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.

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.

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. 2008. 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: 8.673MN @ 410s Stage 2: At Altitude: 110kN @ 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.

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.

Other Considerations Extreme Magnitudes Reliability: Likelihood that launch vehicle will perform as expected and deliver payload into required orbit. Typically 0.91-0.95 (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. 1990  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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 ~ 10-10. No viable systems based on proven physics. Distribution A: Approved for public release; distribution unlimited.

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 1.0-1.5km/s 1. Potential Energy Assist Launch from higher initial altitude. LEO: Orbits mostly kinetic energy 100km Space Tower: Added 0.968 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.

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.

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.

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!

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: robert.h.frisbee@jpl.nasa.gov or andrew.ketsdever@edwards.af.mil