Bryan Palaszewski

Glenn Research Center, Cleveland, Ohio

Solid Hydrogen Experiments for Atomic

Propellants: Particle Formation, Imaging,

Observations, and Analyses

NASA/TM—2005-212897

November 2005

AIAA–2003–4688
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Bryan Palaszewski

Glenn Research Center, Cleveland, Ohio

Solid Hydrogen Experiments for Atomic

Propellants: Particle Formation, Imaging,

Observations, and Analyses

NASA/TM—2005-212897

November 2005

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

39th Joint Propulsion Conference and Exhibit

cosponsored by the AIAA, ASME, SAE, and ASEE

Huntsville, Alabama, July 20–23, 2003

AIAA–2003–4688
Acknowledgments

I’d like to thank the people who assisted in the construction and execution of the experiments conducted at

NASA Glenn Research Center: Maureen Kudlac (GL), Henry Speier, Joan Hoopes, Ernie Bell, Donald Metcalf, and

Robert Vanek. Thanks to Hugh Aylward for the image capturing and pixel measurements of the particle sizes.

Thanks to Brienne Curley (of the University of Toledo) for her assistance with data extraction and plotting. I’d also

like to thank John Cole, head of Space Transportation Research (STR) aspects of the NASA Advanced Space

Transportation Program (ASTP), led by NASA Marshall Space Flight Center. Extensive cooperation with the

U.S. Air Force Research Laboratory (Edwards, CA) was also a critical part of this testing.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov
 

NASA/TM—2005-212897 1

Solid Hydrogen Experiments for Atomic Propellants: 

Particle Formation, Imaging, Observations, and Analyses 

Bryan Palaszewski 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Summary 

 This report presents particle formation observations and detailed analyses of the images from 

experiments that were conducted on the formation of solid hydrogen particles in liquid helium. Hydrogen 

was frozen into particles in liquid helium, and observed with a video camera. The solid hydrogen particle 

sizes and the total mass of hydrogen particles were estimated. These newly analyzed data are from the test 

series held on February 28, 2001. Particle sizes from previous testing in 1999 and the testing in 2001 were 

similar. Though the 2001 testing created similar particles sizes, many new particle formation phenomena 

were observed: microparticles and delayed particle formation. These experiment image analyses are some 

of the first steps toward visually characterizing these particles, and they allow designers to understand 

what issues must be addressed in atomic propellant feed system designs for future aerospace vehicles. 

Introduction 

 For over 70 years, the promise of atomic propellants has been investigated (refs. 1 to 35). Using 

atoms of boron, carbon, or hydrogen, maintained at cryogenic temperatures, very exciting advances in 

rocket propellants and airbreathing fuels can be realized. Atomic propellants are composed of atomic 

species stored in a large number of cryogenic solid molecular hydrogen particles. These particles are 

stored in liquid helium to prevent the recombination of the atoms into molecules. Once the hydrogen is 

warmed as it flows, and the atoms are allowed to recombine, the recombination energy heats the hydrogen 

and helium to high temperatures, and the resulting gases can be directed in a traditional convergingdiverging nozzle to create thrust and, theoretically, high specific impulse (refs. 1 to 6). 

 Over the decades, many details of the physics of storing such propellants have been analyzed and 

experimentally determined. Current research is underway with a team from the U.S. Air Force, NASA, 

the Department of Energy, university, industry, and small business partners (ref. 7). The extensive data 

that have been amassed over many decades have shown increasing storage densities for atoms in solid 

cryogenic storage media and that there may be future breakthroughs that allow more routine use of atoms 

for fuels. 

 Characterizing solid hydrogen particles is required before any practical propellant feed system can 

be created. Solid hydrogen particles were selected as a means of storing atomic propellants in future 

launch vehicles (ref. 8). When storing atoms of boron, carbon, hydrogen, or other atomic materials, a 

solid hydrogen particle is preferred (ref. 8). Very low temperature (<4 K) cryogenic particles have the 

ability to control the atoms’ lifetimes and prevent them from recombining. The particles and the 

atoms must remain at this low temperature until the fuel is introduced into the engine combustion 

(or recombination) chamber. 
 

NASA/TM—2005-212897 2

Why Atomic Propellants? 

 In the future, rocket and airbreathing propulsion systems may be able to gain great benefits from the 

enormous power of atomic propellants. A summary of atomic hydrogen rocket gross lift-off weight 

(GLOW) is shown in figure 1 (ref. 1). Using a 15 wt% atomic hydrogen fuel, the GLOW of the launch 

vehicle can be reduced by 50 percent over the National Launch System (NLS) that uses oxygen/hydrogen 

(O2/H2) propellants. Reductions in GLOW can also be translated into significantly increased payload 

capacity (refs. 1 to 8). The baseline rocket and payload weight for the comparison is an O2/H2 rocket 

taking 96 000 kg of payload to Earth orbit. For the atomic hydrogen fuel, the oxidizer to fuel (O/F) ratio 

is 0.0, using the fuel as a monopropellant. Additional analyses and suggested optimal fuel selections for 

atomic rocket vehicles are presented in references 1 to 6. 

Solid Hydrogen Experiments 

 Solid hydrogen particle formation in liquid helium was experimentally investigated. Experiments 

were planned to visually characterize the particles, estimate their masses, and estimate their production 

efficiency. The particle sizes were estimated from video image analyses, similar to those presented in 

references 9 to 11. The work presented here is the detailed analyses of the Phase II (2001) video images, 

which precisely measured the hydrogen particle sizes. The Phase II testing of 2001 was done to improve 

the visualization of the particles over the previous Phase I tests performed in 1999 (refs. 3, 4, and 9). 

Estimates of the solid hydrogen particle sizes were made. Using the particle size analyses and Dewar 

gas outflow data, a solid hydrogen production efficiency was estimated. 

Figure 1.—Atomic hydrogen gross lift-off weight (GLOW) for monopropellants

 10-, 15-, and 50-wt% H (no He added) compared with National Launch 

 System (NLS) using O2/H2 propellants. Payload for all vehicles is 96 000 kg.

 Isp is specific impulse.

0

500

1000

1500

2000

2500

NLS: Isp 430 s

wt% H = 10 15 50

H: Isp 612 s H: Isp 750 s H: Isp 1282 s

Propulsion technology

GLOW, 1000s of kg

Propulsion

Payload
 

NASA/TM—2005-212897 3

Nomenclature 

ASTP Advanced Space Transportation Program 

FCC face-centered cubic 

FOV field of view 

GLOW gross lift-off weight 

H atomic hydrogen 

HCP hexagonal close packed 

H2 molecular hydrogen 

He helium 

L depth of Dewar 

NASA National Aeronautics and Space Administration 

NLS National Launch System 

O/F oxidizer-to-fuel ratio 

SMIRF Small Multipurpose Research Facility 

STR Space Transportation Research 

wt% weight percent 

x distance of He free surface from Dewar lid 

x/L nondimensional distance of He free surface from Dewar lid 

2001 Solid Hydrogen Experiments 

 The experiments were conducted in the Small Multipurpose Research Facility (SMIRF, formerly the 

Small Multilayer Insulation Research Facility, ref. 12). The facility has a vacuum tank, into which the 

experimental setup was placed. The vacuum tank prevented heat leaks into the Dewar and the subsequent 

boiloff of the liquid helium. The supporting systems (vacuum, purge, and other systems) maintain the 

temperature and pressure of the liquid helium bath where the solid hydrogen particles were created. 

 The experimental setup included several key components. A small cryogenic Dewar was used to 

contain the helium bath in which the solid hydrogen particles were formed. Figure 2 depicts the helium 

Dewar and the associated liquid hydrogen tank. The Dewar was 0.7112 m (28 in.) in height, with a 

0.6096 m (24 in.) inside depth, and had an inside diameter of 0.3159 m (12.438 in.). The solid hydrogen 

was created from liquid hydrogen at a temperature of 14 to 18 K. To contain the liquid hydrogen, a small 

stainless steel tank was used, which was 0.1524 m (6 in.) in diameter, and 0.6096 m (24 in.) long. As 

shown in figure 2, the tank was mounted above the Dewar. To control the hydrogen flow, a precision flow 

valve was used, and a video camera recorded the particle formation. All flow control for the liquid 

hydrogen, liquid and gaseous helium, and nitrogen purge gases was provided by the SMIRF systems. 

 The field of view (FOV) of the camera versus the distance from the Dewar lid was computed. All 

distances in the Dewar are related to distance from the Dewar lid. Figure 3 compares the camera FOV 

with the Dewar diameter. Once the liquid helium's free surface is at x/L = 0.43, where the distance from 

the Dewar lid x = 0.3159 m (12.0 in.) and the depth of the Dewar L = 0.7112 m (28 in.), the liquid's entire 

surface is in the FOV. For runs, the helium liquid level was maintained at nearly 0.3556 to 0.4064 m 

(14 to 16 in.) from the Dewar lid. This location was chosen based on the knowledge of the camera FOV, 

and the need to observe as much of the liquid surface as possible. 

Table I shows the locations of the silicon diodes for the temperature measurements. These 

temperature measurements were used to establish the location of the helium surface and overall image 

sizes and FOV. As soon as the diode temperature was seen to be above 4 K, the liquid level was assumed 

to be at or near that location in the Dewar. 
 

NASA/TM—2005-212897 4

Liquid 

hydrogen 

tank

Liquid 

hydrogen

valve

Liquid 

hydrogen 

Dewar

Figure 2.—Helium Dewar and liquid hydrogen tank

 arrangement for solid hydrogen tests.

1.374 m

0.511 m

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0 0.2 0.4 0.6 0.8 1.0

Nondimensional distance from Dewar lid, x/L

Diameter of FOV and Dewar, m

FOV diameter, m

Dewar diameter, m

Figure 3.—Camera field of view (FOV) in solid hydrogen

 experiments, 28° half angle. Dewar diameter is

 0.3159 m and length L is 0.7112 m. Distance of He free

 surface from Dewar lid is x.
 

NASA/TM—2005-212897 5

Table I.—Silicon diode locations in helium Dewar 

[Depth of Dewar L is 711.2 mm (28 in.).] 

Name Location below Dewar lid, 

mm (in.) 

SD4 0.0 (0) 

LL1 50.8 (2) 

LL2 101.6 (4) 

LL3 177.8 (7) 

LL4 254.0 (10) 

 

LL5 304.8 (12) 

LL6 355.6 (14) 

LL7 406.4 (16) 

LL8 482.6 (19) 

LL9 558.8 (22) 

0.0 0.2 0.4 0.6 0.8 1.0

Nondimensional distance from Dewar lid, x/L

Temperature, K

Figure 4.—Temperature distribution in helium Dewar in

 Feb. 28, 2001, experiments. Distance of helium free

 surface from Dewar lid is x and length of Dewar, L.

0

50

100

150

200

250

 The Phase II temperature profile in the helium Dewar is presented in figure 4. The diodes have a 

temperature accuracy of ±1 K, and they are attached to a nonmetallic rake composed of circuit board 

material that extended from the Dewar lid into the liquid helium. The diodes were mounted on the rake. 

Circuit board material was used as it had a low thermal conductivity, was readily available, and was 

easily cut to the proper dimensions. A polycarbonate screw attached the top end of the circuit board 

material to a polycarbonate rod. The upper end of the polycarbonate rod was threaded and screwed into 

the underside of the helium Dewar lid. 

Experimental Procedure 

 During the experimental runs, a small amount of liquid hydrogen was dropped onto the surface of the 

liquid helium. The hydrogen flow rate selected was 1/500 liter/s (4.2410–3 scfm) for a run time of 1 to 

4 s. This flow rate was selected by comparing the total heat capacity of the hydrogen versus the helium. 

Selecting a high flow rate would create a very high helium vaporization rate and loss of the liquid helium. 

With the low flow rate, the particles formation is clearly observed, and it eliminates any chance of the 
 

NASA/TM—2005-212897 6

relatively warm liquid hydrogen vaporizing all of the liquid helium in the Dewar. Only a small amount of 

the liquid helium contained in the Dewar vaporized as the hydrogen particles were frozen. 

 In the first step of the hydrogen freezing process, the liquid hydrogen was subcooled to 14 to 18 K, 

near its freezing point, by reducing the pressure. By reducing the pressure, the temperature of the liquid 

hydrogen was reduced. This process allowed the hydrogen to be at a very low temperature, near its 

freezing point. Comparisons of the heat capacity of helium and the heats of liquefaction and fusion 

(solidification) of hydrogen led to the selection of conditioning the hydrogen to a very low temperature 

before releasing it onto the helium surface. Otherwise a large amount of helium would have been used to 

condense the gaseous hydrogen, liquefy it, and then finally freeze the hydrogen into solid particles. Large 

clouds of vapor that are created during the longer runs would have also obscured the formation process 

and thwarted efforts to see the final particles. After the hydrogen particles freeze, they are observed for 

many minutes. 

Test Apparatus Improvements 

 The 2001 testing included a number of improvements to the experimental apparatus used in 1999. 

The lighting in the Dewar was improved with a high-intensity light and a silvered cone to reflect light into 

spaces that were shadowed in the earlier tests. The mass flow into and out of the Dewar is shown 

conceptually in figure 5. The gas composition of the venting gases from the Dewar was measured with 

increased accuracy with the addition of a residual gas analyzer (RGA). A heat exchanger was fabricated 

and added to the gas space above the liquid helium. Using liquid hydrogen as a coolant, the heat 

exchanger was designed to help reduce the temperature of the gas above the liquid surface. The insulation 

and cooling of the precision hydrogen valve was improved. A more reliable operation of the valve was 

needed to assure the proper small amount of hydrogen was introduced into the Dewar. 

Input: liquid hydrogen

Measurements: mass flow rate,

 temperature, 

 pressure

Solid hydrogen

Measurements: video observation

Liquid helium

Measurements: temperature,

 liquid level

Gaseous helium and hydrogen

Measurements: temperature, 

 pressure

Exhaust: vent gases of helium and hydrogen

Measurements: mass flow rate, 

 percent hydrogen,

 other trace gases,

 temperature, 

 pressure

Figure 5.—Experimental measurements made in testing.
 

NASA/TM—2005-212897 7

 Many more runs were conducted in the 2001 testing. Appendix A provides the list of runs for each of 

the 11 days of testing. The experience in the 1999 testing allowed for a much greater frequency of testing, 

and the new operational changes to the experimental apparatus provided more repeatable and reliable 

flow of the liquid hydrogen. 

Experimental Considerations 

 As the liquid hydrogen fell toward the helium surface, it became frozen and particles formed 

immediately after hitting the helium surface. Some of the hydrogen froze as it fell, but some vaporized as 

well. The hydrogen was a jet of fluid: The outer shear layer vaporized, but the central core remained 

liquid for a short time, finally freezing during the drop and as it hit the helium surface. Figure 4 provides 

the temperature profile in the Dewar during the experimental run. It is clear that due to the 50 to 200 K 

temperatures above the liquid helium surface, some hydrogen vaporization is likely to occur. 

 Small clouds of hydrogen were seen forming about the stream of hydrogen falling onto the free 

surface. Additional mass flow rate instrumentation was included to assess the total mass of hydrogen in 

the gas phase versus the solid particles. The temperature profiles of the Dewar will shed light on the 

amount of gas formed, and a thermal and mass balance analysis can be conducted to more accurately 

measure the distribution of gaseous and solid hydrogen in the Dewar. A mass spectrometer was also used 

to determine the mass of hydrogen in the helium gas above the liquid helium. The RGA instrument 

showed that the outflow gases from the Dewar were highly concentrated (95 to 98 percent) in hydrogen. 

 Solid hydrogen is less dense than liquid helium, so the hydrogen particles floated on the surface, 

simplifying the particle imaging. In an operational propulsion system, this buoyancy property will be 

overcome by gelling the helium, thus allowing the hydrogen particles to be suspended in the helium. 

During these tests, it was noted that the frozen hydrogen particles may also serve as an effective gelling 

agent for liquid helium (refs. 9 to 11). 

 Many frames from the videotape of the experiment were captured and analyzed. Table II summarizes 

the timing for the experimental runs, where each solid hydrogen formation run began. There was an 

interval of between 25 and 65 min between runs. These time spans were chosen to allow the particles to 

agglomerate, and to observe any unusual or unexpected properties. A more detailed listing of the events 

from each run are provided in appendix B. Appendix C and figure 6 show typical images from the testing. 

The small particles were allowed to float on the helium surface for at least 25 min before adding more 

hydrogen. During that 25-min minimum time span, they began to seek each other out, agglomerate into a 

larger collection of particles, and minimize their surface energy as they float on the helium. The particles 

also turned from clear or translucent crystals to cloudy crystals, implying a transition from face-centered 

cubic (FCC) to hexagonal close-packed (HCP) molecule packing (ref. 13). After allowing the first batch 

of particles to form, the Dewar pressure was lowered to agitate the liquid helium surface, and the particles 

quickly broke up into their original smaller components. The particles would then again begin to 

agglomerate when the agitation stopped. 

Table II.—Solid hydrogen video event timing,a Feb. 28, 2001 

[Helium level is between 14 and 16 in. below lid during run.] 

First drop begins...................................................... 14:50:59:14 

Second drop begins................................................. 16:00:21:17 a

Times given as hh:mm:ss:fr, where fr is frame number. 
 

NASA/TM—2005-212897 8

Temperature

rake

Particles

Visualization 

rake

Figure 6.—Image of solid hydrogen image (enclosed in circle), 2001. Full mass on helium 

 surface, 0.3159-m-diameter Dewar.

Solid Hydrogen Testing Results 

 Three major observations resulted from these solid hydrogen particle runs: particle sizes, the flows 

into and out of the Dewar, and the particle formation. Additional observations were made of the thermal 

profile and stratification in the helium Dewar. 

Analysis Background 

 The images were taken with a 0.5-in.-lens, charged-coupled device (CCD) black and white camera. 

The illumination in the Dewar was created with 150 W bulb with the light introduced into the Dewar with 

an optical fiber system. All of the observations were done with a black and white video camera, with a 

56° FOV. The video images were recorded on Betacam and VHS tape formats. The Betacam recordings 

were used to obtain better high-definition frames for analysis. To analyze the particles, a commercially 

available photographic manipulation and analysis software package was used. 

 There was one effective height to the liquid level that were used in the image analyses. In order to 

assure the entire free surface of the helium could be visualized, the runs were begun with a helium 

distance x of 355.6 mm (14 in.) from the Dewar lid (x/L = 0.5). The specific particle sizes were then 

estimated, using the algorithm in the following section. 
 

NASA/TM—2005-212897 9

Particle sizes 

 The solid hydrogen particles were analyzed by digitizing the video images, and measuring the sizes of 

the particles. The particle size measurements were corrected for the actual size of the particles using the 

equation 

 

area, particle ( ) = area, Dewar ( )

pixels, Dewar ( ) pixels, particle ( ) (1) 

where 

area, particle is the area of the particle (mm2

) 

area, Dewar is the area of the Dewar free surface (mm2

) 

pixels, Dewar is the number of pixels in the imaged free surface 

pixels, particle is the number of pixels in the imaged particle 

 At the beginning of and during each run, a variety of individual particles are measured. The smallest 

of the particles is identified, as well as a representative set of other larger particle sizes. Figure 6 

illustrates a typical image from the analyses. The circle encompasses a small set of hydrogen particles that 

have agglomerated. The size of the observed particles is limited by the captured image resolution. Once 

the videotape is captured, it is digitized and image analysis software is used to determine the particle 

sizes. It is therefore possible that the image analyses may be unable to detect particles smaller than 

1 pixel. 

 Appendix B contains written video observations from the 2001 testing, and appendix C provides the 

2001 video image data. These data are the raw measurements of the particle sizes from the video 

observations. Appendix D contains the particle sizes in tabular data format. 

 In the Phase II tests of February 28, 2001, small particles were formed during the initial freezing 

process. However, the smallest particles were formed many minutes after the initial freezing of the 

hydrogen. Observations inside the Dewar showed that hydrogen did freeze on the Dewar walls and finally 

came to rest on the liquid helium surface; this phenomenon is the likely cause of the late-forming 

hydrogen particles. This observation is distinctly different from the 1999 freezing experiments, where no 

such very small particles were seen. Figure 7 provides data from the February 28, 2001 tests. The figure 

shows the particle sizes at the start of the first run, how the particle size changes when the surface is 

agitated, and how the size changes again after the addition of more hydrogen at the start of the second 

run. The smallest particle sizes are 1.50 to 2.08 mm (0.06 to 0.08 in.). In these tests, no control was placed 

on the particle formation other than the temperature and pressure of the helium and hydrogen and the flow 

rate of the hydrogen. The simple freezing process is somewhat random, and the particles will vary in size 

simply because of the random breakup of the stream of hydrogen that fell onto the helium during the 

freezing process. The other measurement variation of the particles from the video images was due to all of 

the particles not being perfectly spherical or elliptical; thus an average size was measured. The 2001 

testing included some unique observations, such as tiny scintillating particles curling up in strings to form 

larger millimeter-sized particles. 
 

NASA/TM—2005-212897 10

0

0

5

10

15

20

25

30

35

40

45

50

1000 2000 3000

Time after start of run, s

4000 5000

Particle size, mm

Figure 7.—Solid hydrogen particle sizes versus time, Feb. 28, 2001.

a

b

c

d

e

f

g

h

i

j

k

l

m

Agitate

surface

Second run

begins

Time, hr:mm:ss

Flow rate, standard m3/h

Figure 8.—Gaseous hydrogen flow rate out of Dewar, Feb. 28, 2001. Valve FH127 used.

0.0

0.4

0.6

0.8

1.2

1.6

2.0

14:47:0214:48:2914:49:5514:51:2214:52:4814:54:1414:55:4114:57:0714:00:0015:47:0215:01:26

Flow Out of Dewar 

 The flow meter at the exit of the Dewar exhaust stack records the volumetric flow rate out of the 

Dewar. Figure 8 shows a flow rate versus time plot for a 1-s run (representing a hydrogen flow time of 

1 s) on February 28, 2001. Figure 9 provides similar data for the 2-s run of February 27. The total outflow 

mass for the two runs is quite different and is dependent upon the Dewar gas temperature and the 

conditions for each run. 

 The time for the flow rate to reach equilibrium on the first run of February 28 was about 1 min. The 

view on the camera however shows that the view remains clear with the February 28 run, and is partially 

obscured by clouds of hydrogen for 13 min with the February 27 run. The temperature in the helium 
 

NASA/TM—2005-212897 11

Time, hr:mm:ss

Flow rate, standard m3/h

Figure 9.—Gaseous hydrogen flow rate out of Dewar, Feb. 27, 2001.

0.0

0.4

0.6

0.8

1.2

1.6

2.0

14:53:2314:53:4014:53:5714:54:1414:54:3214:54:4914:55:0614:55:2414:55:4114:55:5814:56:15

0.0 0.2 0.4 0.6 0.8 0.1 0.3 0.5 0.7 .09

Nondimensional distance from Dewar lid, x/L

Temperature, K

23-Feb-01

27-Feb-01

Figure 10.—Temperature distribution in helium Dewar during hydrogen drop, Feb. 23

 and 27, 2001. Distance of helium free surface from Dewar lid is x and length of 

 Dewar, L.

0

20

40

60

80

100

120

140

160

180

200

Dewar for the February 28 run is shown in figure 4. The Dewar liquid and gas temperature was generally 

lower in the run on February 28 than in the runs on February 27. The data for the contrasting earlier runs 

is provided in figure 10. 

Flow Into Dewar 

 During the range of runs conducted in the 2001 Phase II test, the mass flow rate of liquid hydrogen 

into the Dewar is maintained for 1 to 3 s. The nominal flow rate desired was approximately 1/500 or 

1/100 liter/s (4.2410–3 or 2.1210–2 scfm). These flow rates correspond to the precision flow valve being 
 

NASA/TM—2005-212897 12

Figure 11.—Notional liquid hydrogen mass flow rate for

 solid hydrogen tests, 2001. (a) Theoretical. (b) Experi-

 mental planned.

1/500

1/500

Time, s

Time, s

Mass flow rate,

liters/s

Mass flow rate,

liters/s

(a)

(b)

17:06:28

17:06:29

17:06:31

17:06:33

17:06:35

17:06:36

17:06:38

17:06:40

17:06:41

17:06:43

Time, hh:mm:ss

Percent open for hydrogen valve

Figure 12.—Liquid hydrogen valve opening percentage

 versus time for hydrogen flow rate into Dewar, 

 Feb. 28, 2001. Valve FHA01_1 used.

0

2

4

6

8

10

15 and 30 percent open, respectively. Based on the calibration of the precision control flow valve, the 

flow rate is not linear, but somewhat sharply peaked. Figure 11 shows the notional comparison of the 

theoretically desired mass flow rate and the rate physically possible with the apparatus. Figure 12 depicts 

the typical valve percent open data from the liquid hydrogen valve, and it is related to actual flow rate 

from the test configuration. The instantaneous flow rate is therefore significantly different than the 

planned rate, and this difference led to better planning for conducting the experiments. Essentially, the 

integrated mass flow of the hydrogen was compared to the predicted desired mass flow. Accommodations 

were made for the specific mass flow characteristics of the hydrogen valve to assure a very small 

controlled hydrogen flow rate. Although average flow rates could not be used, actual total masses of 

liquid hydrogen flow could be estimated and used in the calculations. 
 

NASA/TM—2005-212897 13

Timing Event Influence 

 During the testing there are several time scales that affect the solid hydrogen formation: The time of 

flow for the liquid hydrogen, the time for the hydrogen outflow to be completed, and the time for the gas 

in the ullage to liquefy and then solidify. The flow of liquid hydrogen in the tank is about 1 to 4 s, the 

time for the outflow to stabilize is about 60 s, and the time for the freezing of the hydrogen gas in the 

Dewar ullage is about 1 to 30 min. 

 The flow of liquid hydrogen is controlled by the precision flow rate hydrogen valve and the pressure 

difference across it. This flow rate is computed using standard techniques. During the testing, care was 

taken to prevent the hydrogen tank pressure from exceeding the helium Dewar pressure. If the hydrogen 

tank pressure were not properly controlled, a small amount of unplanned leakage of hydrogen into the 

helium Dewar would occur, creating particles before an experimental run was planned to begin. 

 The freezing of the ullage gas was influenced by its temperature and rate of flow into the Dewar. 

These values can be controlled more rigorously in future experiments, but they were not controlled 

precisely in the experimental runs presented here. 

Comparison of 1999 and 2001 Testing 

 There were some interesting similarities and differences between the 1999 and 2001 tests. Figure 13 

shows the 1999 particle size results. Overall, the initially formed particles were 1.8 to 4.6 mm (0.07 to 

0.18 in.) in diameter. These sizes are very similar to those from 2001: 1.0 to 6.4 mm (0.04 to 0.25 in.). 

However, numerous smaller particles were observed in the February 28, 2001, tests. 

 Particle compaction was found in this set of experimental runs, as with the Phase I tests in 1999. The 

compaction trends seen in the Phase I and II testing were similar. The smallest particle sizes were very 

small (almost microscopic) in most cases, and much smaller than those seen in Phase I. The lower Dewar 

temperatures were able to freeze particles onto the walls of the Dewar, and these particles flow down the 

walls and enter the liquid helium. 

Time after start of run, s

Particle diameter, mm

Figure 13.—Solid hydrogen particle sizes versus time, 1999 tests.

0

20

40

60

80

100

120

140

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500

Run 1 Run 2 Run 3
 

NASA/TM—2005-212897 14

 Many additional images and other temperature data are available for analyses, and these additional 

images and data can give more insights into the formation process. Initial analyses of the video and flow rate 

data showed many new phenomena that were not previously observed. These phenomena include the 

formation of what appear to be microscopic hydrogen particles, the formation of long coiled structures of 

hydrogen that curl up to form small particles, and the formation of long bars of solid hydrogen. Additional 

data and video analyses will show the precise conditions under which the new phenomena occur. 

Conclusions 

 The solid hydrogen testing described in this paper was the Phase II testing of a program to 

characterize solid hydrogen particles. The improvements to the test article allowed a better measurement 

of the hydrogen flow rate into and out of the Dewar. These measurements allowed the computation of a 

production efficiency in the earlier analyses of the Phase II tests and showed that the formation efficiency 

is related to many aspects of the event timing for the experiment. 

 The particle sizes formed in the Phase I (1999) and the Phase II (2001) testing were of very similar 

sizes. In the Phase I (1999) tests, the sizes ranged from 1.8 to 4.6 mm (0.07 to 0.18 in.) in diameter. The 

Phase II tests of February 23 and 27, 2001, produced particle sizes of 1.0 to 6.4 mm (0.04 to 0.25 in.). The 

testing analyzed in this paper (Feb. 28, 2001 tests) produced numerous tiny particles in the range of 1.50 

to 2.08 mm (0.06 to 0.08 in.) in size. These tiny particles were also observed with other phenomena, 

which included the presence of microscopic scintillating particles. These tiny and microscopic particles 

appear to form from hydrogen that froze onto the Dewar walls and slid into the liquid helium. 

Concluding Remarks 

 The data analyzed thus far shows that the formation process must happen slowly to allow for the most 

efficient solid hydrogen particle formation. A fast flow will create a large cloud of hydrogen, much of 

which will go out of the vent stack. Additional data analyses will reveal the best flow rates for future 

experiments. 

 Future propulsion systems using atomic rocket propellants with solid hydrogen will likely require 

massive facilities for creating particles and many complex processes to trap atoms. Though the 

complexities seem daunting, the potential of these propellants is great, and the capacity for reducing 

vehicle lift-off weight and increasing payload capacity is theoretically unmatched. In some future vehicles 

and energy systems, atomic propellants in solid hydrogen may allow us to store and controllably release 

large quantities of energy, and allow the final human expansion into the solar system. 

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Conference, Cocoa Beach, FL, 1999. 
 

NASA/TM—2005-212897 15

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20013233, 2001. 

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19. Sater, J.D.; Pipes, J.; and Collins, G.W.: Cryogenic D–T Fuel Layers Formed in 1-mm Spheres By 

Beta-Layering. Lawrence Livermore National Laboratory Report Number UCRLJC128031ABS 

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Livermore National Laboratory Report Number UCRLJC125981ABS, 1997. 

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Livermore National Laboratory Report Number UCRLJC125121ABS), vol. 31, 1997, 

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Infrared Illumination. Lawrence Livermore National Laboratory Report Number 

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Lawrence Livermore National Laboratory Report Number UCRLJC130037ABS, 1998. 
 

NASA/TM—2005-212897 16

29. Fajardo, M.: Cryosolid PropellantsThe Last “Revolutionary” HEDM Concept. Proceedings of the 

High Energy Density Matter (HEDM) Contractors Conference, Scientific Special, M. Berman, ed., 

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(HEDM) Contractors Conference, Scientific Special, M. Berman, ed., 2000. 

31. Talroze, V.L., et al.: Measurement of the Hyperfine State of the Hydrogen Atom as it Interacts in the 

Gas Phase With CO, SO2, CO2, and N2O Molecules. Akademiia Nauk SSSR, Doklady, vol. 227, 

1976, pp. 407410. 

32. Gordon, E.B., et al.: Change in the Hyperfine State of the Hydrogen Atom During Its Collisions With 

Unsaturated Hydrocarbon Molecules in the Gaseous State. Soviet PhysicsJETP, vol. 36, no. 2, 

1973, pp. 212215. Translation. (Previously cited in Zhurnal Eksperimental'noi i Teoreticheskoi 

Fiziki, vol. 36, no. 2, 1973, pp. 401406.) 

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Rev. Letters, vol. 21, no. 5, 1968, pp. 291294. 

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Lett., vol. 62, no. 20, 1989 pp. 23612364. 

35. Stoof, Henk T.C.: Few-Body Collisions in a Weakly Interacting Bose Gas. Ph.D. Thesis, Technische 

Universiteit Eindhoven, 1989. 
 

NASA/TM—2005-212897 17

Appendix A—Liquid Hydrogen Flow Rate-Related Data for 

Solid Hydrogen Testing in 2001 

 Hydrogen flow valve-on time and experiment timing from the 2001 tests are presented in the table 

below: 

Date: Number Time(s) for runs start Mass flow rate, H2 

 of runs (H2 drop), (% open), on-time (s) 

 (approximate, hh:mm:ss) 

 (if available, hh:mm:ss:fr) 

_____________________________________________________________________

Feb. 23, 2001 1 run 1, 12:18:00 30%, 2 sec (1/100 th liter/s) 

Feb. 27, 2001 1 run 1, 14:55:00 30%, 2 sec (1/100 th liter/s) 

Feb. 28, 2001 3 runs 1, 14:51:00 15%, 1 sec (1/500 th liter/s) 

 2, 16:00:35 15%, 1 sec (1/500 th liter/s) 

 3, 17:06:30 15%, 1 sec (1/500 th liter/s) 

March 1, 2001 3 runs 1, 13:29:00 15%, 1 sec (1/500 th liter/s) 

 2, 14:58: 00 15%, 1 sec (1/500 th liter/s) 

 3, 16:19:30 15%, 1 sec (1/500 th liter/s) 

 

March 9, 2001 2 runs 1, 12:40:00 15%, 1 sec (1/500 th liter/s) 

 (approx.) 

 2, 14:13:00 15%, 1 sec (1/500 th liter/s) 

March 12, 2001 3 runs 1, 12:16:00 15%, 2 sec (1/500th liter/s) 

 2, 14:16:00 15%, 3 sec (1/500 th liter/s) 

 3, 15:52:00 15%, 4 sec (1/500 th liter/s) 

March 26, 2001 3 runs 1, 14:24:00 15%, 1 sec (1/500 th liter/s) 

 2, 15:27:00 15%, 2 sec (1/500 th liter/s) 

 3, 16:10:00 15%, 1 sec (1/500th liter/s) 

 

March 27, 2001 5 runs 1, 11:36:00 15%, 3 sec (1/500 th liter/s) 

 2, 12:41:00 15%, 2 sec (1/500 th liter/s) 

 3, 13:26:00 15%, 2 sec (1/500 th liter/s) 

 4, 14:43:00 15%, 3 sec (1/500 th liter/s) 

 5, 15:44:00 15%, 4 sec (1/500th liter/s) 

March 28, 2001 4 runs 1, 12:57:00 15%, 4 sec (1/500 th liter/s) 

 2, 14:01:00 15%, 3 sec (1/500 th liter/s) 

 3, 14:34:00 15%, 3 sec (1/500 th liter/s) 

 4, 15:47:00 15%, 1 sec (1/500 th liter/s) 

March 29, 2001 3 runs 1, 12:27:00 15%, 4 sec (1/500 th liter/s) 

 2, 13:27:00 15%, 1 sec (1/500 th liter/s) 

 3, 14:26:00 15%, 1 sec (1/500 th liter/s) 

April 2, 2001 7 runs 1, 11:42:00 15%, 1 sec (1/500 th liter/s) 

 2, 12:55:00 15%, 1 sec (1/500 th liter/s) 

 3, 13:40:00 15%, 1 sec (1/500 th liter/s) 

 4, 14:20:00 15%, 1 sec (1/500 th liter/s) 

 5, 15:09:00 15%, 1 sec (1/500 th liter/s) 

 6, 16:09:00 15%, 1 sec (1/500 th liter/s) 

 7, 16:44:00 15%, 1 sec (?) (1/500 th liter/s) 

 

NASA/TM—2005-212897 19

Appendix B—Solid Hydrogen Video Observations: February 28, 2001 

 The following lines present the observations recorded during two solid hydrogen formation runs on 

February 28, 2001: 

February 28, 2001: Run 1 

14:16:00 Begin H2 tank filling 

14:41:00 Preparing to drop H2

14:47:4:01 Beginning of taping 

14:50:59:14 Run 1 begins. Drop H2 onto helium surface, very clear, with little vapor created. 

 Mass flow rate = 1/500th liter per second, for 1 second 

14:51:00:19 Particles are visible. View is clear, no clouds present 

14:51:04:23 Particle are moving 

14:51:04:26 Good image for “smallest” particles 

14:51:15:10 Also, good image for small particles 

14:51:19:06 Massive number of small tiny particles 

14:51:25:02 Swarms... tiny particles. Height in the Dewar is 14 to 16 inches, below the Dewar lid. 

14:52:44:06 Good image of agglomerate. Swarms still persist. 

14:52:50:17 Good agglomerate image 

14:53:54:06 Tiny particle image (good for image capturing) 

14: 55:49:00 Hydrogen agglomerate trapped at visualization rake 

14:56:26:00 Good image of hydrogen agglomerate for area calculation 

14:56:55:06 Also, good image for area analysis 

15:05:23:12 Particles have not yet agglomerated. Surface is somewhat dynamic! 

15:06:57:18 Hydrogen agglomerate breaks up into many smaller pieces. They are about 0.5 to 0.75 

 inches in diameter 

15:09:45:11 Lots of motion on the surface, with many smaller agglomerates 

15:10:31:03 No tiny particles noticed 

15:16:06:23 Particles (hydrogen agglomerates) still not agglomerating fully. Agglomerates seem to 

 be compacting over time. 
 

NASA/TM—2005-212897 20

15:17:36:17 Good image for area measurement, with 4 or 5 masses present 

15:17:35:29 Good image for area analysis 

15:18:10:19 Hydrogen agglomerate continues to break up, and re-agglomerate. 

15:23:23:02 Pressure reduced in Dewar, particles (agglomerates) are agitated. 

15:23:27:03 Good edge, field of view (FOV) shot (bottom left of image) 

15:23:27:15 Another good edge, FOV shot (bottom left of image) 

15:23:30:01 FOV is clear 

15:23:30:02 Good edge, FOV shot, near the top left of the image. 

15:24:05:21 FOV shot, good edge (center right edge) 

15:24:23:02 Small, tiny particles are observed, in the center of the FOV. 

15:26:25:10 Particles, and small agglomerates, are dispersed over the entire surface 

15:29:47:28 Helium surface is very quiescent 

15:32:25:12 Many tiny particles are visible again, mostly at the center of the FOV 

15:34:57:13 Tiny particles persist, almost all particles seem to repel each other with some electrostatic 

 charge. 

Thought (or observation): Particle freeze onto the wall of the Dewar, slide down creating a charge on the 

particle. Probably a whimsical thought. 

15:36:21:07 Many more tiny particle visible 

15:39:50:03 Most of the particle agglomerate 

15:43:17:14 End of tape 1. Particles still persist in motion, changing shape and configuration 

 

Tape 2: 

15:43:42:09 Beginning of Tape 2 

15:44:00:06 Good image for area measurement 

15:46:19:21 Boiling occurring at the temperature rake 

15:49:38:26 Particle flies off the Dewar wall, onto the helium surface. It departs from the left center 

 of the image. 

15:50:28:13 Boiling occurring at the temperature rake, cloudy surface beginning. 
 

NASA/TM—2005-212897 21

15:53:02:24 Particles hover and stick to the visualization rake. 

15:57:22:19 Particles still not fully agglomerated 

15:59:34:02 Good image for area calculation. There are seven particles and /or agglomerates 

16:00:21:17 Solid hydrogen formation - Run 2 of the day begins. One can see the particles forming. 

 Mass flow rate = 1/500th liter per second, for 1 second 

 

16:00:21:21 New small particles form, about 1/16 inch in diameter (they appear dark at formation). 

16:00:22:16 Larger particles appear (about 1/8 inch in diameter) 

16:00:24:08 New particle mix with “old” ones (ones from the previous run) 

16:00:36:00 It is quite cloudy at the top of the FOV 

16:01:17:16 Good image of “complete” large agglomerate. Tiny particles are swarming. 

16:01:46:08 Many tiny particles attach to the larger mass (or agglomerate). 

16:02:54:16 The agglomerate is stabilizing on the visualization rake, and there are still many tiny 

 particles grabbing onto the main agglomerate. 

16:03:24:23 Fog clearing at the top of the FOV 

16:04:35:08 Micro-particles swirl incessantly, in a boiling frenzy 

16:05:27:07 Very good image for image capturing. There are still many micro-particles. 

16:05:40:06 Very good image for image capturing. 

16:06:08:10 Agglomerate breaks away from the visualization rake. 

16:06:45:10 Agglomerate rotates, elements reconfigure themselves. Location is near the visualization 

 rake. 

16:07:00:03 Only a small number of micro-particles are visible. 

16:07:38:04 Agglomerate pulsates. 

16:08:06:03 Visible darkening or aging of the agglomerate 

16:08:41:20 Small particles appear, as if from out of nowhere. 

16:08:46:11 Particles collide, and stick to the agglomerate 

16:09:42:09 Agglomerate lazily moves about the visualization rake 

16:09:57:07 Small waves appear on the surface, and the image is very clear. 
 

NASA/TM—2005-212897 22

16:11:21:14 Good image for image capturing. 

16:11:58:10 A ‘smile’ appear in the solid hydrogen. The agglomerate takes on the vague structure of 

 a Cleveland Indians “Chief Wahoo.” 

16:12:51:22 Quiet surface 

16:17:17:01 Agglomerate moves lazily about the visualization rake 

16:22:21:02 Reduce the Dewar pressure to agitate particles 

16:22:25:07 Edge of the FOV is evident (near the upper left of the image) 

16:22:26:05 Agitation completed. Pressure is restored. Dozens of small and intermediate particles 

 are visible. Good image for capturing. 

16:22:33:14 Particles are very numerous, and moving about. Most seem to be 10’s of millimeter in 

 diameter. 

16:23:21:19 Particles are moving rhythmically across the helium surface and beginning to 

 reagglomerate. 

16:23:51:06 Small millimeter-sized particles are evident. Good image for capturing. 

16:24:31:00 No swarms of tiny particles are present. Surface calming down. 

16:26:04:21 The particles (large ones) cluster at the bottom and top of the FOV. 

16:29:45:19 Bali dancer persist from the agitation. There appears to be a waving hand in the center of 

 the FOV. 

16:31:00:18 Many small particles reappear (they are millimeter sized and smaller). 

16:31:22:09 Strings appear. They are likely to be long chains of solid hydrogen micro-particles. 

16:31:33:23 An amorphous, diaphanous mass appears, near and above the center of the FOV. 

16:34:20:01 Good image for image capturing. 

16:34:48:28: More diaphanous sheets appear. They are above the center of the FOV and to the bottom 

 right of the center of the FOV. 

16:35:17:05 Most of the particles are ‘hiding’ behind the visualization rake. 

16:35:36:11 Good image for detecting small particles. 

16:37:45:16 Agglomerate is nearly complete. It begins rotating on the visualization rake. 

16:38:42:05 Good image for image capturing. 

16:39:09:24 GREAT image – capture it! Agglomerate is free of the visualization rake. 
 

NASA/TM—2005-212897 23

16:39:52:08 Another clear view 

16:43:40:19 Agglomerate continues rotation, with small (!) agglomerate at each rake. 

16:44:19:24 Smaller agglomerate from the temperature rake breaks free. 

16:44:50:00 Great agglomerate image 

16:47:54:00 End of Tape 2. 
 

NASA/TM—2005-212897 24

Appendix C—Solid Hydrogen Images 

 The following images show video observations recorded during the solid hydrogen testing on 

February 28, 2001. The images correspond to timing observations in appendix B. 
 

NASA/TM—2005-212897 25
 

NASA/TM—2005-212897 26
 

NASA/TM—2005-212897 27
 

NASA/TM—2005-212897 28
 

NASA/TM—2005-212897 29

Appendix D—Solid Hydrogen Particle Sizes 

 The following table presents the solid hydrogen particle sizes obtained from video observations of the 

two runs described in appendix B made on February 28, 2001. 
 

NASA/TM—2005-212897 30
 

NASA/TM—2005-212897 31
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AIAA–2003–4688

E–14317

WU– 713–74–10–00

36

Solid Hydrogen Experiments for Atomic Propellants: Particle Formation,

Imaging, Observations, and Analyses

Bryan Palaszewski

Solid hydrogen; Atomic species; Propellants; Feed systems; Gelled; Fuels; Cryogenics

Unclassified -Unlimited

Subject Categories: 15, 16, 20, and 28

Prepared for the 39th Joint Propulsion Conference and Exhibit cosponsored by the AIAA, ASME, SAE, and ASEE,

Huntsville, Alabama, July 20–23, 2003. Responsible person, Bryan Palaszewski, organization code RTB, 216–977–7493.

This report presents particle formation observations and detailed analyses of the images from experiments that were

conducted on the formation of solid hydrogen particles in liquid helium. Hydrogen was frozen into particles in liquid

helium, and observed with a video camera. The solid hydrogen particle sizes and the total mass of hydrogen particles

were estimated. These newly analyzed data are from the test series held on February 28, 2001. Particle sizes from

previous testing in 1999 and the testing in 2001 were similar. Though the 2001 testing created similar particles sizes,

many new particle formation phenomena were observed: microparticles and delayed particle formation. These experiment

image analyses are some of the first steps toward visually characterizing these particles, and they allow designers to

understand what issues must be addressed in atomic propellant feed system designs for future aerospace vehicles.