Propellant Technologies: A Persuasive Wave of Future Propulsion Benefits

A White Paper

Bryan Palaszewski

MS 60-4
NASA Glenn Research Center
Cleveland, OH 44135

February 1997

Summary

All of the new initiatives in NASA Strategic Enterprises require some type of propulsion system for their success. Propellant technologies are the most crucial of these technologies, as they can make the space vehicles safer, more operable, and higher performing. Five technology areas are described and their benefits for future vehicles are briefly discussed.

Introduction

Space exploration and utilization require vehicles that are operable, safe, and reliable. Technologies for improving rocket performance are also desirable. As space missions become more ambitious, the needs for reducing and cost and increasing the capability of rocket systems will increase. Propellant technologies have the power to make space flight more affordable and deliver higher performance.

Why Propellant Technologies?

There have been many extensive investments in propellants over the last 60 years. Many of these ideas related to propellants have languished, with only minimal funding and interest from the major projects of the nation. These ideas represent the creativity of thousands of individuals developed over many years. Now, it seems prudent to take advantage of this enormous national investment and implement the most attractive of these propellants. Investing in propellant technologies can provide benefits across the board to all major programs and NASA Enterprises (Ref. 1)

The Technologies

Five major areas have been identified for fruitful research. The five areas are Monopropellants, Alternative Hydrocarbons, Gelled Hydrogen, Metallized Gelled Propellants, and High Energy Density Propellants. During the development of the NASA Advanced Space Transportation Plan, these technologies were identified as the most likely to have high leverage for new NASA vehicles for each of the Enterprises.

The five technologies are described and their applications and their effect on future missions is discussed.

Monopropellants

Current spacecraft and satellite users and manufacturers are looking for more environmentally benign, safer propellants. Safer propellants can reduce costs by eliminating the need for self-contained atmospheric protective ensemble (SCAPE) suits (Ref. 2) that are needed for toxic propellants. Also, extensive and prohibitive propellant safety precautions, and isolation of the space vehicle from parallel activities during propellant loading operations can be minimized or eliminated (Ref. 3). If used on these satellites, the costs for operating the vehicles will be lowered, in some cases dramatically. Monopropellant testing of hydroxyl ammonium nitrate (HAN)-based fuels has begun to show promise and will soon be adopted for on-board propulsion systems on communications satellites and LEO satellites and constellations (Ref. 4). Other monopropellants using gelled fuels can also improve performance and increase safety (Ref. 5).

A new Small Business Innovation research (SBIR) special (or focused) topic, "Fuels and Space Propellants for Reusable Launch Vehicles," (Ref. 3) has been established by NASA and one of it's subtopics was formulated to promote the development and commercialization of monopropellant rocket fuels. This SBIR topic has fostered the teaming of small business with large industry, universities, and government laboratories.

Alternative Hydrocarbons

The regenerative cooling of spacecraft engines and other components can improve overall vehicle performance. Endothermic fuels can absorb energy from an engine nozzle and chamber and help to vaporize high density fuel before entering the combustion chamber (Refs. 6 - 10). Other types of related hydrocarbons can increase fuel density and reduce the overall mass of the vehicle structure, tankage and related thermal protection systems.

A new SBIR special (or focused) topic, "Fuels and Space Propellants for Reusable Launch Vehicles," (Ref. 3) has been established by NASA, and one of the subtopics was created to promote the development and commercialization of hypersonic airbreathing vehicle fuels.

Gelled Hydrogen

The benefits of gelled hydrogen have been known for many years and experimentally proven in the past (Refs. 11 - 15) There are five major benefits: safety increases, boiloff reductions, density increases with the attendant area and volume related mass reductions for related subsystems (thermal protection system, structure, insulation, etc.), slosh reductions, and specific impulse (Isp) increases (in some cases).

Safety can be significantly increased with gelled fuels. A higher viscosity reduces the spill radius of the gelled hydrogen and limits the potential damage and hazard from a fuel spill. Another important advantage is the potential for leak reduction or elimination. The leak paths from the feed systems would be minimized and the possible explosion potential would be reduced.

Boiloff reduction is another feature of gelled hydrogen. The boiloff reductions are up to a factor of 2 to 3 over ungelled liquid hydrogen (Ref. 11, 12). This feature will assist in long term storage of hydrogen for upper stages that must sustain on-orbit storage or long coast times. Also, lunar flight and interplanetary missions with large hydrogen fuel loads will derive a benefit.

Significant density increases are possible with gelled hydrogen. A 10% density increase is possible with 10% added ethane or methane. These gellants are introduced into the hydrogen as frozen particles that form a gel structure in the hydrogen. References 11 and 12 provides some additional analyses of gelled hydrogen density and performance and some additional discussion of its benefits.

Specific analyses of the performance gains for various missions are dependent on the vehicle and mission design. Systems analyses performed for higher density hydrogen vehicles have shown that the reductions of the gross lift off weight (GLOW) for increased density hydrogen are very significant. In cases where another high density hydrogen, slush hydrogen was used, the density increased by 16%, the GLOW was reduced by 10.2%, or 102,000 lbm. For airbreathing vehicles, such as the National Aerospace Plane (NASP), the estimated reduction in GLOW for slush hydrogen was from 20 to 50%. Thus, a gelled hydrogen with a 10% density increase may deliver a significant fraction of these GLOW reductions and other subsystem mass savings. Supporting references for these analyses are provided in Ref. 11.

Metallized Gelled Propellants

Metallized gelled rocket propellants have been considered for many different applications (Ref. 16-18, and see attached extensive bibliography on gelled propellants). While operational usage has not yet come to fruition, there are many technology programs that are underway to eliminate the unknowns with gelled propellants and the propulsion systems that will use them. Numerous studies have shown the potential benefits of gelled fuels and oxidizers. Technology programs to prove the combustion performance of gelled propellants have been conducted most recently by the U.S. Army Missile Command, with their industry and university partners, for tactical missile applications. The NASA Lewis Research Center and its partners have investigated O2 /H2 /Al and O2 /RP-1 /Al for NASA missions and conducted experimental programs to validate elements of the combustion and fuel technology. Gelled and metallized gelled hydrogen and RP-1 have been emphasized because hydrogen and RP-1 are typical propellants for NASA launch vehicles and upper stages. Derivatives of these propellants are therefore preferred to minimize the incremental risk for a newly introduced propulsion concept. Gelled hydrogen technology is emphasized in this paper. It's likely applications would be for rocket powered launch vehicles and upper stages, rocket based combined cycle airbreathing vehicles, and combination (rocket and airbreathing) propulsion options.

High Energy Density Propellants

New technologies in atom formulation and physics of material manipulation has led to the discovery and synthesis of materials that can be used in rocket propellants (Ref. 3, and see the attached extensive bibliography on high energy density materials).

Using these propellants is more complex than traditional propellants because of their unique chemistry. While the abovementioned monopropellants are often simpler fuels with additives that are traditional molecules which are stable in storage, the high energy species must be formulated very meticulously because they are not occurring in nature. These formulations offer increased energy density, but they must be manufactured and stored in a stabilizing medium. This medium may be solid hydrogen particles that surround the newly created atoms or molecules and isolate them. Next generation RLV propulsion systems can use these frozen hydrogen particles in a cryogenic liquid carrier, such as helium (Ref. 19).

These fuels are the penultimate step in the development of higher performance, higher density propellants. These more advanced propellants will require longer development times, so they would not be the first propellants to be commercialized. Near term aspects related to these high energy species might be the production methods of the atoms or species, the cryogenic feed system components, such as superinsulation, valves and other flow control components, feed lines, cryogenic storage, and leak detection systems.

Conclusions

Using improved propellants can lower operations cost, simplify spacecraft processing, and make space flight more accessible and affordable. Other capabilities that are enabled with these propellant technologies are better vehicle cooling, reduced cryogenic boiloff, reduced vehicle structural mass, reduced thermal protection requirements, and improved safety.

References

1) "NASA Strategic Plan," NASA Headquarters, Washington, DC, February 1996.

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3) SBIR Focused Topic Web Site: http://www.lerc.nasa.gov/WWW/TU/launch/foctopsb.htm, Briefings to SBIR Convocation, April 1996.

4) Nieder, E., Harrod, C., Rodgers, F., Rapp, D., and Palaszewski, B., "Metallized Gelled Monopropellants, " NASA TM 105418, April 1992.

5) Jankovsky, R., "HAN-based monopropellant assessment for spacecraft," AIAA Paper 96-2863, NASA-TM-107287, AIAA/ASME/SAE/ASEE Joint Propulsion Conference Lake Buena Vista, FL July 1-3,1996.

6) Ianovski, L., Moses, Clifford, "Endothermic Fuels for Hypersonic Aviation," Central Inst. of Aviation Motors, In AGARD, Fuels and Combustion Technology for Advanced Aircraft Engines, September 1993.

7) Nixon, A., "Endothermic Fuels as Heat Sinks for Hypersonic Flight," Space/Aeronautics, Vol. 47, May 1967, P. 112- 114.

8) Lander, H., and Nixon, A., "Endothermic Fuels for Hypersonic Vehicles," AIAA 5^th Annual Meeting and Technical Display, Philadelphia, PA., Oct. 21-24, 1968.

9) "Cooling of Advanced Engines by Endothermic Reactions of Hydrocarbon Fuels," Shell Development Co., In AGARD Aircraft Fuels, Lubricants, and Fire Safety, Aug 01, 1971.

10) Petley, Dennis H., and Jones, Stuart C., "Thermal Management for a Mach 5 Cruise Aircraft Using Endothermic Fuel," AIAA Paper 90-3284, AIAA/AHS/ASEE Aircraft Design, Systems and Operations Conference, Dayton, OH, Sept. 17-19, 1990.

11) Palaszewski, B., "Gelled Hydrogen: A White Paper," unpublished manuscript, February 1997, (see Ref. 12 for an excerpt from this white paper).

12) Adams, S., Starkovich, J., and Palaszewski, B., "Nanoparticulate Gellants for Metallized Gelled Liquid Hydrogen with Aluminum," AIAA 96-3234, presented at the 32nd AIAA/ASME/SAE Joint Propulsion Conference, Lake Buena Vista , July 1996.

13) "Characteristics of a Gelled Liquid Hydrogen Polyphenylene Oxide (PPO) Foam Open-Cell Insulation System," General Dynamics, Report Number GDCA 632-3-169, Contract NAS8-27203, February 15, 1973.

14) McKinney, C.D., and Tarpley, W., "Gelling of Liquid Hydrogen," Technidyne, Inc., Contract Number NAS3-4186, NASA CR-54967, RR 66-49, June 1, 1966.

15) Akyurtlu, A., and Akyurtlu, J. "Evaluation of On-board Hydrogen Storage Methods for High-Speed Aircraft," Hampton University, NASA-CR-187755, January 31, 1991.

16) Palaszewski, B. and Powell, R., "Launch Vehicle Propulsion Using Metallized Propellants," NASA-Lewis Research Center, AIAA 91-2050, presented at the 27th AIAA/ASME/SAE Joint Propulsion Conference, Sacramento, CA , June 24-27, 1991, also in AIAA Journal of Propulsion and Power, Vol. 10, No. 6, Nov.-Dec. 1994, pp. 828-833.

17) Palaszewski, B. and Rapp, D., "Design Issues for Propulsion Systems Using Metallized Propellants," NASA-Lewis Research Center, AIAA 91-3484, presented at the AIAA/NASA/OAI Conference On Advanced SEI Technologies, Cleveland, OH , September 4-6, 1991.

18) Smith, A. L. and Anderson, R. E., "Propulsion System Hazard Evaluation and Liquid/Gel Propulsion Component Development, Volume 1," Technical Report CR-RD-RP-90-2, Contract Number DAAH01-86-C-0110, January 1990.

19) Palaszewski, B., "Atomic Hydrogen Propellants: Historical Perspectives and Future Possibilities," NASA-Lewis Research Center, AIAA 93-0244, presented at the 31st AIAA Aerospace Science Meeting, Reno, NV, January 11-14, 1993.

Contact:

Bryan Palaszewski
NASA Glenn Research Center
MS 60-4
21000 Brookpark Road
Cleveland, OH 44135
(216) 977-7493 Voice
(216) 977-7545 FAX
bryan.a.palaszewski@grc.nasa.gov

SBIR Focused Topic Web Site
http://sbir.grc.nasa.gov/launch/foctopsb.htm

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Airbreathing Combustion

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