Single stage to orbit

A single-stage to orbit (or SSTO) launcher describes a as-yet theoretical class of spacecraft designed to place a load into orbit as a self-contained vehicle without the use of multiple stages. An SSTO would take off, fly into orbit, and then return to land as a single unit. In theory this can dramatically reduce operational costs, safety considerations, and turnaround time. However it is also said to be very difficult to build an SSTO, for reasons outlined below.

No actual SSTO launchers have been constructed - current launches are either performed by multi-stage expendable rockets, or the Space Shuttle which is assisted by fuel tanks and solid fuel rockets that are jettisoned during the climb. Several testbed spacecraft have been designed and constructed, most notably the DC-X[?] and Roton[?].

Proponents of the SSTO system claim a general reduction in launch costs on the order of ten times. This has generally fallen on deaf ears, as the same claims were made of the Space Shuttle in the early 1970s. Nevertheless the small-scale tests to date do seem to suggest that the operational costs of a SSTO would be dramatically reduced, if not ten times, something fairly close to it.

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 taken almost completely 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 that 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, it will tell you, and if not, add fuel and go 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 increases their mass fraction[?] and 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 have the same sort of mass fraction as a staged design, it must use every weight saving trick in the book. At one time this appeared to be basically impossible, but the rapid advances in materials technology and decrease in weight for auxiliary systems like flight computers has slowly reduced that to "potentially possible". Its unclear today whether or not a useful SSTO can be built, although the chances get better every year.

SSTO Examples

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 spacecraft 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 SDI 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, the ship was dropped on landing and exploded. 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.

See also HOTOL, space transport and spacecraft propulsion.

Note:

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.

However this is exactly what previous systems have done, and it appears any hope of lowering the cost by reducing the technical complexity of the individual systems is unlikely to help much at all. Most of this appeared to be based on simple engine systems using "low-tech" fuels, but this is exactly what the Russians and Chinese do, without the huge cost savings the big dumb booster[?] proponents claimed.