1. An Overview of Advanced Concepts for Space Access
Andrew Ketsdever
Marcus Young
Jason Mossman
Anthony Pancotti
44th Joint Propulsion Conference and Exhibit
July 21-23, 2008
Distribution A: Approved for public release; distribution unlimited.

2. Introduction
AFRL Advanced Concepts Group performed critical review of advanced technologies for space access.
Room for improvement?
Technologies Considered:
Analysis performed for advanced concepts (15-50 years) is not sufficiently accurate for more than semi-qualitative comparisons.
Qualitatively consider known missions: microsat to LEO and large comsat to GEO.
Thrust Efficiency
Energy Efficiency
Payload Mass Fraction
Cost/kg
($1000/kg)
Cost/
Energy Cost
97% (SSME)
0.2
.01
10s
.0001
Using Propellant
Propellantless
Nuclear
Space Tug
Beamed Energy
Advanced Chemical
Hypersonic Air Breathing
Electromagnetic (Rail)
Elevator
Space Platforms and Towers
Gravity Modification and Breakthrough Physics
Launch Assist
Distribution A: Approved for public release; distribution unlimited.

3. Existing State of the Art
Advanced launch concept must be more than just a new solution.
Must yield system level performance improvements over SOA.
Microsat to LEO
Large Comsat to GEO
Orbital Minotaur IV
Boeing Delta IV Heavy
Reduces microsat launch costs by
reusing Peacekeeper boosters.
4 stage all solid propellant rocket.
First flight scheduled for Dec
7 successful Minotaur I flights…
Developed as part of EELV program.
Reduce costs by 25%.
Increase simplicity and reliability.
Increase standardization.
Decrease parts count.
Stage 1: 3 CBCs RS-68 (LH2/LO2).
Stage 2: 1 RL-10B-2 (LH2/LO2) .
First flight Nov. 20, 2002.
Performance:
Thrust: I: 2.2MN, II: 1.2MN, III: .29MN.
1750kg to LEO.
Minotaur I ~ $30,000/kg.
Stage 1: Sea Level: 410s
Stage 2: At Altitude: 462s
22,950 kg to LEO.
~$10,000/kg.
“Advanced Concepts” have not aided most recent generation!
Distribution A: Approved for public release; distribution unlimited.

4. Launch Costs
Technologically feasible to launch 130,000kg to LEO (Ares V).
What else is important?
Isp: Propellant cost represents small fraction of overall…
Responsiveness: Years/months  Weeks/days?
Cost($/kg): Limitation on type and amount of payload.
Major focus on reducing launch cost (1/10).
Improved performance (STS): Not successful.
Reduced performance (EELV): Not quite successful.
Distribution A: Approved for public release; distribution unlimited.

5. Other Considerations Extreme Magnitudes
Reliability: Likelihood that launch vehicle will perform as expected
and deliver payload into required orbit.
Typically (Sauvageau, Allen JPC 1998).
2/3 due to propulsion elements.
Upper stages less reliable.
Increasing would decrease insurance costs, improve RLV competitiveness.
Availability: Fraction of desired launch dates that can be used.
Responsiveness: Time from determination of desired launch to actual launch.
Currently measured in months/years.
Desert Storm: Sept  Launch Feb. 1992!
Ideal to have weeks/days/hours capability.
Extreme Magnitudes
SSME: P=6GW dthroat=600cm2  10MW/cm2.
Saturn V: Height: 116m, Diameter: 10m, Mass: 6.7 million pounds.
Distribution A: Approved for public release; distribution unlimited.

6. Nuclear powered upper stage
Propellant: Nuclear
Nuclear materials have extremely high energy densities.
Fission: 7 x 1013 J/kg at 100% efficiency.
Fusion: 6 x 1014 J/kg at 100% efficiency.
~107 – 108 > chemical
Benefit practical launch systems?
History
Nuclear fission rockets first proposed in the late 1940s.
Variety of concepts exist with Isp from 800s to > 5000s.
Typically use hydrogen working gas.
Nuclear propulsion enabling for large interstellar missions.
Launch concepts exist.
NERVA upper stage.
Primary concerns: system mass, system cost, allowable temperatures, socio-political.
Large size limits applications to large payloads.
Nuclear powered upper stage
Orion
Distribution A: Approved for public release; distribution unlimited.

7. Propellant: Nuclear Tug
Nuclear fission propulsion can enable space tugs.
Reduce the requirements for launch systems?
Example: mtug (no payload) of 22,000kg, DV = 4.178km/s.
Where is breakeven?
Significant investments required to reduce specific mass of nuclear systems.
Distribution A: Approved for public release; distribution unlimited.

8. Propellant: Laser Beamed Energy
Chemical Propulsion: energy and ejecta same material (neither fully optimized).
Beamed Propulsion: energy stored remotely so ejecta could be optimized.
Lasers and microwaves are both proposed for beamed energy launch.
Both lasers and microwave sources are under continuous development.
More emphasis on laser propulsion.
Laser propulsion was first introduced by Kantrowitz in 1972
1. Heat Exchange
Laser  heat exchanger  flow
Exotic heat exchangers are required.
2. Plasma Formation
Form plasma in a nozzle to reach high operating temperatures.
Have high accuracy pointing requirements.
3. Laser Ablation
Removal and acceleration of propellant via laser ablation.
More thrust than PLT, but must carry propellant.
4. Photon Pressure
Pressure from photons directly used for propulsion.
Bae’s PLT has shown 3000x amplification.
Still requires higher powered lasers.
Coupling
Generation: 1MW  1GW
Transmission
Generation
Laser beamed propulsion will take significant money to develop and deploy and will only service mSat launches in foreseeable future due to required power levels.
Distribution A: Approved for public release; distribution unlimited.

9. Propellant: mwave Beamed Energy
Source: Parkin and Culick (2004):
300 gyrotron sources (140GHz,1MW)  1000kg to LEO.
Transmission: Frequency very important.
Atmospheric Propagation.
Breakdown.
Coupling Efficiency.
Generator Size.
Coupling
Plasma Formation
(Oda et al, 2006) Gas discharge formed at focus of beam. Plasma absorbs beam energy.
Heat Exchanger
(Parkin and Culick) Heat exchanger & hydrogen propellant yield 1000s, payload mass fraction 5-15%.
Both laser & microwave beamed energy propulsion systems require significant source (>1GW) and coupling development to yield viable systems for microsatellite launches.
Overlap with other source applications.
Distribution A: Approved for public release; distribution unlimited.

10. Propellant: HEDM
Performance of chemical rocket is critically dependent on propellant properties.
Problem: High Isp typically low density.
Goal: Find high Isp, density propellant
1. Strained ring hydrocarbons.
2. Polynitrogen
3. Metallic Hydrogen (216MJ/kg).
Difficulties
Molecules containing high potential energy are typically less stable.
Dramatically more expensive (difficult to manufacture, less alternative uses).
Require new nozzle materials/techniques.
Wide range of potential materials yielding both near-term and far-term potential improvements, but with similar technological challenges: less stable, higher operating temperatures.
Distribution A: Approved for public release; distribution unlimited.

11. Propellant: Hypersonic Air Breathing Vehicles
Oxidizer mass fraction >> payload mass fraction for existing launch systems (30% vs. 1.2% for STS).
Can atmospheric oxygen be used instead?
Thrust-to-Weight
SSME: 73.12
Scramjet ~ 2
Alternative technologies show significantly higher Isp, but over a limited range of Mach number.
Multi-stage systems are required.
Parallel systems suffer from volume and mass constraints.
Combined cycle systems require significant development to integrate flowpaths.
Distribution A: Approved for public release; distribution unlimited.

12. Combined Cycle Launch Vehicles RBCC and TBCC
Rocket Based Combined Cycle (RBCC)
Turbine Based Combined Cycle (TBCC)
Rocket-ejectorRamjetScramjetRocket
TurbojetRamjetScramjetRocket
Both technologies are under development at the component/initial integration stages.
Basic demonstration of scramjets has been shown, but survivable, reusable vehicles have not.
Development will probably require decades, but may yield a revolutionary launch technology.
Could be viable for both launch scenarios
X-51
X-43A
Distribution A: Approved for public release; distribution unlimited.

13. Electromagnetic Launch: Railguns
Multiple proposed EM launch technologies: railgun, coilgun, maglev.
Suffer from similar limitations… Only railguns will be discussed.
Technical Challenges
Maintain rail integrity.
Useful high gee payloads must be developed.
Pulsed power system must be developed.
Aero-thermal loads
Now: Ei=10MJ,m=3.2kg,Vmuzzle=2.5km/s
64MJ (6MA) System Ready > 2020
Navy
Direct Launch Requirements
Vmuzzle > 7.5km/s
E > 10GJ (35GJ muzzle, 44GJ input for 1250kg)
L > 1km
Estimated costs: System cost > $1B, 10,000 launches  $530/kg.
Potential for cost savings for microsatellites or small ruggedized payloads in the very far term.
Distribution A: Approved for public release; distribution unlimited.

14. Space Elevator From Liftport
Cable running from Earth’s surface to orbit.
Idea originated with Tsiolkovsky in 1895.
No stored energy required.
Technical hurdles:
Require extreme tensile strengths.
Carbon nanotubes?
High power requirements.
Cost.
Micrometeoroid/orbital debris impact.
Weather interactions.
Atomic oxygen/radiation belts.
Ribbon to Counterweight
Beamed Power
Climber
From Liftport
Significant economic/technical challenges in the short term.
Long term possibility…
Distribution A: Approved for public release; distribution unlimited.

15. Space Platforms and Towers
Physical structures reaching from the earth’s surface to 100km and above.
Idea has been around for awhile
More recently several different configurations have been proposed.
Solid
Inflatable
Electrostatic
Launching from 100km yields only a small amount of the total required mechanical energy
Going from <1km to >100km yields significant technological challenges
Extreme materials properties.
Winds
World’s Tallest Structure
Energy benefit at 100km is small making the development costs difficult to justify.
Burj Dubai
(May 12, 2008: 636m of 818m)
Distribution A: Approved for public release; distribution unlimited.

16. Gravity Modification and other Breakthrough Ideas
Large number of breakthrough physics concepts exist.
Some are based on unproven physics.
Modification or complete removal of gravity (reduce Ep).
Tajmar and Bertolami (J. Prop. Power 2005): “gains in terms of propulsion would be modest (from these concepts) and lead to no breakthrough”
Inertial mass modification: increase propellant mass as it is expelled out of vehicle for increased thrust.
Gravitational mass modification: lead to direct DV reduction. ~1.4km/s if m 0. GEO 13km/s  3 km/s.
Gravitomagnetic fields: Lorentz force analog for gravity. Interact with Earth’s magnetic field to produce thrust. For most configurations very small thrust levels are produced.
Some proven physics yields currently unusable systems.
Casimir force: force is very small and not applicable for launch.
Antimatter: convert all mass to energy during annihilation.
Specific energy density of ~ 9x1016 J/kg. Currently limited in production rate, cost, and storage. Energy return is ~
No viable systems based on proven physics.
Distribution A: Approved for public release; distribution unlimited.

17. Launch Assist: Effects
Can reviewed concepts provide a fraction of required DV instead of all of it?
Consider only first stage launch assist technologies.
Must provide system level performance benefit.
7.5-11km/s
km/s
1. Potential Energy Assist
Launch from higher initial altitude.
LEO: Orbits mostly kinetic energy
100km Space Tower: Added MJ/kg (26% potential, 2.9% total).
2. Kinetic Energy Assist
Launch with initial velocity
Need several km/s to be worthwhile.
Encounter problems with high-speed low altitude flight.
3. DV Loss Assist
Launch from higher altitude.
Typically represents several % of total energy.
Distribution A: Approved for public release; distribution unlimited.

18. Launch Assist: Technologies
1. Air Launch
Fixed Wing
Balloon
Both feasible only for msat launch.
Pegasus launcher exists, isn’t any cheaper, possible other mission benefits.
2. Electromagnetic Launch
Railgun
Coilgun
Maglev
Both gun technologies potentially feasible only for msat launch. Need to increase DE by > 1000x.
3. Gun Launch
Gas Dynamic
Light Gas Gun
HARP gun fired 180kg projectile at 3.6km/s. Next gen could place 90kg in LEO.
SHARP gun 5kg projectile at 3km/s.
Distribution A: Approved for public release; distribution unlimited.

19. Conclusions Significant room for improvement in launch technology.
Wide range of concepts proposed and being investigated.
No obvious winners.
mSat  LEO
Comsat  GEO
Challenges
Nuclear
Mass, Cost, Socio-Political
Space Tug
Significant reduction in specific mass of nuclear system required.
Beamed Energy
Generated power levels. Tracking. Coupling.
HEDM
Stability. Toxicity. Cost. Nozzle Materials.
Hypersonics
Scramjets: thermal load. Rapid combustion. Lifetime. High thrust-to-weight. Significant atmospheric flight.
Electromagnetic
Power source. Rail integrity. High gee payloads. Rail integrity. Aerothermal loads.
Elevator
Long defect free nanotubes, atomic oxygen, micrometeoroids, weather, vibrations.
Platforms
Same as elevator. Must define mission benefit.
Breakthrough
No demonstrated phenomena with sufficient propulsive force.
Launch Assist
High gee payloads. Power sources. Aerothermal.
Distribution A: Approved for public release; distribution unlimited.

20. Conclusions II Significant number of remaining technical challenges.
Solving any single challenge may not enable complete systems, but may have broad effects.
High gee payloads & upper stages.
High temperature nozzles.
Very high power instantaneous power levels.
Lightweight power systems.
Additional concepts are required!

21. Announcing the 2008 Advanced Space Propulsion Workshop (ASPW 2008)
When: Week of October 6, 2008 (TBD)
Where: Pasadena California
Sponsors: NASA Jet Propulsion Laboratory
& Air Force Research Laboratory (Edwards)
Contact: or