The Future of Space Propulsion

Significant challenges and advances in space propulsion have occurred over the past ten years. The end of the Cold War had significant impacts, both positive and negative. Operational systems now have to operate well beyond their design life. Parts and technology obsolescence are having a significant impact, as are an aging workforce and limited opportunities to develop new systems. The advent of asymmetric warfare is fully upon us. The military has to pro - ject power anywhere on the globe. In addition, there is concern that U.S. space assets are vulnerable in this new world. Furthermore, changes in acquisition policy driven by incredible cost and schedule overruns on nearly every space acquisition program will have significant impacts on the future of propulsion.
Legacy spacelift vehicles have been replaced by systems such as this Atlas V. (United Launch Alliance/Karl Ronstrom
In spacelift, the U.S. once held nearly 80% of the launch market — today we hold only 20%. The legacy spacelift vehicles built upon the U.S.’s historical ICBM systems are gone and have been replaced by the Delta IV and Atlas V. A number of programs encountered numerous problems with parts and technology obsolescence — the significant downside of keeping some systems operational well beyond their intended lifetimes.

The U.S. has made great strides in technologies for space in the past 10 years. The Integrated High Payoff Rocket Propulsion Technology (IHPRPT) program has reached some significant milestones. Scramjet propulsion saw its first successful flight. Numerous physics-based modeling, simulation, and analysis efforts were started to address industry shortfalls when trying to design outside the empirical database of the past 50 years. Many commercial companies have tried their hand at entering the spacelift business with small, “cheap” launch vehicles. Microsatellites have been trying to get a foothold on space and appear to be making some headway. If we are to understand what the future of propulsion holds, we need to understand the recent past. The future holds many great opportunities, but just as many technical challenges.

Recent History

The goal of the IHPRPT program is to double U.S. rocket propulsion capability. IHPRPT is a cooperative effort among the Air Force, Navy, Army, NASA, and Office of the Secretary of Defense. IHPRPT is continuing to generate great technological advances that are leading to revolutionary capabilities. The Integrated Powerhead Demo (IPD) successfully demonstrated liquid oxygen/ liquid hydrogen (LOX/LH2) boost engine technologies that resulted in 100 missions between overhaul and 200-mission life capability for future LOX/LH2 engines. The first U.S.-built Hall Effect (electric propulsion) thruster is currently flying. Advances in solid propulsion technologies developed under the IHPRPT program are yielding greater than 50% increase in payloads for small launch vehicles.

Hypersonics shows promise for future tactical and spaceflight applications. NASA flew the first U.S. liquid hydrogen scramjet engines in 2004 on their X-43 Hyper-X research vehicles. The scramjet engines on these vehicles were designed for operating durations of only a few seconds, at flight speeds of Mach 6.8 and 9.6. The Air Force Research Laboratory (AFRL) and the Defense Ad vanced Research Projects Agency (DARPA) are on schedule to fly the first X-51, a hydrocarbon-fueled scramjet test vehicle, in 2009. The X-51 will demonstrate the ability to transition over a range of Mach speeds. Accomplishing the X-51 program goals will require powered flight durations of several minutes.

Technology and component obsolescence has been a major problem since the end of the Cold War. Another fallout of the end of the Cold War was to keep systems flying two to three times beyond their design life. This exacerbates the parts obsolescence issue. Technology and component obsolescence will continue to be a challenge for the future.

Asymmetric warfare is another challenge. Many adversaries see the U.S. as the dominant power in the world and know they cannot compete head-to-head without going bankrupt. The U.S. depends heavily on its space assets and has learned a great deal since the first Gulf War 18 years ago. Commanders in the field know about space and want the capabilities space can provide, and they want it under their control. Adversaries see opportunity in the dependence on space capabilities. AFRL is developing the propulsion technologies to address warfighter needs for responsive spacecraft and responsive spacelift. The AFRL and the Air Force Space Command (AFSPC) have been working over the past seven years to develop the requirements, science, and technology to field a responsive, reusable launch vehicle.


The U.S. will continue to have a need for responsively placing assets into space to support commanders in the field or to quickly replace or gap-fill satellite capabilities that have been degraded or lost. This is a significant capability increase over what can be done today. These revolutionary capabilities are possible through revolutionary developments. The U.S. will also continue to have launch-on-schedule satellites like many of the current systems. These could be launched on either a reusable or expendable launch vehicle. Nothing in the U.S. inventory beats solid rockets for responsiveness or for helping to break the gravity well, and they are currently used in systems like the Minotaur family of launch vehicles. Hydrocarbonengine- based systems are the choice over hydrogen-engine-based vehicles for responsive spacelift.

The AFRL and DARPA are scheduled to fly the X-51, a scramjet test vehicle, next year. Here, a mockup of the X-51 undergoes testing at the Benefield Anechoic Facility at Edwards Air Force Base. (USAF photo by Mike Cassidy)
AFRL technology development efforts like the Hydrocarbon Boost Demo will demonstrate long-life, responsive booster engine technologies far beyond anything currently operating or planned. The technologies will feed all future U.S. hydrocarbon engine developments — responsive, reusable boost, as well as expendable boost.

AFRL also has a roadmap to achieve fully reusable access to space using scramjet/rocket engines to further improve the responsiveness and reduce the cost of future launch systems. The rocket and scramjet engine technologies will be integrated into Rocket-Based Combined Cycle (RBCC) engines, optimized for efficient operation at a variety of flight conditions. Independent analyses have identified significant value in a different type of RBCC — one that uses scramjet engines for flight at speeds from Mach 5 to 10+, and then transitions to rocket operation for ascent to orbit.

Will the current commercial spacelift efforts continue or will they too pass into history as many others have? These efforts can be broken into two classes: those trying to put satellites into orbit, and space tourism. Space tourism companies are taking their passengers suborbital, a much “easier” job than trying to put the same weight into orbit. Both groups will benefit greatly from work under the Hydrocarbon Boost Demo, and eventually the scramjet development efforts.


Freedom to operate in space is of paramount interest to the U.S. The Air Force, Army, and Navy have set out to explore the development of “tactical” satellites that could be under the control of warfighting commanders. These systems require responsive launch vehicles and they have their own propulsion needs. Some of these needs include significant maneuvering and drag makeup, low power consumption, high efficiency, and light weight due to being on such a small satellite bus. Future satellites may require additional propulsion capability to allow them to maneuver away from hostile vehicles. Finally, satellites require more maneuvering capability to enable them to perform more orbit rephasings while maintaining total on-orbit life. This allows the satellites to move back and forth, covering multiple geographic regions of interest.

AFRL is pursuing high thrust-to-power electric propulsion and multi-mode propulsion technologies. Despite the growing interest in microsatellites, they can only perform certain missions and the more typical classes of large satellites flying today will still be developed for the foreseeable future.


Nothing that moves in or through space does so without propulsion. Investment in propulsion research and development will need to continue. Great strides will be made in small spacelift — both solid- and liquidpropulsion- based. Spacecraft propulsion will span the gamut from microsatellites to large satellites. AFSPC and AFRL will continue to develop the necessary longrange plans and identify the necessary science and technology efforts to satisfy the warfighter’s needs. AFRL will continue to conduct technology push, looking for those new ideas that will revolutionize how people view propulsion and what they can do through it.

This article was written by John F. Remen of the Air Force Research Laboratory, Edwards AFB, CA; and Glenn Liston of the Air Force Research Laboratory, Wright-Patterson AFB, OH. For more information, click here .