A single-stage to orbit (or SSTO) vehicle would be a vehicle that could reach orbital velocity without the use of multiple stages. The main problem in constructing such a vehicle is to make the engine efficient and the vehicle structure lightweight enough, to make it unnecessary to drop away parts of the vehicle, whilst still delivering a reasonable payload to orbit.
It is believed by some that reusable vehicles, particularly SSTO reusable vehicles, would lead to much reduced costs for access to space and allow aircraft-like operations.
No actual SSTO launch vehicles have been constructed - current orbital launches are either performed by multi-stage fully expendable rockets, or like 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 research spacecraft have been designed and constructed, most notably the DC-X, the X-33, and the Roton SSTO; but none of them have been anywhere near an orbital launch. So far, SSTOs have been unsuccessful due to technical and/or economic difficulties.
Many in the aerospace community have come to the conclusion that the best way to solve the problems of high cost is with an entirely self-contained and preferably largely or completely reusable vehicle. The idea behind such a vehicle is to reduce the processing requirements.
This is not the case for staged vehicles, which typically have complex "range safety" requirements as the stages detach and fall back to earth. Range safety is one of the main reasons that NASA launches from Florida, where the rocket's flight path takes it out over open water almost immediately. It also causes issues for Russian launches- certain orbital inclinations are inaccessible since stages would fall on areas of dense habitation.
An SSTO craft might allow inclusion of "all-aspect abort," meaning that the craft could abort the mission at any point in the launch cycle. The lack of such abort modes on the Shuttle requires incredible failure avoidance costs and massive overhauls. Soyuz possesses all-aspect abort, and has not seen a loss of crewmember over several decades.
Combine these advantages with more reliable systems and a more fully automated maintenance system, and the cost of operations is considerably reduced. If there is anything requiring attention, only then will the system require maintenance. If not, the only requirement is to add propellants and fly again.
The SSTO problem Edit
An SSTO vehicle has one major problem - it needs to lift its entire structure into orbit. To reach orbit with a useful payload the rocket requires careful and extensive engineering to save weight. This is much harder to design and engineer. A staged rocket 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.
Single stage rockets were once thought to be beyond reach, but the rapid advances in materials technology and construction techniques have shown it to be possible. For example, calculations show that the Titan II first stage, launched on its own, would have a 25 to 1 ratio of fuel to vehicle hardware. It possesses a sufficiently efficient engine to make orbit, but without carrying much payload.
It is now clear that it is possible to build an SSTO vehicle, but it is less clear whether or not a reusable SSTO with an economic payload can be built. The chances get better as components like guidance avionics become lighter and new materials and construction techniques are developed.
Dense versus hydrogen fuelsEdit
It might seem entirely obvious that hydrogen would be the fuel of choice for SSTO vehicles. When burned with oxygen, hydrogen gives the highest specific impulse of any commonly used fuel; around 450 seconds, compared with up to 350 seconds for kerosene.
However, hydrogen is relatively expensive to work with. It is deeply cryogenic, it escapes very easily from the smallest gap, it has a wide combustible range giving easy accidental ignition, it burns with a dangerously invisible flame, it tends to condense air which in turn is highly flammable with many common materials, it has a massive coefficient of expansion for even small heat leaks... the list of practical difficulties goes on and on. All of these issues can be dealt with, but usually with extra manpower and hence higher cost. Furthermore, and most significantly, the density of liquid hydrogen is much lower than other fuels, about 1/7 of the density of kerosene, for example.
This means that, while tanks for kerosene can fairly easily be 1% of the weight of their contents, hydrogen tanks struggle to weigh even 10% of their contents. This is due partly to the low density, but is also a result of the additional insulation that is required to minimize boiloff (a problem which does not occur with kerosene and many other fuels). The low density of hydrogen further impacts the design of the rest of the vehicle; its low density means that pumps and pipework need to be much larger in order to pump the fuel to the engine. The end result is that the thrust/weight ratio of hydrogen fueled engines is 30-50% lower than comparable engines using denser fuels.
This inefficiency indirectly affects gravity losses as well; the vehicle has to hold itself up on rocket power until it reaches orbit. The lower thrust of the hydrogen engines means that the vehicle must angle its exhaust more steeply, and so less thrust acts sideways. This loss of sideways thrust means that it takes longer to reach orbit, and gravity losses are increased by at least 300 m/s. This may not sound like much, but the mass ratio to delta-v curve is very steep to reach orbit in a single stage, and this makes a 10% difference to the mass ratio, on top of the tankage and pump savings.
The overall effect is that there is surprisingly little difference in overall performance between SSTOs that use hydrogen and those that use denser fuels, except that hydrogen vehicles may be rather more expensive to develop and buy. Careful studies  have shown that some dense fuels (for example liquid propane and LOX—liquid oxygen) exceed the performance of hydrogen fuel when used in an SSTO launch vehicle by 10% for the same dry weight.
One engine for all altitudesEdit
SSTO vehicles use the same engine for all altitudes, which is a problem for traditional engines with a bell shaped nozzle. Dependent on the atmospheric pressure, different bell shapes are optimal. Engines operating in the lower atmosphere have narrower bells than those designed to work in vacuum. Shape of the bell not optimized for the height makes the engine less efficient.
One possible solution would be to use an aerospike engine, which can be effective in a wide range of ambient pressures. In fact, a linear aerospike engine was used in X-33 design.
Comparison with the ShuttleEdit
The continual pressure on the budget of NASA, along with the high cost per launch of the Space Shuttle (a vehicle ironically designed to reduce launch costs), sparked interest throughout the 1980s in designing a cheaper successor vehicle of some sort. Several official design studies have been made, but most are basically smaller versions of the existing Shuttle concept.
Most cost analysis studies of the Space Shuttle have shown that manpower is by far the single greatest expense. The original idea was to have a maintenance schedule comparable to that of a commercial airliner, with a two-week turnaround. The final vehicle required massive amounts of maintenance after every launch. This shift was partly a result of the removal of various abort systems, requiring the vehicle to be made safe via intensive inspection. 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, most notably resulting in equipment requiring constant maintenance.
The end result is a vehicle that is almost completely disassembled 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 Vehicle Assembly Building (VAB), which alone takes weeks. Given that there are 25,000 people working on Shuttle operations, the payroll alone is the Shuttle's single biggest operating cost.
Many in the aerospace community have come to the conclusion that the best way to solve this problem was with an entirely self-contained and reusable vehicle. The idea behind such a vehicle is to reduce the processing requirements from those of the Shuttle.
Early versions of the Atlas rocket can be considered to be expendable SSTOs by some definitions. It is a "stage and a half" rocket, jettisoning two of its three engines during ascent but retaining its fuel tanks and other structural elements. However, by modern standards the engines ran at low pressure and thus not particularly high specific impulse and were not especially lightweight; using engines operating with a higher specific impulse would have obviated the need to drop engines in the first place.
The first stage of the Titan II had the mass ratio required for single stage to orbit capability with a small payload. A rocket stage is not a complete launch vehicle but this demonstrates that an expendable SSTO was probably achievable with 1962 technology.
The Orion project was potentially single stage to Mars (and back!); but this failed due to health concerns over nuclear fallout.
A detailed study into SSTO vehicles was prepared by Chrysler Corporation's Space Division in 1970-1971 under NASA contract NAS8-26341. Their proposal was an enormous vehicle with more than 50,000 kg of 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 unmanned DC-X technology demonstrator, originally developed by McDonnell Douglas for the Strategic Defense Initiative (SDI) program office was an attempt to build a vehicle that could lead to a SSTO vehicle. The 1/3 size test craft was operated and maintained by a tiny crew of three people based out of a trailer, and the craft was once relaunched less than 24 hours after landing. 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 were indeed sound. However, that project ran into repeateded cost overruns, and was eventually cancelled.
Today there is almost no SSTO research in the United States, much to the chagrin of those involved.
There are, 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 on projects like Skylon.
Alternative approaches to cheap spaceflightEdit
Study after study has shown that the primary and most effective cost reduction technique across all vehicles, irrespective of technology is economies of scale. Merely launching a large total quantity reduces the manufactured costs of the equipment, in much the same way that mass-produced motor car costs are low; or large ships are cheaper than small boats per weight of boat.
This has lead many others in the industry to declare that the solution to the launch-cost problem is the exact opposite of SSTO. Whereas reusable SSTOs look to save costs (mainly manpower costs) by making a high-tech vehicle that launches repeatedly, this outlook sees the technical advances as a source of the cost problem in the first place. Instead, this position advocates using pre-existing rocket technology to construct large multi-stage rockets built from cheap off-the-shelf parts which are dumped into the ocean after use. This is known as the "big dumb booster" approach.
This is somewhat similar to what some previous systems have done, using simple engine systems with "low-tech" fuels, as the Russian and Chinese space programs 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.
- A Single-Stage-to-Orbit Thought Experiment
- Why are launch costs so high?, an analysis of space launch costs, with a section critiquing SSTO
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