Having been deeply involved in the U.S. human spaceflight program during both the Apollo and space shuttle eras, I fully agree with Robert Zubrin’s position in the Feb. 22 Space News that NASA needs goals, destinations and milestones [“NASA Needs a Destination,” page 19]. Without a destination, NASA tends to revert to being a politically driven jobs and industrial base make-work program, frustrating the many dedicated government and industry employees working in the space program.
Although there were many great achievements during the shuttle era, such as the Hubble Space Telescope and even the international space station, there was a sense of drift compared with the Apollo era. Designs too often were driven by dividing the pie to keep everyone in business rather than by sound engineering. Constellation suffered from the residual of this mentality, making it vulnerable.
I take exception to one point in Zubrin’s excellent article: his objection to funding technology for high-performance in-space propulsion. While the Variable Specific Impulse Magnetoplasma Rocket (VaSIMR) thruster may not be the ultimate technology, it is a step in the right direction. A new high-performance in-space propulsion approach must be developed for humanity to become space-faring rather than just carrying out robotic probes or one-time visits.
I had the honor to present Rocketdyne’s position on propulsion to Tom Stafford and his committee charged with developing the vision for U.S. President George H.W. Bush’s Space Exploration Initiative.
We concluded that boost from Earth to orbit would remain chemical propulsion: liquid propellant using oxygen with either hydrogen or hydrocarbon fuel, or solid propellant. Current engines — those used for the space shuttle, the Delta 4, the Atlas 5 and even the Apollo-era Saturn 5 — are close to the maximum physics limits, so new booster engine technology advances will be marginal and in areas such as production cost. Nonchemical types of propulsion are low thrust, require a vacuum or pose a severe hazard in a launch failure. The use of an air-breathing first stage provides only a slight benefit. Realistic launch systems, either expendable or reusable, are limited to delivering about one ton in orbit for every 16 tons of vehicle on the launch pad at costs not greatly less than current systems.
Because of the limited benefit of funding technology for launch systems, my presentation to the Stafford committee focused on the great benefit of drastically reducing the required mass in orbit. A large proportion of the required mass relates to the propulsion system for near-term technologies such as chemical propulsion or nuclear propulsion. Another significant proportion for long-duration missions is life support, including radiation shielding. Once in space, propulsion systems having lower mass requirements that are not usable for launch from Earth become feasible, especially those requiring a vacuum or having highly toxic products. By reducing transit times, life support can be reduced.
The results of detailed mission models that I presented to Stafford and his committee showed that the use of continuous low, 0.1 gravity or less, acceleration and deceleration provided reasonable transit times and initial mass in Earth orbit compared with using impulsive thrust with long drift periods, as in Apollo. Achieving a continuous thrust capability required breakthrough advancement of in-space propulsion — specifically, a performance increase of at least 20 times the best chemical rockets like the space shuttle main engine, 10 times Apollo-era nuclear rocket technology, and two to 10 times current electrical propulsion in terms of thrust to mass flow rate is needed.
We identified propulsion concepts to the Stafford committee that had potential for approaching this performance level. The most promising, in order of increasing performance, were: gas core nuclear fission; magnetic confinement fusion; magneto-plasmadynamic, of which VaSIMR is an example; laser initiated fusion; anti-matter initiated fusion; and anti-matter annihilation. Less-advanced propulsion technologies, such as nuclear thermal, have trouble surpassing space shuttle main engine technology chemical propulsion when the mass of the reactor and shielding is considered.
I fully agree with Zubrin that we must avoid playing technology forever or building the pieces before we know the mission, either of which could paralyze the U.S. human spaceflight program. However, an in-space propulsion breakthrough would be enabling for many destinations, particularly Mars. At Rocketdyne, we considered this area so important to the long-term future of humanity in space that even though the Space Exploration Initiative wasn’t funded, we funded joint work with universities such as Penn State and other researchers on company resources for many years after our Stafford briefing. Marshall Space Flight Center and other NASA centers also maintained ongoing investigations of these high-payoff approaches.
The inventor of VaSIMR is certainly an advocate of his specific concept, but his advocacy of the need for advanced in-space propulsion is shared by many. Whatever destination is selected for future human spaceflight, extremely high performance in-space propulsion is a critical enabling element.
Stephen A. Evans
Foothill Ranch, Calif.
