Single-stage to orbit
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A single-stage to orbit (or SSTO) vehicle can reach orbital velocity without the use of multiple stages. It is believed that reusable SSTOs would lead to much reduced costs for access to space and allow aircraft like operations. The main problem in constructing such a vehicle is to make the engine efficient and the vehicle structure lighweight enough to avoid carrying excessive amounts of fuel, and making it necessary to drop away parts of the vehicle while in flight to be able to attain orbital velocity.
No actual SSTO launch vehicles have been constructed - current orbital launches are either performed by multi-stage expendable rockets, or the Space Shuttle which is multi-stage and partially reusable, since it is assisted by a drop tank and solid rocket boosters that are jettisoned during the climb. Several testbed spacecraft have been designed and constructed, most notably DC-X, X-33, and Roton SSTO. So far SSTOs have been unsuccessful due to technical and/or economic difficulties.
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The SSTO argument
The continual pressure on the budget of NASA, and the huge launch costs of the Space Shuttle (a vehicle designed to radically reduce launch costs but which conspicuously failed to do so), sparked interest throughout the 1980s in designing a successor vehicle of some sort. Several official design studies have been made, but in general they are basically smaller versions of the Shuttle.
Most studies of the Shuttle have shown a single problem as the root cause of its high cost -- manpower. Contrary to the original design which was to have an airliner-like maintenance schedule with a two-week turnaround, the delivered vehicle had to be made more and more fail-safe as various abort systems were removed. In addition the policy of using the most technically advanced engines and materials (seen as a NASA duty at the time) backfired in a number of ways, and resulted in equipment that requires constant maintenance.
The result is a vehicle that has to be almost completely taken apart after every mission. The engines are removed and rebuilt, large amounts of the structure are taken off for testing, and the entire refurbishing cycle takes months. Even without these problems the system still requires the various parts - the Orbiter, SRBs and ET, to be collected and assembled in the VAB, which alone takes weeks. Given that there are 25,000 people working on Shuttle operations, the payroll alone is the single biggest cost in flying it.
Many in the space community came to the conclusion that the best way to solve this problem was an entirely self-contained, reusable vehicle. The idea is that such a vehicle would require much less processing than the Shuttle, whose individual parts have to be collected back together and re-assembled.
Another advantage would be the inclusion of "all-aspect abort", meaning that the craft could abort at any point in the launch cycle. This is not the case for staged vehicles, which typically have complex "range safety" requirements as the stages fall off and fall back to earth. That is one of the main reasons that the US launches from Florida, where the rocket is out over the water almost immediately. The lack of such abort modes on the Shuttle is what leads to the incredible failure avoidance costs and massive overhauls.
Combine this with more reliable systems and a fully-automated maintenance system, and the cost of launching goes down considerably. If anything does need to be looked at, the system will inform maintenance workers. If not, add fuel and fly again.
The SSTO problem
On the downside, an SSTO craft is technically much harder to make. Staging is used because it greatly reduces the total mass that flies all the way into space; the rocket is continually shedding fuel tanks and engines that are now dead weight. This makes the overall job much easier.
Without staging, the rocket needs to lift everything into orbit all the way. That means that in order for the rocket to be able to reach orbit at all, and allow an acceptable payload, it must use every possibility to save weight. At one time it appeared to be basically impossible, but the rapid advances in materials technology and decrease in weight for auxiliary systems like flight computers has slowly changed that to "potentially possible". It is unclear today whether or not a useful SSTO can be built, although the chances get better every year.
Examples
Early versions of the Atlas rocket can be considered an expendable SSTO by some definitions. It is a "stage and a half" rocket, jettisoning two of its three engines during ascent but not jettisoning any fuel tanks or other structural elements.
The most detailed study into SSTO was prepared by Chrysler Corporation's Space Division in 1970-71 under NASA contract NAS8-26341. Their proposal Shuttle SERV (http://www.astronautix.com/lvs/shueserv.htm) was an enormous vehicle with more than 50,000 kg payload, utilizing jet engines for (vertical) landing. While the technical problems seemed to be solvable, NASA preferred a winged design that led to the Shuttle as we know it today.
The closest approach to a real SSTO vehicle was the unmanned DC-X technology demonstrator, originally developed by McDonnell Douglas for the Strategic Defense (anti-ICBM) program office. The craft was operated and maintained by a tiny crew of people based out of a trailer, and the craft was once turned around in less than 24 hours. Although the test program was not without mishap (including a minor explosion), the DC-X demonstrated without any doubt that the maintenance aspects of the concept was indeed sound.
The program later ran out of money in the midst of a test series during a general downsizing of the Strategic Defense Initiative budget. At that point NASA took the ship for their own testing. Immediately they decided to go against the entire program concept, and added a number of new and advanced features that were of little import to a system that is proving management goals, not technical ones. After a rebuild to include new lightweight tanks and fuel lines, one of the ship's retractable landing struts failed on landing and the craft exploded after toppling over. NASA declined to fix it, releasing a report that blamed the entire concept for the failure and suggesting that any such quick-turnaround system was doomed.
Some have suggested this was an Not Invented Here issue, as NASA already had its own SSTO project underway, the X-34. However that project ran into continued cost overruns, and NASA finally gave up on it. Today there is almost no US research into SSTO, much to the chagrin of everyone involved.
There are, however, a number of efforts around the world to study SSTO, and several have recently progressed to active funding. Primary among these are the Japanese Kankoh-maru project and recent work in Europe on behalf of the ESA.
Alternative approaches to cheap spaceflight
Many others in the industry declared that the solution to the launch-cost problem is the exact opposite of SSTO. Whereas SSTO looks to save costs on manpower by using various technical advances, this group claims that it is in fact the technical advances that have led to this problem in the first place. Instead they propose non-advanced rockets built from off-the-shelf parts, dumping it in the ocean after it flies. This is known as the "big dumb booster" approach.
This is similar to what some previous systems have done, using simple engine systems with "low-tech" fuels, as the Russians and Chinese still do. Although these nations' launchers are not as cheap as they could be, they are significantly cheaper than their western counterparts.
The two stage to orbit approach is also of considerable interest in this field.
See also
External links
- A Single-Stage-to-Orbit Thought Experiment (http://www.spacefuture.com/archive/a_single_stage_to_orbit_thought_experiment.shtml)
- Why are launch costs so high? (http://www.ghg.net/redflame/launch.htm#theory11), an analysis of space launch costs, with a section critiquing SSTO