Author Topic: Why the lack of SSTO projects?  (Read 22381 times)

Offline Vultur

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Why the lack of SSTO projects?
« on: 01/19/2017 06:20 AM »
There are a ton of "new space" companies right now, and many of them are working on or planning new launch vehicles - Blue Origin, Virgin Galactic, and many smaller operations like RocketLab. But there seems to be a lack of interest in SSTOs, except for Skylon, which while incredibly cool and possibly world-changing if successful has much more "novelty factor"/technical risk/development cost than a nearly-off-the-shelf standard "pure rocket" VTVL SSTO.

OK, nearly-off-the-shelf may be an overstatement, but I think every or nearly every part needed is demonstrated now.

The mass fractions needed have been demonstrated since the early 60s (and in fact well exceeded, given that modern engines have far better performance than Titan II's or Mercury-Atlas's).

 Vertical landing used to be a huge unknown, but  SpaceX and Blue Origin have demonstrated that it's now quite solvable.

The TPS IMO is the remaining large uncertainty -- but it may also be solved now. Depends how good PICA-X really is (that data isn't public, right?)

Offline savuporo

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Re: Why the lack of SSTO projects?
« Reply #1 on: 01/19/2017 06:34 AM »
Huge investment with massive risk of unproven system for what market ? Investors need the chance of enormous upside - where does the upside come from ? Has to be something that existing rockets can't do equally well.
Orion - the first and only manned not-too-deep-space craft

Offline Hobbes-22

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Re: Why the lack of SSTO projects?
« Reply #2 on: 01/19/2017 07:07 AM »
IIRC a Falcon 9 first stage can get to orbit on its own, but with 0 payload. So any low-risk developments would be pointless, which leaves the high-risk path of Skylon.

Online Bynaus

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Re: Why the lack of SSTO projects?
« Reply #3 on: 01/19/2017 07:19 AM »
The ITS spaceship or at least the ITS tanker could be considered a first step towards a VTVL SSTO. If that works out, perhaps SpaceX could (should?) even branch off a variant which can be used for intercontinental point-to-point flights.

Other than that, its an application in search for a market with high upfront costs. Not really what investors are looking after...

Offline Nilof

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Re: Why the lack of SSTO projects?
« Reply #4 on: 01/19/2017 08:52 AM »
The big issue with SSTO's is that their smaller payload hurts economic viability if they aren't reusable, and reuse is an all-or-nothing endeavour. This runs counter to the incremental development strategy that most of the new space actors are aiming for.
« Last Edit: 01/19/2017 12:47 PM by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Katana

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Re: Why the lack of SSTO projects?
« Reply #5 on: 01/19/2017 12:10 PM »
The big issue with SSTO's is that their lower payload hurts economic viability if they aren't reusable, and reuse is an all-or-nothing endeavour. This runs counter to the incremental development strategy that most of the new space actors are aiming for.
Even reuseable SSTO are inferior to full reuseable TSTO on payload and booster life. Only the second stage have to suffer orbital reentry in reuseable TSTO.

Offline Oli

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Re: Why the lack of SSTO projects?
« Reply #6 on: 01/19/2017 12:51 PM »

With SSTO you have several times the GLOW for the same payload even with crazy ITS-like mass fractions (meaning a much larger/heavier vehicle with a lot more engines).

Offline Proponent

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Re: Why the lack of SSTO projects?
« Reply #7 on: 01/19/2017 01:01 PM »
I see two fundamental barriers to the development of SSTO:  development risk and demand.

The development risk comes from the great sensitivity of payload to vehicle mass.  Consider the Atlas V 401, which can loft about 9400 kg to 400 km circular due East from the Cape.  The dry mass of the first stage is 21,054 kg.  Typically, 1 kg of mass added to a first stage reduces LEO payload by about 0.1 kg.  So imagine that the first stage runs over its mass budget by 1%, i.e., 210 kg.  Then the payload will be reduced by about 21 kg, i.e., 0.22%.

Now look at the second stage, which has a dry mass of 2243 kg.  On a final stage, a 1-kg overrun on the mass budget deducts 1 kg from the payload.  So if the second stage is overmass by 1%, then the payload takes a hit of 22 kg, i.e., 0.24%.

For an SSTO, we might hope (following Bruce Dunn: see the 3rd attachment to this post; I'm working with the un-sub-cooled lox/RP-1 case), that a vehicle with a dry mass of 46,620 kg could orbit a payload of 16,640 kg.  If the vehicle exceeds its mass budget by 1%, then the payload drops by 456 kg, i.e., 4.9%.

In other words, an SSTO is about 20 times more sensitive to the mass budget than is a conventional TSTO launch vehicle.  And that's using a very optimistic SSTO design (payload is about 2% of GLOM; more regularly one sees figures of maybe 0.5%).

The other thing about an SSTO is that its performance to a slightly more challenging orbit -- a little higher or at an non-optimal inclination -- will be really lousy, because the vehicle itself makes up so much of the mass injected into orbit.  Any foreseeable Earth-based SSTO will fly to and from a particular low-Earth orbit.  So there will have to be a lot of traffic to that one destination.  I'm thinking of a rate measured in launches per week, at the outside, rather than launches per year.  The traffic might be generated by a busy space station or propellant depot or by a low-cost, reusable tug which takes (a lot of) payloads to other orbits, or a combination of these.  But none of them exists yet or is even on the horizon.

EDIT:  Deleted redundant "we might hope" in 4th paragraph.
« Last Edit: 01/20/2017 11:49 AM by Proponent »

Online Robotbeat

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Re: Why the lack of SSTO projects?
« Reply #8 on: 01/19/2017 01:34 PM »
If I were starting a medium/small sat launch company, I'd consider an expendable SSTO to prove initial capability.

You could launch all the way to orbit without any staging events and with everything ground lit (and checked out before launch commit and clamp release). No deployments, no fairings, no recontact worries, no ullage motors, almost nothing.

Operationally, you'd end up using staging to get good performance, but for initial tests, expendable SSTO might be an interesting option.

« Last Edit: 01/19/2017 01:58 PM by Robotbeat »
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Offline scienceguy

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Re: Why the lack of SSTO projects?
« Reply #9 on: 01/19/2017 08:43 PM »
Are there any studies of SSTO's made of more modern, lighter materials like carbon fiber?
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Offline Toast

Re: Why the lack of SSTO projects?
« Reply #10 on: 01/19/2017 09:59 PM »
Are there any studies of SSTO's made of more modern, lighter materials like carbon fiber?
VentureStar and Skylon. VentureStar was cancelled, Skylon is still in development. Both are liable to start "lively" discussions around here.

Online Robotbeat

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Re: Why the lack of SSTO projects?
« Reply #11 on: 01/19/2017 10:14 PM »
And ITS, which Musk says is barely an SSTO in some variant by itself.

But yeah, X-33 was the venturestar sub scale prototype and had a carbon fiber tank which caused major headaches.

DeltaClipper, too. DC-X (its subscale prototype), was partly carbon fiber and DC-XA had aluminum-lithium alloy for tank structure.

Even Shuttle had carbon fiber parts.
« Last Edit: 01/19/2017 10:20 PM by Robotbeat »
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Offline Darkseraph

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Re: Why the lack of SSTO projects?
« Reply #12 on: 01/19/2017 10:25 PM »
The lack of projects is mostly due to the withdrawal of government support for such projects. The Sabre Engine is being slowly funded and that is pretty much it. SSTOs are expensive bleeding edge machines that are nowhere near as technologically mature as TSTO-RLV's. They would take possibly decades and billions to just get to a demonstration system. I can't see private companies jumping at this by themselves.

"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." R.P.Feynman

Online Robotbeat

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Re: Why the lack of SSTO projects?
« Reply #13 on: 01/19/2017 10:41 PM »
The lack of projects is mostly due to the withdrawal of government support for such projects. The Sabre Engine is being slowly funded and that is pretty much it. SSTOs are expensive bleeding edge machines that are nowhere near as technologically mature as TSTO-RLV's. They would take possibly decades and billions to just get to a demonstration system. I can't see private companies jumping at this by themselves.
What is ITS, if it's not SpaceX trying to jump into what is essentially a SSTO?

And by the way, Virgin Galactic Branson wants long distance point to point transport in a rocket ship, which is nearly the same performance as SSTO.

Government can and likely will encourage and help such projects along, but I'm not sure we need to rely on government to lead any such efforts.
« Last Edit: 01/19/2017 10:46 PM by Robotbeat »
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Offline Darkseraph

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Re: Why the lack of SSTO projects?
« Reply #14 on: 01/19/2017 11:01 PM »
^ The dearth of SSTO projects in the private sphere relative to TSTO speaks for itself. Even the bleeding edge ITS is being designed as a two stage system. Technologies that are essential for SSTO tend to be very beneficial for TSTO and the gradual development and maturation of those in more modest two-stage systems may make it much more credible in the future for a private company to attempt SSTO. Or a future program similar to Darpa's XS-1 for SSTO could jump start it.. 

AFAIK, Branson is investing in more practical supersonic jets, not point-to-point suboribtal travel any time soon.
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." R.P.Feynman

Online Robotbeat

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Re: Why the lack of SSTO projects?
« Reply #15 on: 01/19/2017 11:12 PM »
He recently mentioned point to point. In ADDITION to the much slower supersonic jet.
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Offline Katana

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Re: Why the lack of SSTO projects?
« Reply #16 on: 01/20/2017 03:02 AM »
Are there any studies of SSTO's made of more modern, lighter materials like carbon fiber?
VentureStar and Skylon. VentureStar was cancelled, Skylon is still in development. Both are liable to start "lively" discussions around here.
Skylon is also shifting to TSTO.

Online A_M_Swallow

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Re: Why the lack of SSTO projects?
« Reply #17 on: 01/20/2017 05:32 AM »
Reusable SSTO is viable on the Moon and possible on Mars. Concentrate your effort on those.

Online ChrisWilson68

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Re: Why the lack of SSTO projects?
« Reply #18 on: 01/20/2017 06:24 AM »
The answer to the question in the title is simple: it's a bad idea.  Take any credible SSTO system using whatever optimistic technological progress you like for the foreseeable future and you can take those same technologies and make a TSTO system that gives you a better performance/cost and performance/risk trade-off.

Staging is a fantastic technology.  Yes, there are some costs and risks associated with staging, but with Earth's gravity, chemical engines, and foreseeable materials technology, removing staging just introduces greater costs and risks in other parts of the system for an equal payload.

That's true for expendable systems (expendable TSTO is better than expendable SSTO), and it's true to an even greater extent for reusable systems (reusable TSTO is better than reusable SSTO).
« Last Edit: 01/20/2017 06:25 AM by ChrisWilson68 »

Offline KelvinZero

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Re: Why the lack of SSTO projects?
« Reply #19 on: 01/20/2017 06:50 AM »
For a long time I wondered the exact opposite of the OP:

Why are there these hugely extravagant NASA reusable vehicle projects that fail instead of just doing an incremental approach, eg starting with TSTO and reusable boosters?

Then SpaceX came along, and Blue Origin, but I really wonder why we couldn't have done this almost mundane approach first, way back in the 70s.

Admittedly I was mostly thinking of things like the fly-back booster that the airforce was looking at. And I never considered that a commercial company would do this rather than NASA which specifically has budgets for developing things beyond the current state of the art, and doesn't need commercial success at each step.

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