NASA/TM-2010-216246 AIAA-2010-1904

A Densified Liquid Methane Delivery

System for the Altair Ascent Stage

Thomas M. Tomsik

Glenn Research Center, Cleveland, Ohio

Wesley L. Johnson, Todd D. Smudde, and Mark F. Femminineo

Kennedy Space Center, Kennedy Space Center, Florida

Andrew R. Schnell

Marshall Space Flight Center; Huntsville, Alabama

May 2010
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NASA/TM-2010-216246 AIAA-2010-1904

A Densified Liquid Methane Delivery

System for the Altair Ascent Stage

Thomas M. Tomsik

Glenn Research Center, Cleveland, Ohio

Wesley L. Johnson, Todd D. Smudde, and Mark F. Femminineo

Kennedy Space Center, Kennedy Space Center, Florida

Andrew R. Schnell

Marshall Space Flight Center; Huntsville, Alabama

Prepared for the

SpaceOps 2010 Conference

sponsored by the American Institute of Aeronautics and Astronautics

Huntsville, Alabama, April 25-30, 2010

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

May 2010
Level of Review: This material has been technically reviewed by technical management.

Available from

NASA Center for Aerospace Information National Technical Information Service

7115 Standard Drive 5301 Shawnee Road

Hanover, MD 21076-1320 Alexandria, VA 22312

Available electronically at http://gltrs.grc.nasa.gov
A Densified Liquid Methane Delivery System

for the Altair Ascent Stage

Thomas M. Tomsik

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Wesley L. Johnson, Todd D. Smudde, and Mark F. Femminineo

National Aeronautics and Space Administration

Kennedy Space Center

Kennedy Space Center, Florida 32899

Andrew R. Schnell

National Aeronautics and Space Administration

Marshall Space Flight Center

Huntsville, Alabama 35812

Abstract

The Altair Lunar Lander is currently carrying options for both cryogenic and hypergolic ascent stage

propulsion modules. The cryogenic option uses liquid methane and liquid oxygen to propel Altair from

the lunar surface back to rendezvous with the Orion command module. Recent studies have determined

that the liquid methane should be densified by subcooling it to 93 K in order to prevent overpressurization of the propellant tanks during the 210 day stay on the lunar surface. A trade study has been

conducted to determine the preferred method of producing, loading, and maintaining the subcooled,

densified liquid methane onboard Altair from a ground operations perspective. The trade study took into

account the limitations in mass for the launch vehicle and the mobile launch platform as well as the

historical reliability of various components and their thermal efficiencies. Several unique problems were

encountered, namely delivering a small amount of a cryogenic propellant to a flight tank that's positioned

over 350 ft above the launch pad as well as generating the desired delivery temperature of the methane at

93 K which is only 2.3 K above the methane triple point of 90.7 K. Over 20 methods of subcooled liquid

methane production and delivery along with the associated system architectures were investigated to

determine the best solutions to the problem. The top four cryogenic processing solutions were selected for

further evaluation and detailed thermal modeling. This paper describes the results of the preliminary trade

analysis of the 20 plus methane densification methods considered. The results of the detailed analysis will

be briefed to the Altair Project Office and their propulsion team as well as the Ground Operations Project

Office before the down-select is made between cryogenic and hypergolic ascent stages in August 2010.

Introduction

Densified propellants have frequently been identified and studied as a viable approach for substantial

mass savings on launch vehicles as well as alleviating long duration cryogenic storage system (Refs. 1 to

5) problems. Several different densification systems were proposed for the Space Shuttle's liquid oxygen

and liquid hydrogen external propellant tanks (Ref. 6). Densified propellant systems were never

implemented however for actual use in Shuttle operations (Ref. 7). The interest in the technology

remained keen though, and propellant conditioning systems were designed, built and demonstrated in the

1990s. For example, large scale densification units were developed for handling liquid hydrogen and

liquid oxygen for the X-33 Reusable Launch Vehicle program (Refs. 8 to 10). Figure 1 shows the X-33

NASA/TM-2010-216246
Four-Stage GH2

Compressor

Figure 1.—X-33 liquid hydrogen propellant densification unit demonstration at the NASA Glenn Research Center S40 Test Facility.

LH2 propellant densification skid at a NASA test facility. The unit was designed to process 800 gpm of

densified LH2. Different approaches and alternative cryogenic systems were similarly proposed by others

in the aerospace community for the Second Generation Launch Vehicle and Next Generation Launch

System projects (Refs. 11 to 13). The mass savings benefit for such a propulsion system carrying

densified propellants can vary from as high as 20 percent reduction in launch vehicle mass at gross lift-off

to nearly a few percent, albeit a very significant few percent. In general compared to liquid methane,

liquid hydrogen becomes more difficult to densify in a practical method (Ref. 14) due to its very low

normal boiling point (20 K) and thus would drive the requirement for even lower temperatures for

densification.

NASA's Constellation program has several propulsion systems that are candidates for propellant

densification. The best candidate is the liquid methane/liquid oxygen ascent stage of the Altair lunar

larder. The densification state condition allows the heat leak that flows into the methane tanks through

the struts, penetrations, and multilayer insulation (MLI) during the mission to be absorbed by the

subcooled liquid propellant. The propellant undergoes a subsequent rise in temperature but without the

need to vent as tank pressure remains approximately constant during this "warm-up" period. Tank

pressure will gradually rise within the prescribed design pressure limits of the tank as the propellant nears

saturation. Initial CFD modeling studies show that the energy absorbed by the liquid methane over the

entire 210 day mission would not exceed thepropellant system thermal absorption capacity of the fuel

and oxidizer tanks (Ref. 15). Thus a key derived requirement of the cryogenic propellant densification

system is to base the design temperature for the production and delivery of 93 K subcooled liquid

methane stored on-board Altair prior to lift-off. A successful design to this requirement enables the

development of the mass and performance attributes of a viable cryogenic LOX/LCH4 ascent stage for

Altair.

The Altair is to be launched as part of the payload of the Ares V heavy lift launch vehicle. The Ares

V will deliver Altair and the Earth Departure Stage to Low Earth Orbit (LEO), where they will meet up

with the Orion crew vehicle. The two will rendezvous and dock before the Earth Departure Stage (EDS)

is reignited to perform the Trans Lunar Insertion (TLI) segment. The EDS becomes expended then

separates from the Lander and as the Lander approaches the Moon, it performs the Lunar Orbit Insertion

(LOI) maneuver as illustrated in Figure 2.

NASA/TM-2010-216246
100 km

Low Lunar

M MM Orbit l^ }

Ascent Stage

Lander Expended Performs LOI

r

Earth Departure

Stage Expended Service

Module

Exp ded

1

Direct Entry ,^

Land Landing ^ O®,^^

r

W

U

Ares V

4

0

Low

Earth -

Orbit

II

M

LU

Figure 2.—Typical lunar reference mission deploying Altair.

Currently the Ares V is over 100 in and is limited in height by the Vehicle Assembly Building

(VAB) at the Kennedy Space Center, where the Ares V will be assembled. The servicing of the cryogenic

ascent stage will have to be done at a height of approximately 100 in unless a method can be conceived to

allow for the storage vessel and transfer system to be located on the actual vehicle access./umbilical tower.

Historically, cryogenic propellants have been transferred to the launch vehicle from extremely large

LH2/LOX spherical storage dewars that are located over 500 m away from the actual launch pad. In this

operation, several thousand cubic meters of NBP propellant can be delivered to the launch vehicle. For

the very small amounts of propellant required by Altair (a total on-board fuel volume of only 1.82 m3)

this would be an extremely inefficient and impractical method of propellant delivery. The remote delivery

approach would in-turn impact system performance and would likely cause the cost of the delivery of the

propellants to negate any advantages gained by them.

A schematic of the Ares V Launch Pad, shown in Figure 3, indicates the relative elevations and

possible locations for a methane conditioning system. The Altair umbilical is about 100 in ft) above

the Mobile Launch Platform (MLP) while the Launch pad surface is 12 in (40 ft) above existing bulk

LOX/LH2 propellant storage tanks. This paper describes the results of a trade analysis made between 26

different conceptual methane densification processes for producing and delivering 1.82 m 3 of subcooled

liquid methane (LCH4) at 93 K to support Altairs' mission to the Moon.

NASA/TM-2010-216246
Figure 3.—Ares V launch pad schematic (not to scale).

Nomenclature

AIAA American Institute of Aeronautics and Astronautics

CFD Computational Fluid Dynamics

CFM Cryogenic Fluid Management

EDS Earth Departure Stage

ETDP Exploration Technology Development Program

ETO ethylene tetroxide

GHe gaseous helium

GRC NASA Glenn Research Center

GSE Ground Support Equipment

ISP Specific Impulse

JT Joule-Thompson

KSC NASA Kennedy Space Center

LAr liquid argon

LCH4 liquid methane

LEO Low Earth Orbit

LH2 liquid hydrogen

LN2 liquid nitrogen

LOI Lunar Orbit Insertion

NASA/TM-2010-216246 4
- ^ l

LOX liquid oxygen

LSS Lunar Surface Systems

LUT Launch Umbilical Tower

MAWP Maximum Allowable Working Pressure

MEOP Maximum Expected Operating Pressure

MLI Multi-Layer Insulation

MLP Mobile Launch Platform

MMH monomethyl hydrazine

MSFC NASA Marshall Space Flight Center

NASA National Aeronautics and Space Administration

NBP Normal Boiling Point

RLV Reusable Launch Vehicle

SMiRF Small Multi-Purpose Research Facility

SLCH4 slush methane

TLI Trans Lunar Insertion

TRL Technology Readiness Level

TVS Thermodynamic Vent System

VAB Vehicle Assembly Building

Vi Vacuum Jacketed

Altair Ascent Stage Design Requirements

The fuel tanks to be serviced by the densification system are the two liquid methane tanks mounted on

the ascent stage of the Altair, as shown in Figure 4. The spherical methane tanks are on the side of the crew

compartment, each paired with a liquid oxygen tank above it. The tanks will be serviced from the launch

umbilical tower that is built onto the mobile launch platform as depicted in Figure 5. There are two

spherical liquid methane tanks each of 1.3 in diameter and volume of 1.07 m3 . Each tank is to be 85 percent

full to allow for the thermal expansion of the propellant as it wanms-up during the mission. This means that

for two tanks, a total of 820 kg of subcooled liquid methane must be delivered to the vehicle.

Figure 4.—Typical cryogenic ascent stage on Altair, spherical liquid methane tanks have purple interior.

NASA/TM-2010-216246
Figure 5.—Ares V and Altair on the launch pad

next to the launch umbilical tower.

The thermal CFD analysis (Ref. 15) behind the LCH4 temperature requirement neglected transient

heating before reaching steady state in Low Earth Orbit (LEO). Prior to this condition being met, the

spacecraft must start off in the ambient atmosphere on the launch pad and be propelled to LEO, meanwhile

the transition that takes place with the MLI from 760 torr to 10 -5 torr is a very dynamic transition. Previous

testing has shown that the effects of this transition, that's akin to "out-gassing" of the MLI, can take from 24

to 48 hr to run their course before steady state is achieved within the propellant tanks. Lumped parameter

models developed by Johnson (Ref. 16) have shown that this thermal transition can generate a propellant

temperature increase of at least 0.5 K. Thus this study has assumed that the delivery of the LCH4 propellant

shall be at 92.5 K, so this temperature will be the actual design requirement used. Methane has a normal

boiling point (NBP) of 111.7 K so that the subcooling process requirement must result in a bulk fluid

temperature 19.2 K below the NBP. The Altair ascent stage design requirements considered in the methane

delivery system trade study are those as shown summarized in Table 1.

The on-the-pad heat leak of a flight like Altair liquid methane tank with 0.5 in. of spray-on foam

insulation (SOFI) underneath at least 60 to 80 layers of MLI will be in the range of 175 to 200 W. This

takes into account the increasing surface area of each layer of MLI as it's installed on the spherical tank.

Such a heat leak is much larger than the on-orbit heat leak of the system due to the effects of gas

conduction at ambient pressure, and in fact it is over two orders of magnitude higher than the expected

on-orbit and lunar heat loads per the studies of Fesmire and Cady (Refs. 17 and 18). The effect of LCH4

tank pressure rise taken from the CFD analysis by Moder et al. (Ref. 15) over the course of the 210 day

mission is shown in Figure 6. Their results confirm that the tank operating pressure design limits will not

be exceeded for the 100 psig helium pre-pressurization case when the tank initially contains subcooled

LCH4 at 96.1 K at launch.

NASA/TM-2010-216246
Stop analysis at 203 days

P = 375 psi (Tank limit)

T = 115.0 K

Simulate 210 day stay

► P = 201.28 psi

T = 115.4 K

TABLE 1.—ALTAIR ASCENT STAGE DESIGN REQUIREMENTS

FOR THE LCH4 CONDITIONER SYSTEM TRADE.

Fuel Tank Parameter Design Requirement

MEOP 2240 kPa (325 psig)

MAWP 3365 kPa (488 psig)

Pre-pressurization (j) 690 kPa (100 psig)

Diameter 1.27 in (50 in)

Vohune, empty 1.07 m3 (37.8 ft)

Material of constriction AL-2219

Tank mass, dry 34 kg (75 lb,,,)

Heat Leaks (per tank $)

On-the-Pad 175 W

Ascent 500 W

On-Orbit and Cruise 1.0 W

Lunar Surface (Ref. 15) 2.0 W

Loaded Propellant (per tank $)

Mass 411 kg (9061b^)

Volume 0.91 in' (32.3 ft')

Temperature 92.5 K (166.5 °R)

Fluid Interfaces. size/location

Vent 9.5 min / Top (3/8 in)

Fill/Drain 50.8 nnn / Bottom (2 in)

Propellant conditioning time 1.0 In

(f) Pre-press with "cold" gaseous helium prior to lift-off.

(1) Ascent stage has two LCH4 fi(el tanks on-board.

400

350

300

Y

v 250

0..

zoo

150

100

U SD 100 150 200

Time [Days] 210 days on lunar

surface

Figure 6.—Long term pressure history for the initially densified LCH4 Altair propellant tank.

(Figure duplicated from Ref. 15 with permission from Moder).

NASA/TM-2010-216246 7
Trade Study Objectives

The main objective of the study was to determine the best method for producing, transferring and

maintaining liquid methane at the required delivery temperature. In order to do that, each method was

analyzed thermodynamically using simple spreadsheet models. These models were used to determine or

validate the feasibility of the method as well as estimate the approximate mass required on the launch

umbilical tower (LUT). Seven metrics were used to determine the ranking of each method relative to the

other systems:

1. Safety (SAFETY): A relative measure of the assumed safety of each concept. This is measured in

terms of required hazards (to Launch Criteria, GSE, Launch Vehicle, and Personnel) of safety for

each concept.

2. Flight Impact (FLTIMPACT): A relative measure of the degree of impact required to the flight

system. This impact is measured in terms of thermal, structural, and mass changes to the vehicle.

3. Ground Impact (GRDIMPACT): A relative measure of the degree of impact required on the

current or planned ground support equipment. This impact is measured in terms of thermal,

structural, and mass additions to the Ground Support Equipment (GSE) such as mass on launch

umbilical tower, footprint, etc.

4. Densification Performance (DENSPERF): A measure of the concept's ability to condition the

propellant. This is measured in terms of the time required to perforin the densification, efficiency,

commodity consumption, simplicity, reliability, etc.

5. Implementation Cost (COST): A relative measure of the perceived costs of each concept. The

perceived costs will be judged using agency cost estimation guidelines for resources required to

operate and maintain the system in an operational environment.

6. Technological Readiness Level (TRL): A NASA defined metric that supports assessments and

comparisons of the maturity of a particular technology for spaceflight. The TRL for each delivery

system concept will be evaluated based on the NASA guidelines originated by Mankins (Ref. 19).

7. Operations Flexibility (OPFLEX): A relative measure of the dependency of the system on other

ground support systems as well as the duration of recovery to be able to support a scrub

turnaround.

Each method was scaled between 1 and 10 for each metric, where 10 represents the best or most

desirable. The sum of the seven scores gives the total score for the conditioning method. The top three to

four highest ranked propellant densification methods will be examined further during a Phase 2 study

using greater fidelity thermal models.

A secondary objective of the launch pad densification study was to determine what technology

development would need to be done by the Exploration Technology Development Program's (ETDP)

Cryogenic Fluid Management (CFM) Project. The CFM project develops technology to TRL level 6,

which would involve a "system demonstration in a relevant environment" (Ref. 19). The team examined

each of the top methods to determine if any technology development work would need to be done to

support the production and loading of densified liquid methane for the Altair ascent stage.

Methane Densification Conceptual Methods

A total of 26 densification or "propellant conditioning" method architectures were analyzed by the

study team. Each of these methods is listed below in Table 2. A simplified schematic was developed for

each densification system and these are shown in Figures 8 to 26. Relevant thermophysical properties for

liquid methane applied in the spreadsheet system modeling and various analyses are provided for

reference in Table 3.

NASA/TM-2010-216246
TABLE 2.—LIQUID METHANE CONDITIONER SYSTEM CONCEPT METHODS EVALUATED IN THE TRADE STUDY

Method

No.

Description

1 Low Temperature Gaseous Helium Bubbling

2 Low Temperature Gaseous Hydrogen Bubbling

3 Self Densification through a Transfer Pipeline TVS

4 Compact Heat Exchanger with Liquid Hydrogen

5 Compact Heat Exchanger with Liquid Argon

6 Heat Exchanger with Vacuum Orifice in Liquid Methane

7 Slush Methane—LCH4 at its Triple Point with 50% Solid CH4

8 Co-current Heat Exchanger through Indirect Transfer Pipeline

9 Modified X-33 RLV Propellant Densification Process with LCH4

10 Modified X-33 RLV Propellant Densification Process with R13

11 Modified X-33 RLV Propellant Densification Process with R14

12 Direct TVS Vacuum Pumping Process Scheme—On the Pad

13 hi-Situ Storage Vessel Densification—External Limpet Coil Heat Exchanger

14 Liquid Nitrogen Pool Boiling Heat Exchanger with Recirculation Loop

15 Liquid Hydrogen Pool Boiling Heat Exchanger with Recirculation Loop

16 Liquid Argon Pool Boiling Heat Exchanger with Recirculation Loop

17 hi-Situ Storage Vessel Densification—Internal Helical Coil Heat Exchanger

18 Direct Contact Cold Heat Sink

19 Regenerative Hydrogen Heat Exchanger

20 Closed Cycle Mechanical Vapor Compression Refrigeration—Joule-Thompson Expansion w/Nitrogen

21 Closed Cycle Mechanical Vapor Compression Refrigeration—Joule-Thompson Expansion w/Argon

22 Closed Cycle Mechanical Vapor Compression Refrigeration—Joule-Thompson Expansion w/Methane

23 Cryocooled Prechilling in Storage Vessel

24 t Direct TVS Vacuum Pumping Process Scheme—On Orbit

25 + Thermal Strap to Hydrogen Tank

26 + Ct ocooled Prechilling of Fli ght Tanks

(T) Evaluated as a baseline reference method only and was not ranked in the trade.

($)Method was determined to be mifeasible and was abandoned from the trade.

TABLE 3.—SOME KEY THERMOPHYSICAL PROPERTIES FOR LIQUID METHANE

Property Methane

Molecular Weight 16.042 kghnol

Normal Boiling Point (NBP) 111.7 K (201.0 °R)

Liquid Density at NBP 442.4 kg/m3 (26.37 lb/fe)

Triple Point Temperature 90.7 K (163.3 °R)

Triple Point Pressure 11.72 kPa (1.70 psia)

Triple Point Liquid Density 451.7 kg/m3 (28.20 lb/fe)

Solid Methane Density 511.0 kg/m3 (31.90 lb/fe)

Heat of Fusion 60.7 kJ/kg (26.10 Btu/lb)

Heat of Vaporization 5 10. 1 kJ/kg (219.3 Btu/lb)

Slush Density at 50% solids 481.4 kg/m3 (30.05 lb/ft')

Liquid Viscosity at NBP 1.17 x 10-4 Pa-s (7.89 X 10-s lb/ft-s)

Liquid Specific Heat at NBP 3.492 kJ/kg-K (0.834 Bt/lb--R)

Liquid Thermal Conductivity at NBP 0.188 W/m-K (0.109 Btr/lrr-ft-°R)

J-T Coefficient., Vapor at 293 K 0.00457 K/kPa (0.0.566 "R/psi)

NASA/TM-2010-216246
80000

70000

60000

50000

LO 40000

30000

20000

10000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Method No.

Notes: Method no. 10 is not shown because system mass exceeds 3.0x106 kg.

Method no. 19 system mass was not estimated during trade study.

Figure 7.—Densification system mass estimates.

Densification System Mass Scaling Estimates

The previously defined Ground Impact (GRDIMPACT) screening metric accounts for weight and

area foot-print impacts of Ground Support Equipment (GSE) located on the Mobile Launch Platform

(MLP). One of the many key attributes of a close-proximity MLP located propellant servicing station is

minimal mass and low total area requirements. Based on the system schematics that were prepared for

each concept densification method, the major equipment components required by the system were

defined. Mass scaling relationships were then developed for typical cryogenic processing equipment.

The majority of the methods utilize typical cryogenic process equipment including dewars, heat

exchangers, vacuum-jacket piping, vaporizers, pumps, valves, cold-gas compressors, and vacuum pumps.

The mass scaling relationships allowed relatively good determination of all major component weights.

These individual component weights were then tallied in order to estimate the total densification system

masses. The results of this analysis for 23 of the densification methods are shown compared in Figure 7.

Total conditioner system mass estimates were found to range from about 4800 to 73,000 kg.

The following subsections of the trade study report provides a brief technical description and some

discussion of each conceptual densification method that was evaluated along with its flow schematic.

Methods 1 and 2: Low Temperature Gaseous Helium or Gaseous Hydrogen Bubbling

Helium bubbling was used on the Saturn V launch vehicle to eliminate pogo effects in the oxygen

feed-lines (Ref. 20). The heat load to the liquid oxygen feed-line was so great that the boiling rate caused

the bubbles to merge together to form slug flow coming out of the feed-line. This created the water

hammer type effects from liquid rushing into the feed-line to fill the voids left from the bubbles. The

bubbles of helium effectively allowed the oxygen to evaporate into the helium bubble through diffusion,

removing heat from the oxygen in the process. In 1959, Clark and Merte (Ref. 21) developed a model for

bubbling heat transfer. This model was used to estimate the amounts of helium required to densify an

Altair liquid methane tank as shown by the schematic in Figure 8. The 2600 liter (690 gal) liquid

methane tank required to till the Altair tanks would weigh 1700 kg and, in addition, carry 1000 kg of

liquid methane at NBP. A 120,000 liter (32,000 gal) liquid helium tank would be required with a mass of

NASA/TM-2010-216246 10
Methods 1 & 2

Propellant Conditioning, Loading &

Maintenance Loop on the MLP

GCH4 GCH4

Vent Vent

or

Alta^ !' Altair

Tank F L Tank

LH2 Vaporizer or

LHe Vaporizer option

MLP

Figure 8.—Low temperature gaseous helium or hydrogen bubbling system schematic.

55,000 kg carrying 15,000 kg of helium. Including piping and valves, the total mass of the system was

estimated at 73,000 kg. This was a much higher system weight than the majority of the other concepts

evaluated, and thus the gHe bubbling method was dismissed via ranking.

Much different than Method 1, a low temperature gaseous hydrogen conditioning system (also see

Figure 8) would only require a 5000 liter liquid storage capacity with a tank mass of 3000 kg carrying

350 kg of liquid hydrogen. Using the same size methane storage tank and vacuum-jacketed piping system

as Method 1, the total mass of the gaseous hydrogen bubbling system was estimated at 6500 kg.

Additionally, any hydrogen that would be absorbed into the liquid methane would slightly increase the

specific impulse (ISP) of the Altair ascent stage. It would also be possible to pressurize the methane tanks

using this same hydrogen system, eliminating the need for helium pressurization.

Method 3: Self Densification through a Transfer Pipeline TVS

A transfer pipeline Thermodynamic Vent System (TVS) uses Joule-Thompson expansion of the

propellant to cool itself. The heat exchanger is placed inside of the transfer pipeline along with a JouleThompson throttling valve at the entrance to control the expansion of the liquid. Depending on the

operational mode, the exit of the heat exchanger can be vented to either the ambient atmosphere or to a

vacuum pump. In this case, in order to achieve 92.5 K in the process fluid, a vacuum pump would be

required to pump the fluid to near the triple point of methane (--91 K) to achieve the required

temperatures. These heat exchangers would be placed along the transfer line between the storage tank and

the flight vessel. Once the vessel was initially filled and cooled to the proper temperature a smaller

maintenance flow would be required to remove the energy that entered the tank as it sat on the pad. A

schematic of the Transfer Pipeline TVS conditioner can be seen in Figure 9. Two rotary piston vacuum

pumps would be needed for redundancy in the system as is used in the Heat Exchanger with Vacuum

Orifice in Liquid Methane (Method 6). The same sized 6 kW electric, 130 kg gas heater would be used

before the vacuum pump. The required methane storage tank would be 7600 liters (2000 gal) and weigh

4100 kg, while containing 3200 kg of liquid methane. Including 250 kg of valves and piping, the total

mass of the system is roughly 8000 kg.

NASA/TM-2010-216246 11
Gr

Method 3

Figure 9.—Self densification through a transfer pipeline thermodynamic vent system (TVS).

Methods 4 and 5: Compact Heat Exchangers with Liquid Hydrogen or Liquid Argon

Researchers at the Kennedy Space Center have been testing a compact, expanded metallic foam heat

exchanger, similar to a carbon unit tested at the University of Central Florida by Chow et al., (Ref. 22).

The expanded metal foam allows for a higher heat flux through the heat exchanger, drastically reducing

the size. The liquid methane could be cooled by such a heat exchanger using either liquid hydrogen or

liquid argon and then pumped into the Altair tanks. A schematic of the process can be seen in Figure 10.

In addition to the same 2600 liter liquid methane storage tank as required by the gHe or gH2 bubbling

options, an 110,000 liter (29,000 gal) liquid hydrogen tank would be required at 50,000 kg carrying

8000 kg of liquid hydrogen. Similarly, a 16,500 liter liquid argon dewar would be required, with a mass

of 8000 kg. The liquid argon itself would have a mass of 23,000 kg. Both compact heat exchanger

methods are excessively heavy at 61,000 for the LH2 and 34,000 kg for the LAr working fluids,

respectively, when compared to the other densification systems analyzed.

Method 6: Heat Exchanger with Vacuum Orifice in Liquid Methane

A vacuum orifice within the liquid portion of the vehicle tank would remove the necessity of subatmospheric pressure inside the Altair tanks. Using a screen in the lower, wetted portion of the tank, it

becomes feasible to remove energy from the liquid through evaporation across a restrictor screen. This

could allow the removal of energy from the liquid without dropping the pressure inside the tank. A

schematic of the vacuum orifice pumping process is shown in Figure 11. The required size for the

vacuum pump is 0.133 m3 /s (283 cfin, 8000 1pm) and such compressors are commercial off-the-shelve.

Three rotary piston vacuum pumps each at 364 kg would provide the necessary gas flow rates as well as

lend redundancy as two pumps will provide enough floe- with a third, redundant pump. In order to make

up the volume of the liquid pumped, a make-up LCH4 tank is necessary. Based on the vacuum size and a

7-hr maintenance period (500 slpm), a 6000 liter liquid methane tank would be required. This storage tank

would weigh about 3500 kg, while the methane in the tank would be roughly 2600 kg. Additionally, a

NASA/TM-2010-216246 12
LCH4

LUT

)rifice

>creen

lyp)

small electric heater would be required to pre-heat the vent gas flow before it entered the vacuum pump

and the power required for the heater would be approximately 6 kW at 130 kg. The total mass of this

system, including VJ lines and valves, has been estimated at 7000 kg.

Methods 4 & 5

Propellant

Maintenance Altair 'rpAltairll Loop on MLP

LCH4 LCH4 I

Vent 

L Tank L Tank

Propellant conditioning

and transfer GSE

Compact SUPPLY Heat

Exchanger

Lin -A

Co-current

LCH4

LUT Pump

Figure 10.—Compact heat exchangers with liquid hydrogen or liquid argon.

Method 6

LCH4

Pump

Figure 11.—Schematic of heat exchanger with vacuum orifice in liquid methane.

NASA/TM-2010-216246 13
GCH

or V<

Recc

Conc

14

t

Loading

arrows

MLP

Method 7 rrun

Figure 12.—Slush methane densification process schematic.

Method 7: Slush Methane—LCH4 at its Triple Point with 50 Percent Solid CH4

Slush methane, would be a 50 percent by mass solid methane with the remainder in the liquid phase at

the triple point of methane (90.8 K). The use of slush methane (SLCH4) would provide enough thermal

margin to allow the Altair tanks to sit for almost 24 hr before servicing would be required to maintain the

liquid temperature below 92.5 K. Slush would be produced in a supply dewar using a vacuum pump and

agitator to prevent the formation of accumulated solidified ice at the fluids' surface. During the SLCH4

production cycle, a thin layer of solid forms at the surface and then breaks-away into the bulk fluid due to

the mechanical action of the agitator. After the production cycle, the slush methane would then be

pumped to the Altair tanks where it would be allowed to sit until launch. A schematic of the slush

methane production process is shown in Figure 12. The supply dewar would be 6000 liters and weigh

3300 kg. The production process would start off with 2400 kg of NBP liquid methane. Two rotary piston

vacuum pumps each rated at 510 m3/hr (300 cfm) would be needed for a total mass of 1400 kg. A 54 kW

electric heater with a mass of roughly 525 kg would be used to warm up the cold vapor before it enters

the vacuum pump inlets. The slush agitator would be 120 kg. Including VJ piping and valves, the total

mass of the slush methane conditioner system was estimated to be around 7000 kg.

Method 8: Co-current Heat Exchanger through Indirect Transfer Pipeline

A co-current or "parallel flow" heat exchanger would allow a triple wall pipe to be used as a heat

exchanger. The inner fluid would be liquid methane while the middle fluid would be liquid nitrogen. The

nitrogen would be saturated at 440 kPa (49 prig) to prevent potential solidification of the liquid methane.

The third or outer pipe would contain super-insulation, providing a vacuum-jacket to minimize the heat

transfer to the surrounding ambient. A portion of the heat exchanger would be used in the recirculation

loop for maintenance of the propellant while it's in the densified condition. Pumps would be required for

NASA/TM-2010-216246 14
Dewar

Grade

GSE

A A

LN2

Pump

Co-current

♦ LCH4

Pump

Method 8

typ. TRIPLE-PIPE

HEATEXCHANGER

SECTION AA

LCH4

inner

tube LN2

annulus LCH4

fluid Pump

MLP Insulation

annulus

Propellant

Maintenance

Loop on MLP

GN2

Vent

For countercurrent

HXER, LN2 supply

feed would be here

Figure 13.—Co-current heat exchanger methane densification system.

both the methane and nitrogen flows. A schematic for the co-current heat exchanger system can be seen in

Figure 13. The total length of the heat exchanger would be 65 m, meaning that it would probably be best

to place the storage tanks on the ground level and pump the fluids up the side of the launch tower to the

Altair tanks. Using stainless steel 316 pipe sizes of 0.75-in. Sch. 10, 1.5-in. Sch. 10, and 2.5-in. Sch. 10,

the total weight of the heat exchanger would be roughly 640 kg. Redundant pumps for each flow would

total around 200 kg. The process would require an 18,000 liter liquid nitrogen dewar of 9000 kg

containing 14,000 kg of liquid. The standard 2600 liter liquid methane tank would be required, totaling

2700 kg of tank and fluid. With 200 kg of other piping and valves the total mass of the system was

estimated at 27,000 kg. However, the majority of the mass that is the two LCH4 and LN2 storage vessels

would not be on the mobile launch umbilical tower (LUT). A counter current system was also considered

as a conditioning method, however an additional 100 in run for the LN2 supply line would be

required up the side of the mobile launch platform to provide coolant to the maintenance section of the

heat exchanger.

Methods 9, 10 and 11: Modified X-33 RLV Propellant Densification Process with LCH4, R13 or

R14 Working Fluids

The Modified X-33 Propellant Densification Process is based on a proven thermodynamic vent

system (TVS), a principle that rejects heat based on evaporation of a subcooled fluid via reduction of the

"working fluids" vapor pressure. Three possible "working fluids" or refrigerants were considered in the

trade study and are listed below in Table 4.

The basic process consists of a heat exchanger, a "cold-gas" compressor, and a recirculation pump.

Historically, two large scale units using this particular technology were designed, built and tested during the

X33 RLV program (Ref. 1) to support a future X-33 flight experiment with high-density L02 and LH2

propellants on-board. The successful demonstrations conducted at NASA's Glenn Research Center test

facilities enabled the technology to progress to a TRL rating of six. Densified methane propellant for the

NASA/TM-2010-216246 15
Altair would be produced by flowing normal boiling point propellant through a heat exchanger that subcools the feed. The liquid-to-liquid heat exchanger contains a bath filled with saturated LCH4 at 91.2 K and

12.41 kPa (164.1 °R, 1.8 psia) that chills the entering methane stream to the required temperature of 92.5 K.

A multistage centrifugal compressor, that's much like a vacuum pump, is used to reject the low-pressure

boil-off gas to an ambient vent, thus maintaining the sub-atmospheric pressure level in the heat exchanger

bath. The contents of the Altair tanks are densifed over a 1-hr time-line by recirculation of LCH4 in a

closed-loop fashion from the top of Altair's propellant tanks through the modified X33 refrigeration unit and

then back into the bottom. Over time, the Altair tanks become less stratified as subcooled LCH4 at the

bottom fill displaces warmer LCH4 that's removed from the top withdrawal line.

TABLE 4.—POTENTIAL REFRIGERANTS FOR SUBCOOLING LCH4 BY THE MODIFIED X33 PROCESS.

Refrigerant Chemical name Chemical NBP, Triple point temp., K

designation formula K

R50 methane CH4 111.7 90.7

R13 ehlorotrifluoromethane CC1F3 191.8 91.7

R14 tetrafluoromethane CF4 145.3 89.4

A schematic of the LCH4 modified X33 densification process using the working fluid liquid methane

is shown in Figure 14. The schematics for the R13 and R14 refrigerants are similar to the "all methane"

system, with the exception of vapor recovery condensers and refrigerant storage drums that would be

required downstream of the compressor, in order to recover and reuse the refrigerant fluid. For the system

sizing analysis, the Altair tanks were assumed to be filled from a LCH4 bulk storage tank supply located

remotely from the MLP. All three concepts would require minimal capacity methane storage dewars

located on the MLP integrated with the refrigeration units in order to provide make-up fluid for the

processes. The estimated system masses for the X33 densifiers using LCH4 and R14 were approximately

5000 kg and 8000 kg, respectively. The spreadsheet model for the system using R13 showed that it was

impractical from a performance and system mass perspective and therefore was omitted from the trade

space via the normal ranking process.

Methods 9, 10 & 11

loop is iocateo on the MAN ono tanK ioaoing is

MLP from GSE

Figure 14.—Modified X-33 RLV propellant densification process with LCH4 working fluid.

NASA/TM-2010-216246 16
—L0H4LoadedMas

—Talk Pre5sure

10 20 30 40 50 60

Time, min

mao

BM0

81QO

U)

BMO

2

U

J

79ao

78a0

770.0

1 fi0

14.0

120

m

1a0 a

W

BO P

RD c

lU

H

4.0

20

0.0

70

Method 12: Direct TVS Vacuum Pumping Process Scheme—On the Pad

The simplest and most direct method of conditionin g the liquid methane on-the-pad would be by an

evaporative cooling method that uses a vacuum pump. This particular technology has been successfully

used in previous propellant conditioning test applications. These prior experiences include subcooling

13,000 gal batches of liquid hydrogen (Ref. 23) and more recently, for conditioning fully loaded tanks of

NBP LCH4 for a Cryogenic Fluid Management (CFM) test conducted at the NASA Glenn Research

Center (Refs. 24 and 25), a program that's also in support of Altair's cryogenic ascent stage.

The Altair tanks would be chilled and then each initially filled with 387 kg of NBP LCH4. The

process begins by initiating a pump-down process on the ullage of the pre-loaded vehicle propellant tanks.

Liquid temperature is reduced due to evaporation of LCH4 using the vacuum pumping system, while

concurrently, tank ullage pressure decreases as indicated by the pump-down model results of Figure 15.

The final ullage pressure needed to sub-cool LCH4 down to 92.5 K is 14.5 kPa (2.1 psia). The analysis

was based on a 1-hr conditioning time employing two rotary piston vacuum pumps each rated for a

displacement of 255 M3 /hr (150 chn). The TVS scheme is an effective approach to subcool the fluid by

direct evaporative cooling, a process that consumes about 0.14 kg of methane vapor boil-off to lower the

temperature of 1 kg of NBP propellant by 19.2 K while maintaining a constant volume. This means that a

small flow of NBP LCH4 make-up flow is added to the tanks during the pump-down. A schematic of the

Direct TVS Vacuum Pumping Process is illustrated in Figure 16. The system is apparently simple to

operate, has good controllability and the equipment requirements including system footprint and overall

mass are relatively low in comparison to many of the other densification methods. This total mass of this

system was estimated to be on the order of 5300 kg.

Figure 15.—Altair tank ullage pressure and total loaded mass for direct TVS

pump-down.

NASA/TM-2010-216246 17
Loading

arrows

GCH4 Method 12

VentLj . "GCH4

MLP — — Maintenance arrows – NBP LCH4 make-up

Figure 16.—Direct TVS Vacuum Pumping Process Scheme - On the Pad.

Method 13: In-Situ Storage Vessel Densification—External Limpet Coil Heat Exchanger

The in-situ or "internal" storage vessel densification process is an indirect cooling method that

combines the LCH4 storage tank, sized to fully load propellant on-board both Altair tanks, with special

heat exchange apparatus integrated on the vessel itself. In this case, the heat transfer apparatus is an

external limpet coil, welded to the outer shell of the stora ge vessel wall, thus creating a full jacket. Limpet

coils are basically longitudinal sections of half-pipe, typically 2- to 4-in. size, of Sch 40 stainless steel

pipe. The design approach renders the system very appropriate as a modular, portable densification unit

that can conceptually be temporarily positioned on the MLP and then removed shortly prior to launch. A

schematic of the in-situ storage vessel densifier with limpet coil is shown in Figure 17. Liquid nitrogen is

used as the cooling fluid, as LN2 is shown flowing under pressure through the limpet coil circuit and then

to vent. A recirculation pump is employed in the design to move densited LCH4 from the densifier

vessel to the Altair tanks and then back to the densifier in a closed loop fashion, thus providing either

constant or intermittent propellant temperature maintenance while Ares V sits on the pad. The method is

operationally flexible in that it may either be operated as a batch cooling system or a continuous process.

The preliminary LCH4 conditioner vessel and LN2 storage capacities requirements were each sized at

2500 liters (650 gal), respectively. The total estimated dry mass of the system, including margin for VJ

piping and valves, was 7400 kg.

NASA/TM-2010-216246 18
Method 13

Figure 17.—In-Situ storage vessel densification – external limpet coil heat exchanger.

Methods 14, 15 and 16: Liquid Nitrogen, Liquid Hydrogen or Liquid Argon Pool Boiling Heat

Exchangers with Recirculation Loops

This method uses a cryogenic liquid pool boiling heat exchanger to reduce the temperature of the

methane on its way to the launch vehicle. A similar concept is being studied at KSC to deliver cold

helium using a liquid hydrogen pool boiler. The recirculation loop allows for the system to absorb all of

the energy that goes into the launch vehicle while sitting on the surface. This system is defined by the

pool boiling heat exchanger schematic in Figure 18. Pool boiling provides high heat transfer rates at

relatively easily obtainable conditions. By transferring the liquid methane through a boiling pool of liquid

nitrogen, liquid argon, or liquid hydrogen, the areas of heat transfer would be much lower than in other

configurations. All pool boilers would require a 2600 liter liquid methane tank that weighs 1800 kg for

the dewar and contains 1000 kg of liquid methane.

A liquid nitrogen pool boiler would require a 1300 liter LN2 dewar and 1300 liter heat exchanger

both with a mass of approximately 1100 kg. A total of 1100 kg of liquid nitrogen would be used during

the process. A pump would be needed for each liquid and would total 300 kg including redundant pumps.

Including valves and piping, the liquid nitrogen pool boiler mass was estimated to be roughly 7000 kg.

A liquid hydrogen pool boiler would require a 1300 liter LH2 dewar and 1300 liter heat exchanger

both with a mass of approximately I 10 kg. A total of 100 kg of liquid hydrogen would be used during

the process. A pump would be needed for the liquid methane, but not the liquid hydrogen and would total

200 kg including redundant pumps. Including valves and piping, the liquid hydrogen pool boiler weight

was estimated to be about 6000 kg.

A liquid argon (LAr) pool boiler would require a 1300 liter LAr dewar and 1300 liter heat exchanger

both with a mass of approximately 1100 kg. A total of 2000 kg of liquid argon would be used during the

process. A pump would be needed for each liquid and would total 350 kg including redundant pumps.

Including valves and piping, the liquid argon pool boiler system mass was estimated to be approximately

7500 kg.

NASA/TM-2010-216246 19
Methods 14, 15 & 16

Figure 18.—Pool boiling heat exchanger with recirculation loop – LN2, LH2 or LAr.

Method 17: In-Situ Storage Vessel Densification—Internal Helical Coil Heat Exchanger

The in-situ storage vessel densification process with the internal helical coil heat exchanger is

identical to the previous Method 13. The primary difference is in the design of the tank heat transfer

surface area. Rather than employing the external limpet coils like Method 13, greater than five percent

mass savings is realized if one uses an internal helical coil that could be constructed of 1/2 in. OD stainless

steel tubing. This tank configuration is also more readily adaptable to a vacuum-jacket installed over the

conditioning tank exterior wall. The simplified flow schematic that defines this process is given in

Figure 19. The LCH4 conditioner vessel and LN2 storage capacities requirements both remain the same

as Method 13, each at 2500 liters (650 gal), respectively. The total estimated dry mass of the system,

including margin for VJ piping and valves, was 7000 kg, which is about 400 kg less than the limpet coil

approach. Finally, both processes would consume about 1900 liters (500 gal) of LN2 to support

conditioner vessel chill-down, the initial LCH4 densification conditioning cycle and propellant

maintenance for four consecutive launch days, which is a system requirement.

Method 18: Direct Contact Cold Heat Sink

The direct contact cold heat sink would utilize a liquid nitrogen filled heat sink. The nitrogen heat

sink would take the form of a tank internal to a methane dewar. This system would rely on natural

convection currents to evenly subcool the entire vessel. In order to prevent the need for a recirculation

system, a gaseous helium system would be in place for pressurization of the flight tanks to return the

liquid methane back to the storage dewar for reconditioning at an appropriate time. The helium would be

cooled by the nitrogen vent gas to minimize the energy input into the methane upon transfer as well as to

provide thermal mass to keep the tank cold. A schematic of the Direct Contact Cold Heat Sink can be

seen in Figure 20. The inner nitrogen tank, would have an MAWP of 100 psig, a volume of 1300 liters

and weigh 1200 kg. The inner LN2 tank would also contain 1900 kg of liquid nitrogen. The methane

storage tank, not including the internal nitrogen tank volume, would be 12,000 liters (3200 gal) for proper

clearances and heat transfer around the inner nitrogen tank. The mass of the LCH4 dewar would be

6000 kg not including 3000 kg of liquid methane. The helium supply would require approximately

500 kg. The total mass impact of this Direct Contact Cold Heat Sink system was estimated at 14,000 kg.

NASA/TM-2010-216246 20
"W7

Dewar

Loading

arrows

Method 17

Figure 19.—In-Situ storage vessel densification – internal helical coil heat exchanger.

Method 18

^- — ► Maintenance arrows — cold GHe

MLP pressurized expulsion back to cold sink

Figure 20.—Schematic for the Direct Contact Cold Heat Sink method.

NASA/TM-2010-216246 21
Method 19

Figure 21.—Simplified flow schematic for the Regenerative Hydrogen Heat Exchanger method.

Method 19: Regenerative Hydrogen Heat Exchanger

This was the least studied methane conditioning approach, therefore, the performance and mass

impacts for this method are currently uncharacterized. A concept schematic for a possible densification

process using this type of heat exchanger is defined by Figure 21. The key attributes of the Regenerative

Hydrogen Heat Exchanger method are the following:

• Porous finely divided metal mass ("thermodynamic sponge")

– Wires

– Spheres

– High porosity metal or metal alloys

• Alternating thermal cycles

• Accepts heat and rejects heat as the warm or cold fluid flows thru the metal media

• Cold fluid: LH2 at 20 K (36 °R)

• Warm fluid: LCH4 at 112 K (201 °R)

Methods 20, 21 and 22: Closed Cycle Mechanical Vapor Compression Refrigeration—JouleThompson Expansion with Nitrogen, Argon or Methane

One single-stage system using a mechanical vapor compression refrigeration cycle was considered in

the trade study. Three pure refrigerants were evaluated with the spreadsheet mass./energy balance models

using the common refrigeration cycle. These working fluids analyzed were nitrogen, argon and methane.

The most simple subcritical refrigeration system approach for densification of LCH4 would include the

following equipment: a multi-stage reciprocating compressor with interstage coolers, a high pressure

discharge line, a gas cooler for the subcritical cycle where the heat of compression is rejected to cooling

NASA/TM-2010-216246 22
Recuperatie

HXer

jr Accumulato

A

Dewar

Fill &

Make-U

Methods 20, 21 & 22

LCH4

Pump

Figure 22.—Closed cycle single-stage mechanical refrigeration with Joule-Thompson expansion for LCH4

densification.

water or ambient air, a recuperative heat exchanger for pre-cooling the working fluid, an expansion

device, an evaporator, a low pressure compressor suction line, and an optional liquid-line./suction-line

heat exchanger that was not used.

To minimize the use of rotating turbomachinery, all concept cycles employed direct Joule-Thompson

expansion valves, as opposed to turboexpanders. Expansion of the pre-cooled high pressure gas stream at

20,700 kPa (3000 psi) would cause a fraction of the working fluid to liquefy. The liquefied refrigerant at

91 K (165 °R) subcools LCH4 to the target temperature of 92.5 K in the evaporative heat exchanger via a

recirculation process through Altair's propellant tanks. The generic mechanical refrigeration process is

defined by the schematic in Figure 22. The LCH4 storage dewar in all cases contained nominally 3,800

liter (1000 gal) of liquid methane capacity. The mass impacts of these systems for each of the working

fluids was driven by the compressor sizing requirements as shown by the perfonnance numbers compared

in Table S.

TABLE 5.—MASS IMPACTS FOR THE MECHANICAL VAPOR COMPRESSION

REFRIGERATION PROCESSES FOR SUBCOOLING LCH4

Refrigerant Compressor power Refrigerant inass flow Total system mass

(BHP) (kg/ln) (kg)

Nitrogen 400 1820 13,700

Argon 350 1870 12,600

Methane 200 310 9,300

Method 23: Cryocooled Prechilling in Storage Vessel

Cryocooled pre-chilling directly in the LCH4 storage vessel would entail using a cryocooler to not

only produce zero-boil-off conditions, but to densify the fluid as well. This has been proposed numerous

NASA/TM-2010-216246 23
times by the aerospace community, but has not been thoroughly tested (Ref. 26). A small pump would be

needed to allow for a circulation loop back to the storage vessel to maintain the flight vehicle fluid

temperatures during maintenance. A schematic of the Cryocooled Prechilling in Storage Vessel system is

shown in Figure 23. A Cryomech AL-600 Gifford McMahon cryocooler provides 600 W of cooling

power at 80 K. Thus four of these would be required to meet the power requirement of 2.4 kW plus two

redundant coolers as backups for an 8 hr chill down period. For a 24 hr chilldown time only three

cryocoolers would be required. Each coldhead has a mass of 50 kg, so each complete system is estimated

to be roughly 300 kg including a compressor and cold lines. A 3500 liter (900 gal) tank of liquid methane

would provide the necessary liquid to allow the cryocooler to run continuously during fill and

recirculation. The tank would have a mass of 3200 kg and contain 1500 kg of liquid methane. The total

system mass has been estimated at 6000 kg for an 8 hr chilldown of the storage tank and 5000 kg for a

24 hr chilldown of the storage tank. This was the only method that was not sized for the 1 hr conditioning

time-line with-out compromising the feasibility of operation.

Method 24: Direct TVS Vacuum Pumping Process Scheme—On Orbit

In order to develop a baseline vehicle impact, it was necessary to do a quick analysis of what the

impact of performing the densification using the flight vehicle hardware would be. It was determined that

the easiest way to do this would be to launch enough therinal mass at the normal boiling point of liquid

methane to do an on-orbit densification by a controlled opening of the vent valve to the vacuum of space.

This is defined by the schematic in Figure 24. It was determined that minimally, 32 kg of extra propellant

would be required per tank. This additional LCH4 corresponds to more or less an 8 percent increase in

propellant mass onboard Altair. The tank diameter would increase by 2 cm, with a mass increase of

1.5 kg. That results in a 5 percent increase in dry mass not including any structural mass increase. This

becomes roughly 77 kg of added mass to service the stage while it's in orbit. Only the dry mass would be

carried to the lunar surface and back into Low Lunar Orbit (LLO) to deliver the astronauts back to the

Orion crew module. The above vehicle impact analysis did not include the mass of additional propellants

needed for the Ares V first stage, but that would probably be negligible in comparison to the total launch

vehicles' mass at lift-off.

Method 23

LCH4 Indicates solid strap/

Pump conductor

Figure 23.—Cryocooled Pre-chilling in Storage Vessel methane densification

method.

NASA/TM-2010-216246 24
GCH4

ON-ORBIT

GCH4

'ent

Loading

arrows MLP

Tanks On the MLP - Loaded Vent Tanks to Vacuum of

with NBP LCH4 Space On Orbit in LEO to

Vent Vent

Subcool LCH4 before TLI

Method 24

Figure 24.—System schematic for Direct TVS Vacuum Pumping - On Orbit.

Method 25: Thermal Strap to Hydrogen Tank

By thermally connecting the Altair liquid methane tanks to a liquid hydrogen tank, it was postulated

that the energy removal from densification could be rejected to the hydrogen tank. A schematic of the

thermal strap can be found in Figure 25. For a 1 hr densification time, it was determined that 7.5 kW

would be needed to achieve densification. Using a UHFC copper strap that was 0.5 in the required

cross sectional area would be 0.06 m' (a 10- by 10-in. block). This was deemed excessive in size, mass,

and attachment area and the method was scrubbed from the trade study.

Method 26: Cryocooled Pre-Chilling of Flight Tanks

By pre-chilling the flight tanks with normal boiling point liquid hydrogen, it was thought that it might

be possible to remove enough energy from the tanks to densify the liquid methane. A schematic of a prechilled flight tank can be seen in Figure 26. However, the 34 kg aluminum AL-2219 tanks would only

provide 450 kJ out of the 27 MJ required to densify the liquid methane. A cryocooler attached directly to

the flight tanks was discussed as well under this concept, but a ground based cooler would suffer from the

same issues as previously discussed with the Thermal Strap method. Thus the cryocooler pre-chilling of

the flight tanks was abandoned as an unfeasible solution.

NASA/TM-2010-216246 25
Figure 25.—Thermal Strap to Hydrogen Tank conditioning method.

Method 26

Indicates solid strap/

conductor

Cryocooler 

f7

 y Make-Up

LCH4

LCH4

Pump

Figure 26.—System schematic for Cryocooled Pre-chilling of Flight Tanks.

NASA/TM-2010-216246 26
Technical Note on Pump Reliability

It is often pursued by launch pad designers to minimize the amount of rotating machinery on the

launch pad. The main reason given for this practice is the reliability issues of the machinery. An effort

was made during the study to gather all of the historical infonnation from the existing rotating machinery

on the pads. The Space Shuttle liquid oxygen pumps have required corrective action 14 times in 214

loading attempts. Not one of the 14 times has the second pump failed thus causing a scrub. The reliability

for an individual LOX pump was calculated to be 93.4 percent. The reliability of two pumps each having

a 93.4 percent reliability is 99.6 percent. The reliability of two pumps is estimated to be sufficiently high

enough to permit the use of liquid transfer pumps as required for the methane densification systems. For

three pumps at 93.4 percent reliability, the probability of two pumps failing is less than 2.5 percent.

Phase I Method Screening Results

After the densified methane delivery methods were technically evaluated both operationally and for

performance, a down select screening analysis was made using a Kepner-Tregoe trade matrix (Ref. 27).

The average scores (X) of all the methods for each of the seven metrics were calculated based on the

individual scores generated by five technical reviewers. The average scoring data for the various metrics

are reported in Table 6 while the scale range that was used is described below.

Scale Range: 1 to 10

Highest Score: 10 (most desirable)

Scale Definitions

– 10: minimal impact%cost, optimal performance

– 7: minor impact cost, acceptable performance

– 4: major impact/cost, marginal perfonnance

– 1: extreme impact/cost, failed performance with slim possibility of improvement

– 0: unacceptable, all zeros will be thrown out of trade space

The rankings of each system were then compiled into a ranking spreadsheet where the weight of each

individual metric was a variable. The term used for the weighting variables were referred to as

Importance Factors (a,) for each of the seven metrics. Using a variety of different weighting scenarios that

ranged from each k being equally weighted (k = 1/7) to COST and TRL Vs being omitted (kc = 0.0 and

^ T = 0.0) with a higher emphasis on the SAFETY metric, the dependence of the weighting factors became

minimized. There were five Importance Factor distributions that were run in the screening analysis as

shown by the values applied in Table 7. The following expression provides the mathematical relation for

computing the total score (0) for each methane conditioner method based on their average scores (X) plus

factoring in the different values of k.

(1)=XS - kl+XF • kl +XG • XG +XD ' kD+XC • kC +XT ' a^T+XO' kG

where,

(D = total method score

X = average metric score

X = importance factor for metric

subscripts for metrics

S = SAFETY

F = FLTIMPACT

G = GRDIMPACT

D = DENSPERF

NASA/TM-2010-216246 27
C = COST

T=TRL

O = OPFLEX

TABLE 6.-AVERAGED SCORE DATA FOR SCREENING METRICS OF THE LCH4 CONDITIONER METHODS.

CONCEPT SAFETY

XS

FLTIMPACT

XF

GRDIMPACT

XG

DENSPERF

XD

COST

Xc

TRL

XT

OPFLEX

Xp

1 Heli urn Bubbling 7.80 6.80 4.20 6.00 4.50 6.50 6.00

2 Hydrogen Bubbling 5.40 8.60 7.80 7.20 6.88 3.00 6.00

3 Li ne TVS 5.60 7.90 5.80 6.60 5.75 4.00 6.50

4 Compact H/E - H2 5.30 7.80 5.20 6.80 6.00 4.50 6.75

5 Compact H/E- Ar 7.70 7.80 5.20 7.40 5.00 4.50 6.00

6 Vacuum Orifice 6.50 6.90 7.80 6.20 7.50 4.00 6.75

7 Slush Methane 5.80 7.90 7.60 8.10 6.33 6.00 7.63

8 Co-current H/E 7.40 8.10 5.00 7.50 5.67 7.50 6.75

9 Modified X-33, LCH4 6.50 7.80 8.40 7.40 6.75 7.00 8.00

10 Modified X-33, R13 4.00 7.80 3.00 4.60 5.00 4.00 6.75

11 Modified X-33, R14 4.00 7.80 5.60 6.80 5.67 5.00 6.75

12 Di rect TVS 6.10 7.20 8.40 7.80 7.67 7.50 7.00

13 In-SITU Limpet 7.50 7.40 5.80 6.80 7.00 7.00 6.75

14 LN2 H/E 7.10 7.80 5.80 6.90 5.75 4.50 6.75

15 LH2 H/E 5.20 7.70 7.00 6.90 5.75 4.50 6.13

16 LAr H/E 7.00 7.80 5.30 6.80 5.38 4.50 7.00

17 1 n-SITU Coil 7.20 7.20 6.40 7.50 7.00 5.50 7.06

18 Direct Contact Cold Heat Sink 7.20 6.80 5.50 6.20 7.50 5.50 5.50

19 Regenerative H2 H/E 4.00 6.00 4.00 4.33 0.00 3.50 3.67

20 Closed Cycle Refrig. JT- N2 5.75 7.25 4.25 7.00 3.50 4.00 6.33

21 Closed Cycle Refrig. JT-Ar 5.50 7.50 4.50 7.00 3.50 4.00 6.00

22 Closed Cycle Refrig. JT-CH4 5.00 7.25 4.00 6.75 3.50 4.00 5.33

23 Cryocooler 8.50 7.00 9.00 6.50 5.00 4.50 5.50

TABLE 7.-IMPORTANCE FACTOR DISTRIBUTIONS

Importance

factor

distribution

SAFETY

^,s

FLTIMPACT

^F

GRDIMPACT

kG

DENSPERF

^-D

COST

^'C

TRL

XT

OPFLEX

ko

Stun

of the k

1 0.14 0.14 0.14 0.14 0.14 0.14 0.14 1.00

2 0.2 0.2 0.2 02 0.0 0.0 0.2 1.00

3 0.5 0.13 0.13 0.12 0.0 0.0 0.12 1.00

4 0.25 0.125 0.125 0.125 0.125 0.13 0.125 1.00

5 0.167 0.167 0.167 0.167 0.167 0.0 0.167 1.00

Table 8 shows the final ranking results for the highest top four scoring LCH4 conditioner methods

based on the various ^. scenarios that were used. Based on these relative rankings and total score data, it

was decided to carry four densification methods into the second phase of development. It was found that

for each k scenario, four basic systems came out on top: the Modified X-33 (Method 9) and the Direct

TVS (Method 12) systems were generally the top two ranked methods while the Cryocooler) Storage

NASA/TM-2010-216246 28
Vessel Densification (Method 23), the In-Situ External Limpet Coil (Method 13) and In-Situ Infernal

Helical Coil (Method 17) systems were also readily visible, especially from the safety perspective.

TABLE 8.-TOTAL SCORE VALUES (0) FOR TOP FOUR HIGHEST RANKED LCH4 CONDITIONER METHODS

huportance Total score Method Total score Method Total score Method Total score Method

factor ranked ranked ranked ranked ranked ranked ranked ranked

distribution no.l no.l no.2 no.2 no.3 no.3 110.4 no.4

Modified Shush In-situ 1 7.41 X-33, LCH4 7.38 Direct TVS 7.05 Methane 6.89 Limpet

Modified Slush 2 7.62 7.41 7.30 Cryocooler 7.30 X-33, LCH4 Methane Direct TVS

Modified X- Compact 3 7.77 Cryocooler 7.20 7.15 7.12 33, LCH4 H/B- Ar In-situ Coil

Modified In-situ Co-current 4 7.29 7.22 6.97 6.91 X-33. LCH4 Direct TVS Lim>et H/E

Modified Slush 5 7.48 7.36 7.23 7.06 X-33. LCH4, Direct TVS Methane In-situ Coil

From the down select, four conditioning methods were chosen for further evaluation during the

Phase 2 study. The scope of the Phase 2 study will include the development of higher fidelity thermal

models using CFD methods or their equivalent. Additionally, a cost analysis will be performed based on

FY2012 projected dollars for the installation of the system, including all safety equipment as required for

launch pad operational use. It is however unclear at this juncture if the Altair methane tanks would be

capable of structurally handling the sub-atmospheric internal pressure (2 psia) that's required for the

Direct TVS conditioning system without significant impact. This issue will be explored further during

Phase 2 as well.

Study Conclusions

While storable propellants are still considered the baseline for the Altair ascent main engine, there is

much interest in switching to a liquid oxygen./liquid methane (LOX/LCH4) engine to increase

performance and thereby increase payload delivered to the lunar surface. Furthermore, the

implementation of subcooled and densified LCH4 provides the capability for these flight tanks to perform

throughout the entire mission with-out the need to vent. In support of the Altair ascent stage down-select

between cryogenic (LOX./Methane) and hypergolic fuels (ETO/MMH), a Phase I conceptual study was

conducted on the best ways to produce and deliver the liquid methane (LCH4) on-board the Altair vehicle

in a densified state. The required temperature for densified liquid methane of 92.5 K (166.5 °R) prior-toliftoff is associated with a saturation pressure of 13.8 kPa (2 psia).

During this first phase of the engineering study, 26 different conceptual methods of production and

delivery of densified LCH4 were analyzed with simple thermodynamic and mass models that were

developed to check the feasibility of the system architectures. The densification methods were then

ranked based on their safety, performance, flight and ground impacts, TRL, cost, and operational

flexibility by team members from Fluid Systems (GRC and KSC), Shuttle Cryogenics (KSC) and

Constellation Program (MSFC and KSC) disciplines.

The top four ranked methods were down-selected for further review and development of higher

fidelity thermal modeling. Those methods chosen for further study during a Phase 2 effort were: a

Modified X-33 densification system using liquid methane (Method 9); a Direct TVS on-the-pad vacuum

pumping process system (Method 12); Cryocoolers integrated into a LCH4 storage tank (Method 23); and

a secondary fluid heat exchanger via In-Situ Limpet Coil (Method 13) or In-Situ Helical Coil (Method 17)

integrated into a liquid methane storage tank. The locations for each of these system architectures that

would service Altair prior to launch would be locally mounted and potentially transportable GSE set-ups

NASA/TM-2010-216246 29
on the Mobile Launch Platform during propellant servicing. All of these methods have been briefly

documented in this report for future reference to the aerospace community, while the conceptual design

and operational details of the top four fuel conditioning methods selected will be expanded upon during

the Phase 2 on-the-pad methane conditioning study. Some of the densification methods described in this

paper are novel approaches not reported-on in the literature. In conclusion, the study team has high

confidence that the four down-selected methane conditioning systems are judged to be the best solutions

for Altair.

References

1. Tomsik, T.M., "Recent Advances and Applications in Cryogenic Propellant Densification

Technology," NASA,/TM-2000-209941, March 2000.

2. Moran, M.E., Haberbusch, M.S., and Satornini, G. A., "Densified Propellant Technology: Fueling

Aerospace Vehicles in the New Era," AIAA-97-2824, July 1997.

3. LeMoyne, R., "Fundamental Analysis of Cryogenic Propellant Densification," AIAA-2008-2500,

July 2008.

4. Lak, T.I., Lozano, M.E., and Tomsik, T.M., "Advancement in Cryogenic Propulsion System

Performance Through Propellant Densification," AIAA Paper 96-3123, Presented at the 32 nd JPC,

Lake Buena Vista, FL, July 1996.

5. Notardonato, W.U., "Densification of In-Situ Produced Propellants for the Human Exploration of

Mars," AIAA-2002-4296, July, 2002.

6. Fazah, M.M., "STS Propellant Densification Feasibility Study Data Book," NASA TM-108467,

September 1994.

7. Greene, W.D., and Boxx, D.L., "Propellant Densification for Shuttle: The SSME Perspective,"

AIAA-2002-3602, July 2002.

8. Tomsik, T.M., "Performance Tests of a Liquid Hydrogen Propellant Densification Ground Support

System for the X33!RLV," NASA TM-107469, AIAA Paper 97-2976, Presented at the 33rd JPC,

Seattle, WA, July 1997.

9. Greene, W.D., Knowles, T.B., and Tomsik, T.M., "Propellant Densification for Launch Vehicles:

Simulation and Testing 1999," AIAA Paper 99-2335, June 1999.

10. Nguyen, K., Knowles, T.E., Greene, W.D., and Tomsik, T.M., "Propellant Densification for Launch

Vehicles: Simulation and Testing," AIAA-2002-4293, July 2002.

11. Radebaugh R., Arman, B., Larson, K., Bradley, P., and Prazair Inc., "Pulse Tube Refrigerator for

Hydrogen Densification," AIAA-2002-3600, July 2002.

12. Lak, T.I., Lozano, M.E., and Neary, D.A., "Propellant Densification Without Use of Rotating

Machinery," AIAA-2002-3599, July 2002.

13. Haberbusch, M.S., Culler, A.J., DePhilips, D.P., Nguyen, C.T., Skaff, A.F., and Yeckley, A.J., "A

New Densified Propellant Management System for Aerospace Vehicles," AIAA-2002-4294, July

2002.

14. Hardy, T.L., and Whalen, M.V., "Technology Issues Associated With Using Densified Hydrogen for

Space Vehicles," AIAA 92-3079, July 1992.

15. Barsi, S., Moder, J., and Kassemi, M., "Numerical Investigation of L02 and LCH4 Storage Tanks of

the Lunar Surface," AIAA-2008749, Presented at the 2008 Joint Propulsion Conference and

Exhibit, Hartford, CT, July 2008.

16. Johnson, W.L. "Launch Hold and Ascent Effects on Cryogenic Propellants," Level 1 Model

Development Report, prepared for the Exploration Technology Development Program, Cryogenic

Fluid Management Project, September 25, 2009.

17. Augustynowicz, S.D. and Fesmire, J.E., "Cryogenic Insulation System for Soft Vacuum," in

Advances in Cryogenic Engineering, Vol. 45, Kluwer Academic/Plenum Publishers, New York,

2000, pp. 1691-1698.

NASA/TM-2010-216246 30
18. Cady, E., Cronick, J. et al. "LSTCS Final Scientific and Technical Report," NASA Contractor Report

No. NNM08AA22C, September 30, 2009.

19. Mankins, J.C., "Technology Readiness Levels—A White Paper," Prepared for the Advanced

Concepts Office, Office of Space Access and Technology, NASA Headquarters, Washington DC,

April 6, 1995.

20. Larson et al. "Cooling of Cryogenic Liquids by Gas Injection," Advances in Cryogenic Engineering,

Vol. 8, pp. 489-520.

21. Clark, J.A. Merte Jr., H., et al., "Pressurization of Liquid Oxygen Containers," Progress Report No.

18 UMRI Project 2646, Univ. of Mich. Research Inst. (October 1959).

22. Wu, W., Du, J. H., Lin, Y. R., Chow, L. C., and Notardonato, W., "Design and Experiment of

Compact and Effective Carbon Foam Recuperative Heat Exchangers," Journal of Thermophysics and

Heat Transfer, Vol. 23, No. 2, April–June 2009.

23. Jurns, J., Tomsik, T.M., and Greene, W.D., "Testing of Densified Liquid Hydrogen Stratification in a

Scale Model Propellant Tank," (1999 Cryogenic Engineering and International Cryogenic Materials

Conference, Montr6al, Qu6bec, Canada), NASA/TM-2001-209391, September 2001.

24. Doherty, M.P., Gaby, J.D., Salerno, L.J., and Sutherlin, S.G., "Cryogenic Fluid Management

Technology For Moon and Mars Missions," AIAA-2009-6532, AIAA SPACE 2009 Conference and

Exposition, Pasadena, CA, September 14-17, 2009.

25. NASA Glenn Research Center, Image Gallery Website, "Liquid Methane Tank,"

http://www.nasa.gov/centers/genn/multimedia/images,gallery/if 54_ methanetank.html, October 2009.

26. Salerno, L.J., "Reducing Launcher Mass with Zero Boil-Off or Densified Propellant Storage,"

presented at the French Cryogenic Society Meeting, Septieme Journees de Cryogenie et

Supraconductivite, Aussois, France, May 2003.

27. Kepner, Charles, H. and Tregoe, Benjamin, B., "Kepner-Tregoe Decision-Making Website,"

http://wwNv.decision-making-confidence.com/keener-treaoe-decision-makina.html

NASA/TM-2010-216246 31
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13. SUPPLEMENTARY NOTES

14. ABSTRACT

The Altair Lunar Lander is currently carrying options for both cryogenic and hypergolic ascent stage propulsion modules. The cryogenic

option uses liquid methane and liquid oxygen to propel Altair from the lunar surface back to rendezvous with the Orion command module.

Recent studies have detennined that the liquid methane should be densified by subcooling it to 93 K in order to prevent over-pressurization

of the propellant tanks during the 210 day stay on the hinar surface. A trade study has been conducted to determine the preferred method of

producing; loading, and maintaining the subcooled, densified liquid methane onboard Altair from a ground operations perspective. The trade

study took into account the limitations in mass for the launch vehicle and the mobile launch platform as well as the historical reliability of

various components and their thermal efficiencies. Several unique problems were encountered, namely delivering a small amount of a

cryogenic propellant to a flight tank that is positioned over 350 ft above the launch pad as well as generating the desired delivery

temperature of the methane at 93 K which is only 2.3 K above the methane triple point of 90.7 K. Over 20 methods of subcooled liquid

methane production and delivery along with the associated system architectures were investigated to determine the best solutions to the

problem. The top four cryogenic processing solutions were selected for further evaluation and detailed thermal modeling. This paper

describes the results of the preliminary trade analysis of the 20 plus methane densification methods considered. The results of the detailed

analysis will be briefed to the Altair Project Office and their propulsion team as well as the Ground Operations Project Office before the

down-select is made between cryogenic and hypergolic ascent sta ges in August 2010.

15. SUBJECT TERMS

Densification; Methane; Cryogenic equipment; Cryogenic rocket propellants; Ground support equipment

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