Space launch evolution and revolutionby Eric R. Hedman
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| The Holy Grail of low per passenger cost launcher concepts keeps coming back to scramjet engines. |
The US military has expressed an interest in space-based solar power generation. Space-based solar power is just on of many possible space-based industries that could benefit from a dramatic drop in launch costs. Even with advances in solar cell efficiencies and lightweight large structures, launch costs are going to be an impediment to lifting significant structures into orbit. If space tourism is to ever grow into significant numbers to low Earth orbit, launch costs are probably going to have to come down more than reusable rockets can bring them.
The US Air Force is currently funding the X-51 WaveRider program as the next step in determining the feasibility of scramjet propulsion. The X-51 WaveRider is designed to test the ability of the scramjet engine to accelerate from Mach 4.5 to Mach 6 or 7 and cruise at that speed. It will test a number of technologies that include the engine, the airframe, and high-temperature materials, among others. It is scheduled for test flights starting towards the end of 2008 and stretching into 2009. In approximately two years the data on these flight tests will be in. NASA’s Ames Research Center is commencing a project to test and develop high-temperature materials for hypersonic flight.
After the X-51 program is completed, it will be the time to see just how the flight envelope can be extended into something for practical use. Now is the time to start brainstorming as to what the follow-up could be. The fun part of brainstorming before trying to narrow down to something likely to succeed is that there should be no limits to how outlandish the ideas are. This includes different concepts of mixing ideas already in use or under development for other projects. I think there are probably unlimited ways you could mix scramjet engines, solid fuel rocket engines, liquid fuel engines, and hybrid engines. There are probably many new ways of making each of these types of engines that can make them perform better at a lower cost and more reliably.
When I attended the School of Engineering at the University of Wisconsin thirty years ago, they had an engineering expo that was put on every two years. Students either working alone or in small groups would work with a faculty advisor on a project that would take usually the full two years to develop. The students came up with an amazing array of creative new ideas in incredibly diverse areas. The ideas ranged from new types of robotic manipulators to an attempt at thought control of an electronic system by measuring brainwaves. I’m sure many of these ideas came from brainstorming sessions.
The expo, and one of my classes, inspired me to propose developing a rocket engine that would now fall under the category of a hybrid engine (a term I had never heard of at the time). The engine was to be powered by burning powdered metal with oxygen. When the expo was on I was taking a metallurgy class that was covering a section on using powdered metals for forming objects. When the professor explained that powdered metals were dangerous to work with because they had a huge surface area that could react extremely quickly and energetically if anything triggered the combustion, I started to think that maybe it could be used as rocket fuel. I asked the professor if that might be a possibility. He said the biggest difficulty would probably be finding a way to control the combustion, but he thought it was theoretically possible. I didn’t know back then that small aluminum pellets had long been used to aluminize fuel in solid fuel rockets like the Poseidon missile significantly enhancing their performance.
In cooperation with three other engineering students I started to sketch ideas. The ideas included concepts to push metal powders into a combustion chamber using augers to powder-filled tubes that would be pushed in by a piston or gas pressure. It was late in the spring and we planned to get together in the fall and do more brainstorming. When I came back in the fall two of the other students didn’t return to campus. The idea died on the spot. Even if the idea would work in creating a rocket engine, I don’t know if it ever could be turned into something practical with advantages in cost, performance, or reliability over existing technology. Mixing technologies to create a solution needs to pass the test of improving performance, improving reliability, and/or reducing costs to make a significant contribution.
The ideas I had almost thirty years ago on how to burn powdered metals in an engine are in hindsight probably not practical, but a number of things have changed since then. The hybrid engine used on SpaceShipOne has a very interesting design that uses solid rubber as the fuel with liquid oxidizer. It may be worth checking out to see if aluminizing the rubber with pellets before casting it in the rocket casing would significantly increase the thrust of the engine.
| Any reduction in cost while maintaining or improving reliability will be welcome, but a major reduction in costs might be needed to create a real growth market in orbital spaceflight including supporting opportunities like space-based solar power. |
In brainstorming a new vehicle concept, such as the follow up to the X-51, there should be no initial ideas off-limits. Looking at a hybrid of propulsion types for a multi-staged vehicle should explore every option that might be reasonable. Neither vertical nor horizontal takeoff should be ruled out at the beginning. If the vehicle takes off horizontally, a rocket assist should not be ruled out. It works for C-130 transports with JATO rockets on short fields with heavy loads until enough speed can be reached to get sufficient lift. If the vehicle is capable of horizontal flight, in-air refueling shouldn’t be ruled out until a cost-benefit analysis of the concept can be carried done. It is time for massive brainstorming with no initial limits on concepts by NASA, the DOD, and industry because the results of the X-51 tests are going to be in soon.
The SpaceX Falcon 9 and the Rocketplane Kistler K-1—whose future is currently in question—are supposed to have reusable stages. The question for both vehicles, in terms of long-term reduction in launch costs, is how much will it cost to return the stages to the launch site and refurbish them for the next launch? Another question to be answered on these stages is how many flights will these boosters be able to handle? How much will it cost to thoroughly inspect the stages between flights? A number of factors go into launch costs that have to be minimized to reduce the cost of a flight significantly. Only time will tell if either of these vehicles will fly and then deliver launch costs significantly lower than other vehicles with similar payload capacity. Any reduction in cost while maintaining or improving reliability will be welcome, but a major reduction in costs might be needed to create a real growth market in orbital spaceflight including supporting opportunities like space-based solar power.
If a frequently reusable vehicle can be developed that requires minimal refurbishment between flights and only requires a minimal launch crew, access to space could finally be reduced in price significantly. To accomplish this will require thinking “outside of the box” (an overused, trite, but applicable phrase). Making the connection to seemingly unrelated technologies may be involved. Success in anything starts with an attitude that you can do it. It is an attitude we have to have as we push the boundaries of space.