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I hear a lot of talk about SSTO, and it got me to wonder, what's so good about them? Doesn't it make sense to get rid of parts that you no longer need for a mission?

The way I see it, if it emptied a tank of fuel, I don't want to be carrying it with me, so I dump it (hopefully in such a way that it can be reused in the future).

So, I ask, what are the advantages of SSTOs?

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I'm very curious too! Many space flights have at least three different modes: Heavy launch through the atmosphere, flight in empty space and landing through the atmosphere. There should be huge benefits from reconfiguring the spaceship between those distinct modes. Even in ocean shipping, some ships are loaded via smaller shuttle ships to the harbor. And some fighter aircrafts use drop tanks. – LocalFluff Jun 19 '14 at 9:22
up vote 11 down vote accepted

The idea behind a SSTO craft is that it can not just get to orbit in one piece, but also back to the ground, refuel and start again. That means after the initial investment of building the SSTO launch system, a launch only costs you the fuel (theoretically - practically you will always have some components which will need to be replaced more or less regularly).

With a multi-stage craft, the spent stages usually sink into the ocean, disintegrate in the atmosphere or end up in a graveyard orbit which means that they can only be used once and need to be rebuild from scratch. Each launch has the price-tag of building the whole launch system.

A middle-ground between both methods is a multi-stage launch system where the spent stages can be recovered, refueled and reused. This seems to combine the advantages of both methods - no wasted hardware and no dead weight into orbit. But it is technically a lot more complex to do. The Space Shuttle was the first launch-system which tried this. Its solid rocket boosters were recovered and refueled. Unfortunately the execution was flawed: Refilling the boosters with solid fuel was almost as expensive as rebuilding them.

A newer system which tries this is the reusable version of the Falcon 9. SpaceX is currently developing a lower stage which returns to the launch-site autonomously. The system uses liquid-fuel, which is a lot cheaper and a lot easier to get into the booster than solid-fuel. Also, because the stage returns to the launch-site on its own power, an expensive recovery-operation becomes unnecessary.

The downside compared to an expendable launch system is that the lower stage needs to conserve some of its fuel for returning and landing, which means less fuel for the payload. The downside compared to an SSTO is that each stage must be a full-fledged craft capable of autonomous navigation, maneuvering and landing which inevitably means that lots of systems will be redundant.

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Thanks for the answer. If I understand correctly, What you're saying is that if we can recover the separated stages and reuse them efficiently, we can get a better solution than SSTOs to the problem of launch vehicle construction costs. – Neowizard Jun 19 '14 at 9:32
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@Neowizard Not necessarily. I added another paragraph. – Philipp Jun 19 '14 at 10:05
    
Note how in early days of aviation there were various airplanes that used either disposable or recoverable gear for start; it was a "stage" not carried up into the air. Some of them were not designed to land or be recoverable - they'd crash into water, or the pilot was expected to eject. The problem was in underpowered engines unable to carry all the redundant weight. Nowadays all airplanes carry their landing gear with them and use it to start too. Personally, I expect SSTOs will become the norm once we get better propulsion/launch systems. – SF. Apr 14 at 4:08
    
An SSTO vehicle that carries a crew into orbit will probably be more like a limousine service to an orbiting platform. It makes sense that vehicles like this should be reusable. Intermediately, a two part cargo delivery system with a self returning first stage makes sense. But when we need to lift really huge stuff to orbit or further out, it might make sense to let some of the parts go. It would be nice if this kind of launch was to send up stuff thas was going to be used for a long, long time. – Howard Miller Apr 17 at 21:34

A distinction should be made between 'Single-Stage Orbiters' and REUSEABLE single stage orbiters. The Space Systems/Loral company investigated a design about 15 years ago called 'Aquarius', that would have put a metric ton and itself into low orbit. It was to be ocean launched and so would have had zero launch facility costs. Each launch would have cost $3 Million.

Think about it: a rocket launching a ton of payload that arrives in orbit fully intact with power and RCS systems still operational. Shuttle cost a billion dollars to launch. That would have paid for 330 Aquarius launches, delivering a cumulative payload of 300 tons - and 660 potentially habitable propellant tanks. I leave it to the reader to imagine the possibilities. . .

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$3m seems unrealistically low, along with the claim that getting the thing out to sea would be free (no ship? no launch crew at all? no assembly building? no dock facility?) – pjc50 Jun 19 '14 at 14:32
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I leave it to the answerer to site some sources. – Ellesedil Jun 19 '14 at 14:47
    
en.wikipedia.org/wiki/Aquarius_(rocket) – MercuryPlus Jun 19 '14 at 15:46
    
The low cost per launch was due to the expected high flight rate of at least 80 units per year, though up to m100 was considered practical. The cost included the two ocean-going barges - one of which was large enough to carry 40 launchers. – MercuryPlus Jun 19 '14 at 15:49
    
www.responsivespace.com/Papers/RS3/SESSION%20PAPERS/SESSION%206/... – MercuryPlus Jun 19 '14 at 15:52

In addition to what's been posted earlier:

Separation events are risky. Every time a multi-stage craft experiences stage separation, there's a chance for things to go wrong. If the stages fail to separate, you won't make it to orbit. If they only partially separate, you might tear the craft apart. If they separate late, the upper-stage engine might take damage from overpressure. If they separate early, the lower stage might not have finished boosting and will crash into the upper stage, damaging it. If they separate unevenly, the upper-stage engine might be damaged or the upper stage may be put into a spin that damages its payload or centrifuges its fuel.

See the Falcon 1 launch history, where launch #2 experienced an unplanned contact between the interstage and the second-stage engine bell (first-stage rotation during separation), and launch #3 failed because residual thrust after MECO collided the first stage into the second (insufficient delay before separation). Other rockets have experienced similar problems. Using fewer stages means having fewer separation events, with SSTO obviously having the fewest possible.

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What most of you seem to miss is the ssto advantage of useing extremely efficient air breathing jet engines to get to upper atmosphere and loads of horizontal speed, so it takes only a small fraction of expensive rocket fuel to boost it up to lower orbit. After all its the fuel that costs most i belive.

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Keep in mind that rocket fuel is only a small fraction of the total launch cost. – Hohmannfan Apr 13 at 20:59
    
But ssto plane only needs a runway. And when talking about launch costs we should not take the cost of the load into account, so getting ssto into upper atmosphere costs as much as getting a passanger plane would – Vilius Apr 13 at 21:03
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Fuel is not the largest cost. Equipment capital, amortized launch risk, skilled personnel, maintenance of launch base equipment, maintenance of reusable launch vehicles, and so on and so forth. All of that has a cost; capital to build the vehicle in the first place, in particular, has an enormous cost, since it takes tens, hundreds, or even thousands of millions of dollars to build a launch vehicle, but only a few hundred thousand dollars to fill even a fairly large rocket with fuel. – Nathan Tuggy Apr 13 at 21:23
    
I should probably also note that while some SSTO designs use air-breathing engines, that's not actually unique to or characteristic of SSTO; the Pegasus is a staged, non-reusable design with an airplane as its first (or zeroth?) stage, and there are SSTO designs that use rockets alone. – Nathan Tuggy Apr 13 at 21:26
    
I agree that fuel is not the most expensive thing, but all the aspects you named applies to any launch vehicle, and when you take all that away and leave the lifting procedure alone, its more efficient. – Vilius Apr 13 at 22:30

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