Space Propulsion Concepts

(Last Updated 6 February 2012)


Exotic Power and Propulsion Concepts by Robert L. Forward – N91-22140
Metallic Hydrogen - Potentially A High Energy Rocket PropellantMSFC-718-ABSTRACT / MSFC-718-PRESENTATION

Exotic Chemical Fuel Concepts

Many of these are known under the term High Energy Density Materials (HEDM).

Metastable Helium

Notes: Has a theoretical ISP of 3,100 seconds. Manufactured via electronically exciting helium atoms via a helium-neon laser. Theoretically, metastable helium can last up to eight years, but so far practical results have a lifetime of one second.


Notes: An excited state molecule with four hydrogen atoms in a tetrahedron-shaped molecule. It initially looked promising, but calculations showed it decayed rapidly, proving why it’s not found in nature.

Spin Polarized Atomic Hydrogen

Notes: Has a theoretical ISP of 2,100 seconds. Has been produced by Daniel Kleppner of MIT in quantities large enough to cause damaging explosions in cryogenic glassware. Unfortunately, the lifetime of SPAH decreases drastically as density increases through increasing three-body recombination collisions. Unless a way around this is found, it will not produce a useable fuel.

Metallic Hydrogen

Notes: Has a theoretical ISP of 1,700 seconds and a density of 700 to 1150 kg/m3 versus 70 kg/m3 for liquid molecular hydrogen. It is an extremely dense form of atomic hydrogen formed under intense pressure. There is strong uncertainty over what happens when you release the creation pressure on metallic hydrogen. Will it rapidly revert to molecular hydrogen, will it be metastable (Diamond is a metastable form of graphite), or can you keep it stable at a substantially lower pressure than that required for creation? Until these issues are resolved, MH is an impractical fuel.
Other issues are that in order to get the 1,700 ISP; combustion chamber temperatures will reach over 5,600 K (>9,600 F), something far beyond any known material’s ability to withstand.
If you dilute the metallic hydrogen with liquid hydrogen, temperatures drop dramatically. (GRAPH)
Current concepts have the metallic hydrogen placed inside a high pressure spherical tank which is placed inside the cryogenic hydrogen tank to maintain low temperatures and maintain a reasonable amount of pressure on it.

Metallized Gel Propellants

Notes: These propellants are known as tripropellants as they consist of a liquid fuel, liquid oxidizer and a metal fuel. The metal fuel is suspended in particulate form as a slurry or gel inside the fuel, oxidizer, or separate carrier fluid.
Various formulations are:

Advantages are:
Safety: The higher viscosity of the fuel reduces the spill radius and hazard from a fuel spill, and may have benefits in leak reduction/elimination.
Boil-Off Reduction: Gelled LH2’s boil off rate is reduced by up to a factor of 2 or 3 over regular LH2. This is a significant advantage in on-orbit storage of propellants and for upper stages which must coast in orbit for long periods.
Density Increases: RP-1 density increases from 773 kg/m3 to 1281 kg/m3 with a 55% Aluminum loading, while LH2’s density increases from 70 kg/m3 to 168.6 kg/m3 with a 60% Aluminum loading. This allows smaller volume tankage to be used for cryogenic propellants, reducing weight by a not-insignificant fraction.
Weight Decreases: Due to the higher viscosity of gelled propellants, the amount of slosh modes in a propellant tank is reduced by a non-trivial amount, thus allowing the elimination of baffle volume and mass from the propellant tank.
Higher Performance: “Straight” O2/LH2 has an ISP of around 479.5 seconds in vacuum, while O2/LH2/Al can have an ISP of up to 485.4 seconds. “Straight” NTO/MMH has an ISP of 321.2 seconds in vacuum, while NTO/MMH/Al can reach up to 366.4 seconds.

Nuclear Propulsion Concepts

Low-Pressure Nuclear Thermal Rocket (LP-NTR)

Notes: NTR concept designed for simplicity. Does not need complex turbopumps due to combustion chamber pressure being 14.7 to 29.4 PSIA. With such low pressures, fuel tank pressure is enough to drive the fuel into the engine. This design however needs extremely large nozzles in order to develop maximum thrust from exhaust gasses.

Particle Bed (PeB) Nuclear Thermal Rocket (TIMBER WIND)

Notes: Brookhaven National Laboratory (BNL) began developing the Particle Bed Reactor concept in 1960. By 1983, Brookhaven had advanced enough to begin designing actual elements of the system itself. In 1985, they began Project PIPE, a program to explore particle bed reactors via pulsed irradiation of PBR fuel elements (where the acronym was derived from).
A subscale particle bed reactor model was tested at Sandia National Laboratory’s Annular Core Research Reactor (ACRR) at Kirtland AFB under the PIPE 1 and 2 tests. PIPE 1 occurred in October 1988 and achieved a peak output of 1.9 MW, while PIPE 2 in July 1989 experienced anomalies due to a blockage of coolant flow and was shut down less than 30 seconds into the test.
The Defense Department did not take long to notice the possible technological and military benefits of particle bed reactors, namely generating large amounts of electrical power in space for various SDIO programs, and in 1987, SDIO led a study on the utility of these reactors that BNL participated in.
Based on the results of the study, on 30 September 1987, the SDIO director’s office requested that a particle-bed NTR program be established and funded as a Special Access Program (SAP) under his office, designated TIMBER WIND.
TIMBER WIND continued as a Special Access program until 3 May 1991 when it was decided to terminate the Special Access measures. The de-SAPing process began on 27 June 1991, and the program was ultimately transferred to the USAF on 1 October 1991, with it going “public” in January 1992 as the Space Nuclear Thermal Propulsion (SNTP) program.
SNTP was managed from the USAF’s Phillips Laboratory, with Grumman Electronics Systems being the prime contractor. Babcock & Wilcox in conjunction with Aerojet General handled the reactor.
The program was defunded and died in 1993.
No details are easily accessible about the TIMBER WIND initial designs; but the paper “Particle Bed Reactor Nuclear Rocket Conceptpresented at the 1990 Nuclear Thermal Propulsion Workshop by Hans Ludewig from BNL gives us some conceptualization of the performance:

High T/W Ratio

High Specific Impulse

Power (MW)

1000 to 5000

500 to 2000

Max Fuel Temp (K)

2500 to 3650

3200 to 3900


800 to 1060

1000 to 1300

Thrust (lbf)

44,960 to 224,800

13,490 to 44,960

Engine Mass (lb)

1,433 to 12,125

6173 to 13,228

Shield Mass (lb)

2,866 to 14,110

8,157 to 17,417

T/W (w/ Shield)

8.6 to 14

2 to 3.2

Chamber Press. (MPa)

7 to 14


Chamber Temp. (K)

2500 to 3500

3000 to 3750

(1.9 MB PDF of DoD OIG Audit of TIMBER WIND Classification)
(480~ kb PDF of Particle Bed Reactor Nuclear Rocket Concept paper)
(620~ kb PDF of GAO Report: Space Nuclear Propulsion: History, Cost and Status of Programs)
Secret Gadgets and Strange Gizmos: High-Tech (and Low-Tech) Innovations of the U.S. Military by Bill Yenne

Solar Propulsion Concepts

Solar Thermal Propulsion (STP)

Description: Large concentrating mirrors are used to gather and focus solar energy onto a light absorber which turns the solar energy into thermal energy. This energy can then be used to run a heat engine to provide electricity, or to heat propellant which is exhausted to provide thrust.
In the mid 1980s, Rocketdyne built a small thrustor for the US Air Force Astronautics Laboratory (AFAL) that generated 4.45 newtons of thrust at an ISP of 820 seconds from a 25 kilowatt solar facility.
AFAL’s design goals for an orbital transfer STP system in the 1990s were around a system based on two 30 m diameter mirrors providing 1.5 MW of thermal power at a 10,000:1 concentration ratio, providing power to two thrustors which would operate at an ISP of 900 and provide 222.5 N of thrust each (445 N in total).

Laser Propulsion Concepts

Double Pulsed Laser Thermal Propulsion (DP-LTP)

Description: The payload sits atop a solid base plate composed of some sort of ablative propellant such as plastic or water ice. A high energy pulsing laser is directed at this plate. The first pulse ablates away a few micrometers, causing a thin layer of gas on the surface of the plate. The second pulse then “explodes” the gas layer, producing thrust. This is repeated at a rate of 100 to 1000 Hz.
Due to the explosive expansion taking place so close to the surface of the plate, no nozzle is needed and thus a LTP-powered vehicle can fly at angles to the laser beam. This also enables transitioning into a near-circular orbit without an apogee kick motor. Additionally, the vehicle can be steered from the ground via moving the laser beam off the center of the base plate, drastically reducing on-board guidance needs.
The numbers indicated that a 20 MW pulsed laser could launch a 150 kg vehicle into LEO with a 20 kg payload. Peak acceleration would have been comparable to chemical rockets. Elsewhere, in other beam intensities, a 1 MW pulsed laser could perform LEO-GEO orbital raising, while a gigawatt-class laser could launch multi-ton spacecraft into orbit.
The double-pulse concept was tested in laboratories under SDIO funding, with tests showing 800 ISP and thrust efficiencies of 10%.
(Diagram of DP-LTP Flat Plate Concept)