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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 at 9:22

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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 at 9:32
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@Neowizard Not necessarily. I added another paragraph. –  Philipp Jun 19 at 10:05

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 at 14:32
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I leave it to the answerer to site some sources. –  Ellesedil Jun 19 at 14:47
    
en.wikipedia.org/wiki/Aquarius_(rocket) –  MercuryPlus Jun 19 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 at 15:49
    
www.responsivespace.com/Papers/RS3/SESSION%20PAPERS/SESSION%206/... –  MercuryPlus Jun 19 at 15:52

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