Why the lack of SSTO projects?

[john smith 19]

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.

2 reasons.

1) NASA under Nixon suffered a 90% reduction in it's budget as a proportion of total government spending.
With such a cut down there was little reason to explore reusing existing stages, especially as the biggest (the Saturn V) had already ceased production by 1968.

2)Because in fact it's not "mundane". It took SX 5 years from F9's first flight to delivering a 1st landing of the 1st stage despite having the Grasshopper test vehicle to practice on and processors about 450x more powerful than those on the early Shuttle flights. That is from no more than a top speed of M10. 

Musk ruled out making the 2nd stage reusable (which is the final velocity an SSTO would have to achieve) at all.

So even partial reusability has turned out to be quite tough. The key seems to be recognizing that engine TVC alone can't do it.  Something it seems none of the 60's stage reuse concepts considered at all.

[Nomic]

Always liked the idea of dual mixture ratio hydrolox for any one crazy enough to try reusable SSTO. Start off running LOX rich, switch to fuel rich at the equivalent of staging. Bulk density similar to methlox, but with full flow staged combustion ISP of 360 at sea level LOX rich going to 460 fuel rich ISP in the vacuum. LOX rich tan mentioned by JonGoff on the other thread is an interesting variation, lower pressure injection of the extra TAN LOX reducing the delta in pumping requirements between fuel rich and LOX rich modes.  Would still pose some interesting pump design problems.

For a smallsat SSTO, the original rotary rocket idea could be an option. Only scales up to 1000kg, maybe less. Wonder if it could be combined with a dual mixture ratio hydrolox, would probably have to sacrifice rotor blade aerofoil performance to get a large enough hydrogen line through it.

[Proponent]

Always liked the idea of dual mixture ratio hydrolox for any one crazy enough to try reusable SSTO. Start off running LOX rich, switch to fuel rich at the equivalent of staging. Bulk density similar to methlox, but with full flow staged combustion ISP of 360 at sea level LOX rich going to 460 fuel rich ISP in the vacuum.

I had a quick look at  variable-mixture-ratio lox-hydrogen SSTO a while ago but was underwhelmed by the results.

[Robotbeat]

Theoretically, it's a nearly perfect solution as oxygen is very dense. This allows you to adjust the mixture to match the velocity. This is how you can get an energy efficiency at lower speeds.

Variable mixture SSTO is especially interesting if you're limited by energy, i.e. If propellant costs are significant.

[john smith 19]

Always liked the idea of dual mixture ratio hydrolox for any one crazy enough to try reusable SSTO. Start off running LOX rich, switch to fuel rich at the equivalent of staging.

The J2 did this. Sacrificing Isp to improve payload seems counter intuitive but it's the concept of putting less KE into the remaining mass of the vehicle.

In a era of much lighter and faster computer control the logical move is to step profile the O2 O:F ratio as you rise.

BTW Aerojet seemed to have been quite fond of this tactic in many of their design studies. I think they went as high as 12:1. It's another of those "low hanging fruit" you should be looking at if you're going to do SSTO.

IRL the joker is you run oxidizer rich, which makes US developers very nervous, and you transition through the stoichometric  (maximum temperature) range. Obviously if you can scan through that range quickly enough without triggering combustion instability that's not a problem but I don't think anyone's ever really done that. 

[Robotbeat]

Maybe introduce the oxygen down-stream for lift-off, like in a thrust-augmented nozzle. That way, the combustion chamber conditions remain largely unchanged.

You'd probably want a separate lower pressure, higher mass flow turbopump just for the thrust-augmenting liquid oxygen.

I think it'd work. Someone should try it!

Edit: the Aerojet patent expires in 4 years...

[KelvinZero]

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.

2)Because in fact it's not "mundane". It took SX 5 years from F9's first flight to delivering a 1st landing of the 1st stage despite having the Grasshopper test vehicle to practice on and processors about 450x more powerful than those on the early Shuttle flights. That is from no more than a top speed of M10.

I just meant basics, eg TSTO reusability before SSTO reusability. Not making it also immediately and always manned to _really_ tie potential for innovation and experimentation in knots. Not having the bizarre configuration of solids burning next to a massive hydrogen tank and engine. 4 things that have to separate sideways instead of 2 or 3 vertically.

And they did have unmanned landings. And that is a good place to invest development anyway. Less effort on the other issues would have solved these ones faster. And I was actually imagining flyback boosters, or in any case not the pretty miraculous thing SpaceX did of taking a fairly conventional rocket without particularly special physical characteristics such as throttling and just using incredible computing power to get a hoverslam type solution working.

I think NASA could have achieved a lot if it was allowed to do a sensible two (or 1.5) stage unmanned launcher, just a workhorse like F9, only concentrate on reuse of the first stage or boosters at first, but know you would not have to go back to the drawing board for the entire rocket if you decide to later consider upper stage reuse, or a "dreamchaser" like space plane in any case.

I know a lot of it was politics.

[ChrisWilson68]

Musk ruled out making the 2nd stage reusable (which is the final velocity an SSTO would have to achieve) at all.

Musk didn't rule it out.  He decided the economics of it don't make sense for now.  That's an important difference.

[Robotbeat]

Indeed. His comments leave open the possibility of 2nd stage reuse, just not right now. From what I've heard from spiiice on reddit, he's kind of obsessed with the idea of 2nd stage reuse.

[clongton]

Basic answer to the basic question:
Chemical propulsion just cannot provide enough power long enough to enter orbit with any kind of meaningful payload. At least in earth's gravity well. It just costs too much money to produce a vehicle that will never be able to pay for itself. You won't see a useful SSTO vehicle until the stupid phobias against nuclear power are lifted.

[john smith 19]

I think NASA could have achieved a lot if it was allowed to do a sensible two (or 1.5) stage unmanned launcher, just a workhorse like F9, only concentrate on reuse of the first stage or boosters at first, but know you would not have to go back to the drawing board for the entire rocket if you decide to later consider upper stage reuse, or a "dreamchaser" like space plane in any case.

The rules under which Shuttle was developed were as follows.
1)No more than $1Bn per year.
2)No money can be rolled over to the next year.
3)There is no increase to allow for inflation (during the 1970's which was a period of high US inflation)
4)There is no contingency allowance for cost overruns.

To put this in perspective that was viewed as enough to develop 1 new rocket engine and 1 new complete "stage" IE Orbiter.

You might almost think Nixon didn't want NASA to succeed in developing a new launch vehicle. :-|

The result being the aircraft + huge RATO packs + monster drop tank architecture.

Once you know that background you realize STS had very few choices to get anything flying. 

[Vultur]

IIRC a Falcon 9 first stage can get to orbit on its own, but with 0 payload.

That's about right (IIRC it was too small to be useful, maybe a few hundred kg, not quite 0). But...

So any low-risk developments would be pointless, which leaves the high-risk path of Skylon.

...that doesn't follow, because F9 first stage is optimized as a first stage. It carries <100 (IIRC) tons of second stage, as well as the payload. If you added that 100ish tons as more fuel (minus whatever the extra tank weight would be) its delta-v would be significantly higher.

IIRC mass ratio of 30 was mentioned for Falcon Heavy side booster, at least at one point. With that tank stretch the mass ratio could end up extremely good.

Huge investment with massive risk of unproven system for what market ?

The investment wouldn't be that huge.

OK, over a hundred million (F1 was $90 million, an F1 payload class SSTO would be more) but on the scale of space projects or large corporations, not so huge.

And the risk is not that big since there is really no 'out there' technology.

A really reusable "gas and go" SSTO would make space cheap - far more so than a semireusable (Shuttle or F9) system could ever achieve, probably significantly more than a two-stage fully reusable.

Basic answer to the basic question:
Chemical propulsion just cannot provide enough power long enough to enter orbit with any kind of meaningful payload. At least in earth's gravity well.

I just don't see the evidence for that. When I run the rocket equation with Merlin 1D's known Isp and the best demonstrated structural mass fractions, I get something very good.

Remember that Mercury-Atlas was almost an expendable SSTO (swap out those engines with modern ones and it would easily be a true SSTO) and it reached orbit in 1961.

You won't see a useful SSTO vehicle until the stupid phobias against nuclear power are lifted.

NERVA style nuclear rockets are too low TWR for practical Earth liftoff, IIRC. A nuclear lightbulb might work but that's way more unknown technology.

Project Orion would do it easily, of course, but there are other objections to that one than the fission products. Wouldn't it fry a whole bunch of existing satellites.

[john smith 19]

The thing with all proposed VTO SSTO's is they sacrifice payload for simplicity.

So far 3-4% of GTOW on 2 (or sometimes more) stages has outweighed the benefits of 1% (or thereabouts) on 1 stage.

That's been the ball park for reusable VOTL SSTO  proposals.

So maybe an expendable SSTO could do better (1.5%? 2%?)

But bottom line More GTOW / less payload --> higher $/lb to orbit.

I could see an existing stage design being adapted to an expendable SSTO, but would the development cost be recouped by the number of launches it would make in it's lifetime?

[Vultur]

The thing with all proposed VTO SSTO's is they sacrifice payload for simplicity.

So far 3-4% of GTOW on 2 (or sometimes more) stages has outweighed the benefits of 1% (or thereabouts) on 1 stage.

That's been the ball park for reusable VOTL SSTO  proposals.

Well 'outweighed the benefits' might not be quite the situation. 2/3STO (or 'halfway' approaches like the Shuttle and Mercury-Atlas where there are discarded parts but the "upper stage" engines light on the ground) have always been chosen historically, yeah.

But there are factors other than pure engineering benefits in those decisions, like risk aversion & "irrational" government budgeting limits (Shuttle), and political pressure to reuse pre-existing hardware (eg SLS).

But bottom line More GTOW / less payload --> higher $/lb to orbit.

I doubt that. All else being equal, sure - but all else is definitely not equal.

Total cost probably doesn't go up linearly with size, even if materials costs do (F1 cost $90 million to develop, the original version of F9 cost $300 million to develop - even if we count much of the F1 cost into F9's because of heritage/experience, that's still about x4 development cost for about x10 size).

So the difference between 1% of GTOW and 3-4% of GTOW is 3-4x vehicle size for the same payload... but less than 3-4x cost.

And a reusable SSTO should be able to fly way more than 4x more often... even 10x would be pretty unimpressive.

EDIT: fixed quote tag

[Robotbeat]

Falcon booster, full thrust and densified should get at least 3-6 tons to LEO equatorial expendable SSTO. That's not insignificant. Particularly the stretched version for FH (that has now changed to non-stretched).

[Katana]

Indeed. His comments leave open the possibility of 2nd stage reuse, just not right now. From what I've heard from spiiice on reddit, he's kind of obsessed with the idea of 2nd stage reuse.

2nd stage reuse is always simpler than reuseable SSTO or shuttle architecture on the same conditions. TPS of large rockets is a great pain.

[Katana]

I think NASA could have achieved a lot if it was allowed to do a sensible two (or 1.5) stage unmanned launcher, just a workhorse like F9, only concentrate on reuse of the first stage or boosters at first, but know you would not have to go back to the drawing board for the entire rocket if you decide to later consider upper stage reuse, or a "dreamchaser" like space plane in any case.

The rules under which Shuttle was developed were as follows.
1)No more than $1Bn per year.
2)No money can be rolled over to the next year.
3)There is no increase to allow for inflation (during the 1970's which was a period of high US inflation)
4)There is no contingency allowance for cost overruns.

To put this in perspective that was viewed as enough to develop 1 new rocket engine and 1 new complete "stage" IE Orbiter.

You might almost think Nixon didn't want NASA to succeed in developing a new launch vehicle. :-|

The result being the aircraft + huge RATO packs + monster drop tank architecture.

Once you know that background you realize STS had very few choices to get anything flying. 

The shuttle program spended more than 50% the total cost of Apollo program and reinvented a launch system sending 100 tons into orbit , only 20% are useful payload.

Even more expensive than resuming the Saturn/Apollo/Skylab series could cost.

Saturn shuttle with F1/J2 engines and shuttle C for cargo are cancelled.

[john smith 19]

2nd stage reuse is always simpler than reuseable SSTO or shuttle architecture on the same conditions. TPS of large rockets is a great pain.

That's certainly an interesting PoV.

Most people who've looked at the problem note a large vehicle (IE one that keeps its tanks) has a large surface area but a low ballistic coefficient, so its entry interface is higher. That means it can lose more energy at a lower rate, enabling a lower temperature TPS. IOW Shuttles problem was not that it was too big, it was too small.

The thing is all SSTO proposals I'm aware of have been designed to do SSTO and to return. They side stepped the control issues of high aspect ratio, relatively flexible standard launch stages. Given SX had to fit grid fins to finally make it possible to work that seems like a smart move.

I look forward to SX demonstrating 2nd stage reuse.  I look forward even more to seeing at what price they will offer such a vehicle at.

[john smith 19]

Well 'outweighed the benefits' might not be quite the situation. 2/3STO (or 'halfway' approaches like the Shuttle and Mercury-Atlas where there are discarded parts but the "upper stage" engines light on the ground) have always been chosen historically, yeah.

But there are factors other than pure engineering benefits in those decisions, like risk aversion & "irrational" government budgeting limits (Shuttle), and political pressure to reuse pre-existing hardware (eg SLS).

Unfortunately IRL those also count as "benefits." 

But bottom line More GTOW / less payload --> higher $/lb to orbit.

I doubt that. All else being equal, sure - but all else is definitely not equal.

Total cost probably doesn't go up linearly with size, even if materials costs do (F1 cost $90 million to develop, the original version of F9 cost $300 million to develop - even if we count much of the F1 cost into F9's because of heritage/experience, that's still about x4 development cost for about x10 size).

So the difference between 1% of GTOW and 3-4% of GTOW is 3-4x vehicle size for the same payload... but less than 3-4x cost.

You have the problem backwards. If you don't have the "Bank of Elon" behind you you have to convince financiers this is a good idea.

They will do due diligence by plugging in that vehicle size into their aerospace cost model, where cost is roughly proportional to GTOW.
Result "Your SSTO will give this much to orbit, but a TSTO will give 2-4x that amount for the same GTOW. Funding refused."
This demonstrates 2 points.

If you do what you always do you'll get what you always got.

Being able to match a TSTO ELV in payload fraction matters. No historical SSTO had managed this.

These cost models are also the ones that reckon the budget to do F1 and up to the first F9 launch would be $2Bn, about 6.6x the known figure.

And a reusable SSTO should be able to fly way more than 4x more often... even 10x would be pretty unimpressive.

EDIT: fixed quote tag

Historically this has been something of a circular argument.  An SSTO with 1/3 to 1/4 of the payload of a TSTO ELV has to fly 3-4x more often. Personally I think it is possible and I don't think there are many payloads that cannot be broken up into reasonable sized modules for on orbit assembly. Of course what "reasonable sized" means is open to wide interpretation. 

[Katana]

2nd stage reuse is always simpler than reuseable SSTO or shuttle architecture on the same conditions. TPS of large rockets is a great pain.

That's certainly an interesting PoV.

Most people who've looked at the problem note a large vehicle (IE one that keeps its tanks) has a large surface area but a low ballistic coefficient, so its entry interface is higher. That means it can lose more energy at a lower rate, enabling a lower temperature TPS. IOW Shuttles problem was not that it was too big, it was too small.

The thing is all SSTO proposals I'm aware of have been designed to do SSTO and to return. They side stepped the control issues of high aspect ratio, relatively flexible standard launch stages. Given SX had to fit grid fins to finally make it possible to work that seems like a smart move.

I look forward to SX demonstrating 2nd stage reuse.  I look forward even more to seeing at what price they will offer such a vehicle at.

Both stages of F9 are large and hollow structures with similar low ballistic coefficient, thus require similar TPS thickness.

But the absolute scale and cost of first stage is much larger, a large stage suffering orbital reentry would increase total cost.

Anyway, a TSTO first stage could live a happy long life with moderate reentry velocity.