Author Topic: Why the lack of SSTO projects?  (Read 23660 times)

Offline IsaacKuo

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Re: Why the lack of SSTO projects?
« Reply #40 on: 01/21/2017 01:53 PM »
Not that it necessarily helps enough, but if we assume ISRU source of propellant in space, a reusable upper stage could actively cancel out its orbital velocity before reentry. This could greatly reduce the TPS mass that must be carried all the way to orbit and back.

Alternatively, ISRU resupplied lox might be used to open cycle cool the TPS during reentry, directing the hot oxygen out nozzles for additional braking thrust. (Lox might be harvested in LEO via atmospheric scooping.) Same practical effect of minimizing TPS mass, but you only need to resupply oxidizer.

Offline Robotbeat

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Re: Why the lack of SSTO projects?
« Reply #41 on: 01/21/2017 02:59 PM »
Better to use nitrogen in that case instead of oxygen. There's like 4x as much nitrogen, and it's nearly inert.
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Offline Proponent

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Re: Why the lack of SSTO projects?
« Reply #42 on: 01/21/2017 04:07 PM »
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.

I just had another look at the mixture-ratio data in Apollo by the Numbers, and I'm a bit puzzled.  First of all, for each burn of each stage, two O/F figures are given:  one corresponding to the start of the burn and the other supposedly the change during the burn.  But the latter figure, however, is generally larger than the former, which implies that O/F more than doubled during the burn.  That seems very unlikely, so I assume the second number is actually the O/F at shut-down.

With that assumption, it seems that the O/F gets leaner (higher) during most if not all burns.  But don't we expect just the opposite, namely getting richer (more hydrogen) to increase specific impulse as the burn proceeds?

It also seems to be the case that the second S-IVB burn is on average leaner than the first (I say "seems to be," because the average during the burn isn't necessarily the average of the intial and final values).  Rocket-equation-wise, this too seems counterintuitive, though maybe it's a way of maximizing stage performance in the face of hydrogen boil-off.

What gives?

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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.

That sounds interesting -- do you have any pointers as where to find these studies?

BTW, attached is a Rocketdyne study of a variable-mixture-ratio lox-hydrogen engine.

Offline IsaacKuo

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Re: Why the lack of SSTO projects?
« Reply #43 on: 01/21/2017 06:23 PM »
Better to use nitrogen in that case instead of oxygen. There's like 4x as much nitrogen, and it's nearly inert.

That's true, assuming you can use liquid nitrogen in one (or both) of the propellant tanks.

Offline Katana

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Re: Why the lack of SSTO projects?
« Reply #44 on: 01/22/2017 02:22 AM »
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.

I just had another look at the mixture-ratio data in Apollo by the Numbers, and I'm a bit puzzled.  First of all, for each burn of each stage, two O/F figures are given:  one corresponding to the start of the burn and the other supposedly the change during the burn.  But the latter figure, however, is generally larger than the former, which implies that O/F more than doubled during the burn.  That seems very unlikely, so I assume the second number is actually the O/F at shut-down.

With that assumption, it seems that the O/F gets leaner (higher) during most if not all burns.  But don't we expect just the opposite, namely getting richer (more hydrogen) to increase specific impulse as the burn proceeds?

It also seems to be the case that the second S-IVB burn is on average leaner than the first (I say "seems to be," because the average during the burn isn't necessarily the average of the intial and final values).  Rocket-equation-wise, this too seems counterintuitive, though maybe it's a way of maximizing stage performance in the face of hydrogen boil-off.

What gives?

Quote
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.

That sounds interesting -- do you have any pointers as where to find these studies?

BTW, attached is a Rocketdyne study of a variable-mixture-ratio lox-hydrogen engine.
Variations of mix ratio are too small to be meaningful.

Offline A_M_Swallow

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Re: Why the lack of SSTO projects?
« Reply #45 on: 01/22/2017 04:06 AM »
For a reusable TSTO can the upper stage and the capsule's service module be merged?

Offline HMXHMX

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Re: Why the lack of SSTO projects?
« Reply #46 on: 01/22/2017 04:23 AM »
For a reusable TSTO can the upper stage and the capsule's service module be merged?

Indeed it could.  There are some interesting configurations that can be explored along those lines.  The "Mainstage" (as I call it) would perform a once-around mission, never entering orbit, while the spacecraft with at least a few hundred m/s of delta V circularizes, raises orbit, etc. on its own.

Offline RotoSequence

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Re: Why the lack of SSTO projects?
« Reply #47 on: 01/22/2017 04:23 AM »
For a reusable TSTO can the upper stage and the capsule's service module be merged?

If you extrapolate the design of SpaceX's Interplanetary Transport System architecture to its logical conclusion, I don't see why you couldn't perform an arbitrary amount of systems integration into a launch vehicle's second stage as weight allows.
« Last Edit: 01/22/2017 04:25 AM by RotoSequence »

Offline Vultur

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Re: Why the lack of SSTO projects?
« Reply #48 on: 01/22/2017 12:30 PM »
\
Unfortunately IRL those also count as "benefits."  :(
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Well, sure, but more for NASA/government projects... I was asking more about the lack of SSTO projects among the "new"/commercial space companies.

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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.

But where does this assumption ("cost proportional to GTOW") come from? It seems not to be true, or at least to be avoidable.

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

But why is payload fraction the relevant metric? Cost per flight is more important.


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.

Yeah but it ought to be able to fly >10x more often - early on. Once the market expands and the technology matures >>100x. 1 flight per vehicle every day or two doesn't really seem that unreasonable... IF you computerize the thing to the max, so there's no human inspections between flights unless the vehicle's computers flag a problem.

High flight rate and low cost per flight will be dependent on low labor costs and therefore heavily automated systems. Also probably not using existing ground infrastructure.

Offline Robotbeat

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Re: Why the lack of SSTO projects?
« Reply #49 on: 01/22/2017 01:22 PM »
In other words, ITS.
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Offline john smith 19

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Re: Why the lack of SSTO projects?
« Reply #50 on: 01/23/2017 06:32 AM »
Variations of mix ratio are too small to be meaningful.
Check the paper
"Enhancing the Saturn V translunar payload capability." by Logsdon & Africano.

The paper describes how NASA increased the payload to the Moon over the life of the Moon launches by 5%, at a time when the cost of that mass per unit to LLO was higher than Gold without hardware changes to the vehicle (essentially by doing, for want of a better phrase, "better maths"). That's about 5 tonnes.

They estimate that 1/2 of that improvement was due to MR control, which could not be applied to the first stage (which probably would have been even better).

MR control can give significant benefits and if you're dealing with a VTO rocket only SSTO you'd better be looking at as much "low hanging fruit" as you can get given the high sensitivity to engine under performance and structural weight growth.

IMHO the paper should be on any LV designers must read list. Sadly I can't find the planned follow up paper on propellant biasing for maximum payload to orbit.  :(
« Last Edit: 01/23/2017 06:33 AM by john smith 19 »
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Offline QuantumG

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Re: Why the lack of SSTO projects?
« Reply #51 on: 01/23/2017 06:40 AM »
2nd stage reuse is always simpler than reuseable SSTO or shuttle architecture on the same conditions. TPS of large rockets is a great pain.

Ummm.. the reentry system of Sea Dragon was the pressure vessel. Never flew, obviously, but I don't think anyone really argues there's much doubt it wouldn't work. Big fluffy things reenter intact even when you don't want them too. It's small things that need painful TPS.
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Offline Nomic

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Re: Why the lack of SSTO projects?
« Reply #52 on: 01/23/2017 08:55 AM »
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.  :(

Aerojets work on the OTV engine touched on using mixture rations up to 13:1, some of the problems discussed  on page 108 onwards, and  solutions including gold platting the MCC (page 115) and platinum baffles. Idea behind the high mixture ratios was to use Lunar regolith as source of ISRU LOX. Don't think anyone has ever run an engine at these sort of mixture ratios though.

Offline john smith 19

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Re: Why the lack of SSTO projects?
« Reply #53 on: 01/23/2017 03:33 PM »
Aerojets work on the OTV engine touched on using mixture rations up to 13:1, some of the problems discussed  on page 108 onwards, and  solutions including gold platting the MCC (page 115) and platinum baffles. Idea behind the high mixture ratios was to use Lunar regolith as source of ISRU LOX. Don't think anyone has ever run an engine at these sort of mixture ratios though.
Exactly.  And check that expansion ratio. 1200.  :o

This is clearly not going to be operating anywhere near sea level.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Robotbeat

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Re: Why the lack of SSTO projects?
« Reply #54 on: 01/23/2017 03:36 PM »
That's the neat thing about Thrust Augmented Nozzles. The expansion ratio for the main engine can be huge, but because you're adding a bunch of propellant downstream when at low altitudes, you don't get flow separation.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Patchouli

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Re: Why the lack of SSTO projects?
« Reply #55 on: 01/23/2017 04:51 PM »
That's the neat thing about Thrust Augmented Nozzles. The expansion ratio for the main engine can be huge, but because you're adding a bunch of propellant downstream when at low altitudes, you don't get flow separation.

One reason I think a SSME with TAN  might be the prefect engine for a simple near term SSTO.
Right now the nozzle is a compromise for sea level and  vacuum performance that uses a lot of tricks to prevent flow separation.
But a TAN nozzle could have an expansion ratio that is better optimized for both sea level and vacuum operation.

« Last Edit: 01/23/2017 04:54 PM by Patchouli »

Offline Nilof

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Re: Why the lack of SSTO projects?
« Reply #56 on: 01/23/2017 05:08 PM »
The other advantage of TAN of course is that it is one way to implement a tripropellant engine. For SSTO's, the increased payload fraction that can potentially be afforded by a tripropellant engine is difficult to understate.

Among the SSTO's that got somewhat serious consideration, I believe the MAKS spacecraft was the one that had the most margin, entirely due to the amazing performance of the RD-701.
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 john smith 19

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Re: Why the lack of SSTO projects?
« Reply #57 on: 01/25/2017 04:39 PM »
Ummm.. the reentry system of Sea Dragon was the pressure vessel. Never flew, obviously, but I don't think anyone really argues there's much doubt it wouldn't work. Big fluffy things reenter intact even when you don't want them too. It's small things that need painful TPS.
True.

Shuttle would have been a very different, and bigger design had they incorporated the ET into its design. It would also have had to deal with the issue of cycle life for the main fuel tanks over the life of the vehicle. Instead of which they opted to recover "just the engines" and deal with a highly temperamental TPS instead.  :(

50 years on and it seems both Arianespace and ULA are planning to adopt the same plan, with a small dense module that will fall deep into the atmosphere before hitting air dense enough to slow it down.

One reason I think a SSME with TAN  might be the prefect engine for a simple near term SSTO.
Not that near term. Look at what proportion of thrust comes from TAN. It's not just filling the bell to stop flow sep. At SL SSME is about 384Kbls. Aerojet were talking 50-100% of that. IE 190-384 Klbs That's a pretty major pump installation that's either got to run off it's own power system or tap the existing SSME turbine drive.
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Right now the nozzle is a compromise for sea level and  vacuum performance that uses a lot of tricks to prevent flow separation.
Which are? The only "trick" I'm aware of the very high chamber pressure, coupled with some care in keeping the interior of the bell clean.
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But a TAN nozzle could have an expansion ratio that is better optimized for both sea level and vacuum operation.
I think the term you're looking for is more adaptable to both SL and vacuum. You can get individual nozzles that are very well adapted to either condition already, the problem is when you want it to do both.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Archibald

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Re: Why the lack of SSTO projects?
« Reply #58 on: 01/25/2017 06:37 PM »
it is a crying shame nothing happened with TAN over the last decade.

Offline john smith 19

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Re: Why the lack of SSTO projects?
« Reply #59 on: 01/25/2017 07:08 PM »
\
Unfortunately IRL those also count as "benefits."  :(
Well, sure, but more for NASA/government projects... I was asking more about the lack of SSTO projects among the "new"/commercial space companies.
Because as "any fule know" a VTO SSTO has to sacrifice payload. A TSTO ELV is normally in the 3-4% of GTOW. Historically the VTO SSTO's that Philip Bono described were expected to hit 1-1.5% GTOW as payload.

That means you either a)Have to fly 3-4 flights to put the same mass in LEO or b) Make it 3-4x bigger.
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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.

But where does this assumption ("cost proportional to GTOW") come from? It seems not to be true, or at least to be avoidable.
Cost data is collected by several organizations and essentially the information is graphed and a best fit curve found, much as data is collected on software development projects to update the COCOMO II cost model for large project development.

There are several problems with these models (as a google on the sci.space.* newsgroups would show you over the last 20 years).
They are mostly based on government LV and aircraft programmes. These tend to over run their costs, which moves the "centre of gravity" of the cost curve upward. I cannot guess what the F35 programme will do for the estimated cost of a fighter programme of X lbs payload and Y mph top speed for example.   :(

For aircraft lowering GTOW is a good idea, as it lightens the landing gear, increases range etc. In LV systems it encourages you to go to LH2. I think the key difference is that in launch the spec is always set at Orbital speed and altitude high enough to not immediately re-enter. Everything below that is sub orbital, and a failure.

These were the models that NASA checked against SX's internal accounts to conclude F1 and F9 up to first launch as a NASA contract under BAU rules would have cost $3.977Bn and $1.695bn under "more commercial" procurement rules.  :(
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Being able to match a TSTO ELV in payload fraction matters. No historical SSTO had managed this.

But why is payload fraction the relevant metric? Cost per flight is more important.
Not to sound repetitive but if you don't have an angel investor then you have to deal with commercial investors. They will hire someone to do "due diligence" who will probably plug your plan for payload and GTOW into one of those models.

As of now there is only 1 design concept that has voluntarily undergone detailed 3rd party inspection and passed.

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Yeah but it ought to be able to fly >10x more often - early on. Once the market expands and the technology matures >>100x. 1 flight per vehicle every day or two doesn't really seem that unreasonable... IF you computerize the thing to the max, so there's no human inspections between flights unless the vehicle's computers flag a problem.
The problem is the budget you're going to need to build that vehicle.  The LV business is strange.
No other transportation system has basically the same company make the vehicles and operate them. Ship builders don't run container liners. Locomotive mfg's don't run railroads. Volvo (who own Mack in the US) don't run a trucking company.

The business model for mfg is completely different from operators (in every other transportation system)  yet in the LV they are basically the same. As long as this model persists it seems impossible for the mfg to ever recoup the cost of their vehicles development through the  open market.  The only reason this has not been a problem is because historically development has been under written by a government, or a consortium of governments.  :(
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High flight rate and low cost per flight will be dependent on low labor costs and therefore heavily automated systems. Also probably not using existing ground infrastructure.
True. 
IIRC there's about 400 staff standing behind each airline takeoff.That's about 2 orders of magnitude smaller than Shuttle (and the size of that standing army is very important to the running costs).

Various techniques were developed (but not deployed) to radically lower inspection and repair costs for shuttle.

In some ways the key thing is to start backward, with the cost target, and then look at what can give you that.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

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