 49-420 Hz, single phase, or 110-350 Vdc. 

208 VAC 3-phase is also available (see Options). 

AC input (maximum): 8A (WL7 case), 12A (WL9 case) 

DC input (maximum): 5.75A (WL7 case), 8.8A (WL9 case). 

DC input may be connected without regard to polarity. 

Inrush current: Cold start, (thermistor limiter) 33A peak @115 VAC (typical); 65A peak @ 

230 VAC (typical). 

Startup Time: 800 mS (typical). 

Input Undervoltage: An input of less than 90 VAC will not damage power supply. 

Power Factor:0.99 (typical) at 115 VAC, 60 Hz and full load. Complies with EN61000-3-2. 

Regulation (in constant voltage mode):

Line Regulation: ±0.05% 

Load Regulation: ±0.05% 

Regulation, Ripple (in constant current mode): 

Line Regulation: ±0.2% or 30 mA. 

Load Regulation: ±0.5% or 100 mA. 

Current Ripple: 0.5% rms. 

Ambient Operating Temperature: 0 to +71 °C. 

Temperature Coefficient (after 30 minute warm-up): 

Voltage mode: ±0.02%/°C (typical). 

Current mode: ±0.1%/°C (typical). 

Drift (voltage mode or current mode): ±0.1% (typical) over 8 hours, after 30 minute warmup. 

Storage Temperature: –40 to +85°C. 

Holdup Time: 20 mS minimum with full load. 

Transient Response: 300 μS to return to ±1% of output setting. Maximum of ±3% output 

excursion following a load step change from 50 to 100%. 

Polarity: Output is floating and may be used in either polarity. 

Remote Sensing: Compensates up to 0.5 V drop per output line. 

Output Adjustment: Voltage and current output adjustments are located on the front. Output 

adjustment may also be controlled by using remotely located potentiometers. 

Output Programming: The output voltage and current may be programmed from 0 to full rating 

by means of control voltage inputs of 0 to +10 Vdc (0 to +5 Vdc for models with option “C5”). 

Voltage mode accuracy: 0.5%. Current mode accuracy: 3% for models with greater than 10 amps 
Microgravity Heat Pipe Flooding Experiment 

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output current and 4% for models with less than 10 amps output current. Accuracy percentages 

do not apply below 5% of output rating. 

Voltage Monitor Terminal: Permits remote monitoring of output voltage, stepped down by a 

ratio of 10:1 (for 3.3v to 90v models) or 100:1 (for 100v to 135v models). Accuracy is 0.5% of 

maximum rated output voltage. 

For models with 0-5v programming option “C5”: Permits remote monitoring of output 

voltage, stepped down by a ratio of 10:1 (for 3.3v to 45v models) or 100:1 (for 48v to 135v 

models). Accuracy is 0.5% of maximum rated output voltage. 

Current Monitor Terminal: For models with greater than 10 amps output current: permits 

remote monitoring of output current, stepped down by a ratio of 100 mV/Amp (accuracy is 3% 

of maximum rated output current). For models with less than 10 amps output current: permits 

remote monitoring of output current, stepped down by a ratio of 1000 mV/Amp (accuracy is 3% 

of maximum rated output current). 

For models with 0-5v programming option “C5”: For models with greater than 45 A output 

current: permits remote monitoring of output current, stepped down by a ratio of 10 mV/A 

(accuracy is 5% of maximum rated output current). For models with less than 45 amps output 

current: permits remote monitoring of output current, stepped down by a ratio of 100 mV/A 

(accuracy is 3% of maximum rated output current). 

Overload/Short Circuit Protection: A short or overload forces the power supply into constant 

current mode, with automatic recovery. 

Overvoltage Protection: Latches power supply OFF, reset by momentarily removing AC input 

power. 

Thermal Protection: Thermostat(s), self-resetting. 

Internal Failure Protection: Provided by internal fuse. 

Output Inhibit: Applying between +3 and +15 Vdc to the Inhibit terminal will disable the 

supply. ‘Output Enable’ is also available (see Options). 

Output Indicator (DC on): Green LED. 

Switching Frequency: 110 kHz (typical). 

EMI: Designed to meet FCC Part 15 and EN55022, Class A. 

Dielectric Withstand Voltage Isolation 

--------------------------------------------- 

Input to output: 4242 Vdc 500 VAC 

Input to case: 2121 Vdc 500 VAC 

Output to case: 750 Vdc 300 VAC 

Cooling: Forced-air cooled; air enters rear of power supply and exits from front cover. 
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DATA ACQUISITION SYSTEM (DAS) 

NI PXI-1052 Chassis DAS Attached to Equipment Rack 

Figure 17 Figure 18 

Embedded Control Module 

 

Figure 19 

The DAS (see figures 10 & 12) is a collection of embedded modules that control and gather data 

from the experiment, which are inserted into a NI PXI-1052 chassis (see Figure 13). The 

gathered data is sent to a laptop computer where it is analyzed and displayed in an appropriate 

manner, i.e. graphs and charts. Types of data that will be gathered are acceleration data, heater

DAS 

Power 

Supplies 
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block voltage and current data, and voltage readings from the thermocouples, which are turned 

into temperatures by the DAS. 

CIRCUIT BREAKERS 

 Circuit Breaker Trip Curve 

 Figure 20 Figure 21 

12 Klixon 0.5 to 20 amp, single–phase, non–compensated, high performance circuit breakers 

(one per heat pipe) prevent the power system from over-loading. These circuit breakers are 

specifically designed for use in airborne vehicles. 

EMERGENCY CUTOFF SWITCH 

Photo of E-Cutoff on Experimental Setup 

Figure 22 

In the event of an emergency, the 120 VAC/DC push-pull emergency cutoff switch can be 

pressed to cut power to the entire experiment. 

Variable 

Transformer 

to Control 

Fan Speed 
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VARIABLE TRANSFORMER 

Specs 

Figure 23 

The Staco Energy Products’ type 291 variable transformer controls the speed of the cooling fans 

(see Figure 22). 

COOLING FANS

3D CAD Drawing Array Specs 

Figure 24 Figure 25 

A 3x3 array of cooling fans cools the heat pipes. 

LAPTOP 

Photo of Laptop 

Figure 26 
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The Dell laptop is mounted atop the experiment. It controls the experiment and records data. 

We will put the computer in the storage bin during takeoff and landing. 

ACCELORAMETER 

Figure 27 

The Silicon Designs 3-axis accelerometer is used to gather acceleration data to be used in 

conjunction with heat pipe temperature data. 

SHIELDED CABLE 

Figure 28 

The shielded 68-conductor cable connects the National Instruments SCB-68 analog output 

connector block (see below) to the DAS. 

ANALOG OUTPUT CONNECTOR BLOCK 

Figure 29 

Analog output connector block for the NI PXI-6704 (DAS module) high resolution analog output 

board for PXI 16-bit, 32 DC analog output. 
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ISOLATED VOLTAGE OUTPUT SIGNAL CONDITIONING MODULE 

Figure 30 

Figure 31 

The 7B22 is a unity gain single-channel signal conditioning output module that interfaces and 

filters a ±10 V input signal and provides an isolated precision output of ±10 V. 

5. STRUCTURAL VERIFICATION 

TACO flight experiment component weights and center of gravity data: 

Thermosyphon Array in Controlled Operation (TACO) ~ Assembled Rack Mass Properties 

 Component Mass 

(lb.) 

Center of Mass 

"X" "Y" "Z" 

(in.) (in.) (in.) 

Flight Rack Top, Sides & Handles 41 0.0 0.0 27.2 

Flight Rack Bottom 19 0.0 0.0 0.0 

Plate / Power Supplies 50 0.0 0.0 6.0 

Computer / Top Plate 10 0.0 0.0 44.0 

NI Data Acq. Unit 45 0.0 0.0 11.5 

Thermosyphon Rack 15 0.0 11.0 26.0 

Cooling Fan Rack 10 0.0 -6.0 30.5 

Front Control Panel 10 0.0 -10.5 35.0 

Power Strip 5 0.0 -10.8 4.5 

Rear Upper Elect. Rack 5 0.0 10.5 20.3 

Rear Lower Elect. Rack 5 0.0 11.0 4.8 

Cooling Air Duct 5 0.0 -5.0 27.0

Plates & Brackets 20 0.0 0.0 16.0 

Electronics & Fasteners 15 0.0 -2.0 16.0 

Thermocouples & Plugs 5 0.0 0.0 16.0 

Cables & Power Cords 10 0.0 0.0 16.0 

Total Assembly 270 0.0 0.0 16.7 
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There are four rack attachment holes on a pattern 20" center-to-center and symmetrical with the 

rack. The coordinate system’s origin is located at the rack’s base, and the center of the square 

pattern of four attachment locations. Note however that in order to satisfy overall loading 

constraints on the total experiment rack, the rack should be oriented so that its “Front” faces 

laterally. 

1) Power Supply (1 of 12) Attachment to Base Plate

Center of Gravity at (x=0,y=0,z=3) relative to power supply base plate Margin of Safety 

Case 1: Horizontal 9g, 4 ~ #6-32 screws in shear 48 

Case 2: Horizontal 9g, prying action, 2 ~ #6-32 screws in tension 10 

Case 3: Up 2g, 4 ~ #6-32 screws in tension 212 

	


 n/a 

2) 12 Power Supplies & Base Plate

Center of Gravity at (x=0,y=0,z=6) relative to experiment rack Margin of Safety 

Case 1: Horizontal 9g, 2 ~ 1/4-20 screws in shear 7 

Case 2: Up 2g, 2 ~ 1/4-20 screws in tension 22 

	


 n/a 

3) National Instruments Data Acquisition Unit

Center of Gravity at (x=0,y=0,z=11.5) relative to experiment rack Margin of Safety 

Case 1: Forward 9g, 10 ~ #8-32 screws in shear 15 

Case 2: Forward 9g, 4 ~ #10-32 screws in tension 12 

Case 3: Up 2g, 10 ~ #8-32 screws in shear See Case 1 

Case 4: Down 6g, 10 ~ #8-32 screws in shear See Case 1 

Case 5: Down 6g, 8 ~ #10-32 screws in shear 17 
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4) Entire Experiment Rack on Attachment Bolts

Center of Gravity at (x=0,y=0,z=11.5) relative to experiment rack Margin of Safety 

Case 1: Forward (Horizontal) 9g, screws in shear 3.1 

Case 2: Forward 9g, prying action, screws in tension 1.3 

Case 3: Up 2g, screws in tension 14 

	


!"

 n/a 

5a Calculation Sheet 

Description: TACO Hinged Thermosyphon Rack Structural Verification 

Project Title: Thermosyphon Array in Controlled Operation Experiment 

Project Number: 09009 

Filename: “TACO Hinged Thermosyphon Rack Analysis 2012-Feb-14.docx” 

Engineer: Ed Sechkar / ASRC Aerospace Corp. 

Date: February 14, 2012

5a. STRUCTURAL VERIFICATION ~ Modified Thermosyphon Rack 

The TACO flight experiment has been modified such that the Thermosyphon Rack is now 

located outside of the overall experiment rack. The Thermosyphon Rack is now attached at its 

lower edge by a hinge assembly, and near its upper edge by a steel bracket and a pattern of bolts. 

The hinge assembly consists of a 1⁄2” diameter stainless steel rod welded to the 2”x2”x1/4” 

aluminum angle which supports the lower ends of the thermosyphons. The rod is welded to the 

angle with six 1⁄4” fillet welds, each approximately 3⁄4” long. At each end of the rod, there is a 

stainless steel strap, 1/32” thick x 0.5” wide, which wraps around the rod and is bolted to the 

overall experiment rack with a #8-32 stainless steel bolt through a 1”x1”x3/8” thick spacer made 

from an extruded aluminum profile. 

The upper ends of the thermosyphons are contained by a 1⁄2” thick aluminum bar. This bar is 

attached to a 1/8” thick steel bracket with four #10-32 bolts. The bracket is attached to the 

overall experiment rack with four #8-32 bolts. 

The following photographs and sketches show the assembly details. 
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Figure 5a-1: Hinged Thermosyphon Rack 

Figure 5a-2: Steel Bracket Attached to Top of Thermosyphon Rack 

Figure 5a-3: Bolts for Fastening Steel Bracket to Overall Experiment Rack 
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Figure 5a-4: Hinge Assembly at Lower End of Thermosyphon Rack 

Figure 5a-4: Lateral View of Hinged Thermosyphon Rack on Experiment Rack 
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Figure 5a-5: Forward View of Hinged Thermosyphon Rack Hinge Assembly 

Figure 5a-6: Forward View of Upper Attachment of Hinged Thermosyphon Rack 
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As listed in Section 5, the Thermosyphon Rack weighs 15 lb. The following table shows the 

margins of safety for each of the load cases considered. 

5a - 1) Hinged Thermosyphon Rack 

Center of Gravity at mid-height and mid-width of thermosyphon rack Margin of Safety 

Case 1: Forward 9g, 6 ~ #8-32 thermosyphon rack screws in shear 28 

Case 2: Lateral 2g, 6 ~ #8-32 thermosyphon rack screws in tension 223 

Case 3: Forward 9g, 4 ~ #10-32 bar screws in shear 27 

Case 4: Lateral 2g, 4 ~ #10-32 bar screws in tension 185 

6. ELECTRICAL ANALYSIS/VERIFICATION 
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LOAD TABLES 

Table 1. Control Load Table 

Power Source Details Load Analysis 

Name : Power Cord PM-A0 NI-PXI Chassis - 1 Amp 

Voltage : 115 VAC, 60 Hz Laptop Power Supply - 0.3 Amps 

Wire Gauge : 14 Fan - 1.5 Amps 

 EPAC Power Supply - 0.2 Amps 

Max Outlet Current : 20 Amps Total Current Draw: 3.0 Amps 

Table 2. Power Supply Array 1 Load Table 

Power Source Details Load Analysis 

Name : Power Cord PM-A1 Acopian Power Supply 1 - 4 Amps 

Voltage : 115 VAC, 60 Hz Acopian Power Supply 2 - 4 Amps 

Wire Gauge : 14 Acopian Power Supply 3 - 4 Amps 

 Acopian Power Supply 4 - 4 Amps 

Max Outlet Current : 20 Amps Total Current Draw: 16.0 Amps 

Table 3. Power Supply Array 2 Load Table 

Power Source Details Load Analysis 

Name : Power Cord PM-A2 Acopian Power Supply 5 - 4 Amps 

Voltage : 115 VAC, 60 Hz Acopian Power Supply 6 - 4 Amps 

Wire Gauge : 14 Acopian Power Supply 7 - 4 Amps 

 Acopian Power Supply 8 - 4 Amps 

Max Outlet Current : 20 Amps Total Current Draw: 16.0 Amps 

Table 4. Power Supply Array 3 Load Table 

Power Source Details Load Analysis 

Name : Power Cord PM-A3 Acopian Power Supply 9 - 4 Amps 

Voltage : 115 VAC, 60 Hz Acopian Power Supply 10 - 4 Amps 

Wire Gauge : 14 Acopian Power Supply 11 - 4 Amps 

 Acopian Power Supply 12 - 4 Amps 

Max Outlet Current : 20 Amps Total Current Draw: 16.0 Amps 
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STORED ENERGY 

 No stored electrical energy. 

ELECTRICAL KILL SWITCH 

 There is an electrical kill switch for our experiment as is outlined in the 

 equipment description. 

ELECTRICAL SHUTDOWN PROCEDURE 

 _____ Verify that the experiment or test sequence has been completed. 

 _____ Set all the power supply Voltage Settings to “0” Volts. 

 _____ Verify on the “Main” Display tab that the voltage, current, and power readings all read “0” for 

each heat pipe. 

 _____ Verify that all the temperature readings begin to drop in temperature. 

 _____ To speed up the cooling down procedure, turn the fan control fully clockwise to the full on 

position. 

 _____ Verify that all the temperatures drop below 50 °Celsius. 

 _____ Turn the fan control fully counter-clockwise to the off position and verify the fans turn off. 

 _____ Turn the key switch counter-clockwise to the off position. 

 _____ Turn off the laptop computer. 

EMERGENCY SHUTDOWN PROCEDURE 

_____Push the E-STOP “kill switch” button and verify that everything turns off except the computer. 

LOSS OF ELECTRICAL POWER 

In the event of electrical power loss (expected, unexpected, or E-STOP), the experiment will fall 

to a safe position with the electrical power control to the heaters set to zero. Upon the return of 

electrical power, the experiment will be in an idle state with the power to the heaters off. 
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7. PRESSURE VESSEL/SYSTEM 

The heat pipes were pressure tested for 490 psi. 

Actual pressure anticipated at the 400 K operating temperature is approximately 60 psi. 

8. PARABOLA DETAILS AND GROUND CREW ASSITANCE 

This experiment will take place over four flights with a focus on 1/6 g and zero g parabolas. 

Note: We prefer to have the 1/6 g parabolas grouped at the beginning of the flight day and the 

zero g parabolas grouped at the end of the flight day. It was found from the 2011 flight campaign 

that 12-15 lunar parabolas were required to get the heat pipe data needed for this research. 

Ground Crew Assistance: None Required 

In flight crew assistance: None Required 

Free floating requirements: None
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9. HAZARD ANALYSIS

HAZARD CAUSE EFFECT Sev/Prob 

RAC 

CONTROLS VERIFICATION DISPOSITION 

Sev Prob RAC 

Electrical 

Shock 

Short circuit in 

test equipment 

Burn, 

shock 

and/or 

death 

IA1 System designed 

per NASA-STD3000 Requirements.

All electrical 

circuits grounded 

Review of Electrical 

drawing. 

GFCI installed in 

electrical circuitry 

IE4 

Temperature 

Extremes 

Heat Pipes 

Operate at 400K 

Burn IIIA2 Overtemperature 

Protection in 

controls. Physical 

barrier prevents 

contact between 

heat pipes and 

operators 

Review of Test 

Equipment. 

IIID5 

Explosion Heat Pipe 

Rupture releases 

steam. 

Burn, noise IIIA2 Temperature 

control prevents 

temperature 

excursions which 

would cause 

pressure inside heat 

pipe to exceed 

pressure test psi. 

Pressure Testing 

Of heat pipes. 

Controller 

Verification during 

ground testing. 

IIID5 

 

10.TOOL REQUIREMENTS

No special tool requirements. 
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11.AIRCRAFT LOADING

A forklift is preferred for loading/unloading. The unit is equipped with 

handles, but at 270 lb, it is still heavy to manually carry with four people. 

The unit is also equipped with pneumatic casters for moving inside the 

fuselage that will need to be removed before flight. Caster features are listed 

below. 

Capacity 200 lb

Mounting Type Plate

Caster Type Swivel

Wheel Material Pneumatic

Floor Material Asphalt, Brick, Carpet, Ceramic Tile, Concrete, Dirt, Hardwood, 

Linoleum, Ribbed, Steel, Terrazzo

Wheel Properties Noise Reducing

Wheel Diameter 6"

Wheel Width 2"

Mount Height 7-1/2"

Pneumatic Wheel Material 4-Ply Rubber

Plate Length x Width 4-1/2" x 4"

Plate Thickness 3/16"

Center-to-Center 3" x 3"

Slotted to 2-5/8" x 3-5/8"

Bolt Size 3/8"

Number of Bolt Holes 4

Frame Material Type Steel

Frame Construction Cold Formed

Frame Finish/Coating Zinc Plated

Application General Purpose

Swivel Construction Rivet Kingpin

Wheel Bearing Type Ball

Swivel Bearings Double Ball

Leg Thickness 1/8"

Grease Fittings Wheel

Wheel Color Gray

Number of Wheels One

McMaster-Carr Name Quiet-Ride

Specifications Met Not Rated

Caster installation will be achieved off of the shipping pallet by sliding the unit to the edge of the 

pallet and installing two casters, rotating and installing a third caster, and blocking the unit by 

tilting and inserting 2x4 and 4x4 wooden blocks under the unit to install the fourth caster. Caster 

removal in the fuselage will be achieved by tilting and inserting 2x4 and 4x4 wooden blocks 
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under the unit with subsequent tilting and removal of the wooden blocks until the unit is at floor 

level. 

12. GROUND SUPPORT REQUIREMENTS

Four 120v power outlets will be required on site in order to warm up the heat pipes. During the 

2011 flight campaign, we were placed in the high bay were we could use the simulated aircraft 

power supplies which was needed for our special ground fault interrupter circuits listed above. 
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13.MATERIAL SAFETEY DATA SHEETS
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14. EQUIPMENT PROCEDURE DOCUMENTATION

EQUIPMENT SHIPMENT 

Use of commercial transportation vendor (e.g., FedEx) to Ellington Field. 

GROUND OPERATIONS 

As mentioned in the ground support requirements. We need access to 4, 115 VAC outlets in 

order to warm-up the experiment before the flight. 

LOADING/STOWING 

Loading: 

A forklift is preferred to load/unload the experiment. The equipment rack is equipped with 

handle bars and removable pneumatic casters. 

Stowing: Laptop Computer. 

PREFLIGHT 

Preflight operations consist of warming up the heat pipes. 4, 115 VAC outlets are required for 

preflight and flight operations. Switch from ground support power to aircraft power is 

acceptable. (Power down during takeoff is acceptable.) 

IN-FLIGHT CHECKLIST 

***Note: The entire experiment is contained within the equipment rack. Except for the fan 

control, it is entirely automated. 

Pre-parabola: 

 _____ Turn on cooling fans 

 _____ Monitor temperatures & heat pipe functionality 

Parabola: 

 _____ Adjust cooling fan speed if necessary 

 _____ Adjust heater power if necessary 

 _____ When heat pipes flood, turn off heaters via laptop 

 _____ Post-parabola: 

 _____ Turn cooling fans up to their maximum speed 

 _____ If heat pipes have returned to pre-flooded state, proceed to pre-parabola checklist. 

 _____ If heat pipes have not returned to pre-flooded state, wait for next parabola. 
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Emergency Procedures: 

The heaters for each heat pipe have a circuit breaker that will trip if electrical problems arise. 

The entire experiment can be shutoff via the emergency stop button, which is located near the 

fan control and laptop (easily accessible). If the data acquisition system (DAS) detects a 

temperature that is too high on a specific heat pipe, it will automatically shut the heaters for that 

heat pipe down. The DAS will continue to function even if the laptop were to freeze up for 

whatever reason. 

POST FLIGHT CHECKLIST 

 _____ Unload experiment from plane 

 _____ Download/Analyze data 

OFFLOADING OF EXPERIMENT 

The experiment will be removed from the plane using a forklift . As stated in the ground 

operations section, the experiment is equipped with handles and removable pneumatic casters for 

movement inside the fuselage. Caster installation will be achieved by tilting and inserting 2x4 

and 4x4 wooden blocks under the unit. 

EMERGENCY/CONTENGINCY 

Utilize Emergency Stop Button. 
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15. ELECTRICAL QUESTIONNAIRE 

For completion by research teams flying on RGO Aircraft through the Flight Opportunities 

Program 

Please answer the following question with verified and measured parameters: 

1. Organization Name:____________NASA GRC FP06__________________ 

2. Contact Name:__Marc Gibson________________________Phone:__216-433-5562___ 

Email:___marc.a.gibson@nasa.gov________________ 

3. Aircraft Electrical Power Requirements (actual measured ): 

a. 115VAC: Normal Current:___16A______ Max Current:____20A____ 

Phase:__Single___ 

b. 28VDC: Normal Current:____N/A_____ Max Current:____N/A____ 

4. Emergency Shut Down Capability with kill switch in easy access location? 

_____YES____ 

5. Grounding: Exposed metal surfaces were adequately grounded to prevent electrical 

shock. Y/N:___Y____ 

6. Electromagnetic Compatibility: 

a. Wireless device usage Y/N: __N__; Model:____N/A___ Manuf:________ N/A 

________ 

b. Frequency:____ N/A ____ RF Transmit Power:___ N/A __ FCC approved: 

Y/N:____ N/A _ 

c. Maximum voltage and current generate if 

applicable:_____75VDC___3.8ADC_______ 

d. Motor usage Y/N: __N___ Motor type:_____ N/A _____Model:______ N/A 

__________ 

Brushed Motor Y/N:__N_____EMI checked Y/N:_____N/A________ 

e. High voltage generator (spark)? Y/N:___N___ Max Voltage and Current:_____ 

N/A ___ 

f. EMI checked Y/N:__N/A__ 
Microgravity Heat Pipe Flooding Experiment 

Test Equipment Data Package 

NASA/TM—2013-216536 89 

7. Battery type: _____ N/A _________ Specification:_____ N/A 

_______________________________ 

8. Circuit breaker Y/N:__Y___ Specification:____2 and_4ADC_, 20A 

ELCI______________ 

9. Potential liquid in contact with electrical circuitry Y/N: __N____ Mitigation Y/N:____ 

N/A _____ 

10. Identified other potential electrical hazard and safety that was not identified in the TEDP 

Y/N: __N__ If yes, provide description 

___________________________________________ 

________________________________________________________________________ 

________________________________________________________________________ 

If you have questions, please contact: Cap V. Nguyen, cap.nguyen@nasa.gov 
NASA/TM—2013-216536 90 

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