Author Topic: Problems of SSTO and technologies to solve these  (Read 57659 times)

Online A_M_Swallow

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Re: Problems of SSTO and technologies to solve these
« Reply #300 on: 04/09/2019 05:44 am »

3% / 0.6% = 5

So a SSTO is likely to be 5 times as big as a TSTO for the same payload. The significantly larger manufacturing costs explains why about 10 flights are needed to make a profit.
Again, please make your assumptions explicit.

A Vertical Take Off and Landing Rocket based SSTO is likely to be 5x bigger than an equivalent VTO TSTO ELV to give the same absolute payload mass.

That limitation does not apply to all SSTO architectures, and as has been pointed out in other threads the assumptions you make (and how they interact) have very significant effects on what looks viable, or not viable.

You can call it science or you can call it economics but the result is the same if you insist on such an architecture.

Other architectures offer other options.

The 3% SSTO figure came from John smith 19's reply #249. I calculated the 0.6% estimate in reply #262 in this thread. If the SSTO is better than 0.6% then it may be more profitable.

Offline Elmar Moelzer

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Re: Problems of SSTO and technologies to solve these
« Reply #301 on: 04/09/2019 06:06 am »
This assumes that development and construction of a larger SSTO will cost 5 times as much as the development and construction of a TSTO with the same payload. I don't think that this is necessarily true.
I also do not think that an SSTO needs to have a large payload in order to be viable.
Again, I think an SSTO capable of launching the equivalent of a Dragon2 to the ISS would be golden.
IMHO a vehicle the size of the Starship or slightly bigger should be able to do that. Looking at how SpaceX is building it's prototypes and the materials used, it could be relatively cheap.
I may be wrong, but I would assume that the biggest expenses are development of the engines and the heat shield and both were/are needed for the TSTO as well. So there would be very little extra cost to building an SSTO version of Starship.

Online rakaydos

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Re: Problems of SSTO and technologies to solve these
« Reply #302 on: 04/09/2019 12:11 pm »

3% / 0.6% = 5

So a SSTO is likely to be 5 times as big as a TSTO for the same payload. The significantly larger manufacturing costs explains why about 10 flights are needed to make a profit.
Again, please make your assumptions explicit.

A Vertical Take Off and Landing Rocket based SSTO is likely to be 5x bigger than an equivalent VTO TSTO ELV to give the same absolute payload mass.

That limitation does not apply to all SSTO architectures, and as has been pointed out in other threads the assumptions you make (and how they interact) have very significant effects on what looks viable, or not viable.

You can call it science or you can call it economics but the result is the same if you insist on such an architecture.

Other architectures offer other options.
Please don't put words into their mouth. The assumption is clearly stated in the same quotation- a SSTO with a .6% payload mass %. It doesn't matter if they achieve that percentage vertically, horizontally, or by teleportation, their post hold up.

Offline john smith 19

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Re: Problems of SSTO and technologies to solve these
« Reply #303 on: 04/11/2019 06:54 am »

The 3% SSTO figure came from John smith 19's reply #249. I calculated the 0.6% estimate in reply #262 in this thread. If the SSTO is better than 0.6% then it may be more profitable.
If you are independently wealthy then you can fund you're own SSTO and find out.

For anyone who has to raise funds from investors the logic I've outlined will ensure you don't get funded.
If you don't believe please develop an outline design, look for funding and let us know how that works out.
[EDIT in fact I encourage everyone who thinks you don't need to match the payload fraction of a TSTO (and hence a vehicle no bigger than a TSTO, which is what the VC's are paying for) to try and raise funding for their design.

If your opinion is correct you should find it fairly easy to raise funds. After all around 18 TSTO ELV's have done so, and your concept is clearly superior

Good luck ladies and gentlemen. I look forward to hearing about your results soon. ]

An interesting question would be how far you could scale down Skylon and still retain its payload fraction of 3% of GTOW.

A "micro launcher" that has no staging and all the benefits of an SSTO with the size (and hence estimated cost) of a TSTO ELV.
« Last Edit: 04/13/2019 05:35 pm by john smith 19 »
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Online A_M_Swallow

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Re: Problems of SSTO and technologies to solve these
« Reply #304 on: 04/14/2019 12:03 am »

The 3% SSTO figure came from John smith 19's reply #249. I calculated the 0.6% estimate in reply #262 in this thread. If the SSTO is better than 0.6% then it may be more profitable.
If you are independently wealthy then you can fund you're own SSTO and find out.

For anyone who has to raise funds from investors the logic I've outlined will ensure you don't get funded.
If you don't believe please develop an outline design, look for funding and let us know how that works out.
[EDIT in fact I encourage everyone who thinks you don't need to match the payload fraction of a TSTO (and hence a vehicle no bigger than a TSTO, which is what the VC's are paying for) to try and raise funding for their design.

If your opinion is correct you should find it fairly easy to raise funds. After all around 18 TSTO ELV's have done so, and your concept is clearly superior

Good luck ladies and gentlemen. I look forward to hearing about your results soon. ]
{snip}

To solve that one I would need to know the dry payload mass fraction. For 10 TSTO flights 10 launch vehicles need manufacturing where as only 1 SSTO vehicles is needed.

Offline daveklingler

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Re: Problems of SSTO and technologies to solve these
« Reply #305 on: 04/14/2019 01:35 am »
Oh dear. More remedial English.

I just noticed this thread and went back a couple of weeks to review it.  I can't help noticing that you have markedly more spelling and grammatical errors in your posts than anyone else. In fact, I'd go so far as to say you make more remedial English errors than everyone else combined.

My point is that "remedial English" is something you should not be emphasizing, unless when you say, "Oh dear. More remedial English," you're lamenting your own difficulties, in which case I'll pat you on the back virtually and say, "Now, now, it's probably just your typing."   ;)

On the topic, the viability of reusable SSTOs is very much an issue of economy.  If they can operate for less than the competition, then they can probably find funding and begin operations in today's environment. If they can't, then they'll have to wait.

I had a long conversation with Max Hunter long ago outside a latrine at a DC-X launch. He did a nice job of convincing me that VTVL reusable SSTOs were absolutely economically viable, and I still haven't seen any data that says differently. At the time he made the argument, the future looked bright for his school of thought, but then along came X-33 and Venturestar to (quite unjustly) discredit it. Humans are susceptible to group-think, and sometimes you just have to wait a generation or two for folks to say, "Hey, the numbers look fine to me," as some are doing on this forum.

Regarding the alleged issue of higher part quality for reusable rockets as opposed to expendables, it's my view that if your expendable's design isn't such that you could probably reuse it several times if you could just get it back, your failure rate is going to put you out of business. I never thought Rocketdyne's philosophy of building engines to just barely meet their mission requirements was at all a good idea.

To wit, a rocket engine should look essentially the same a day after you fired it as it did the day before. Even a Falcon 9 expendable is fired a few times before it ever leaves the launch pad.  As SpaceX's thinking evolved, they concluded that they were almost building completely reusable rockets; why not finish the job? I hope ULA comes to the same conclusion while they still exist.

I think the industry will probably take a little bit more time to get accustomed to the idea that rockets can be truly reusable.  When that paradigm is fully accepted, then fully reusable SSTO's will again find fertile soil.
« Last Edit: 04/14/2019 03:36 am by daveklingler »

Online rakaydos

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Re: Problems of SSTO and technologies to solve these
« Reply #306 on: 04/14/2019 07:15 am »


I think the industry will probably take a little bit more time to get accustomed to the idea that rockets can be truly reusable.  When that paradigm is fully accepted, then fully reusable SSTO's will again find fertile soil.
The problem is that your talks about the viability of reusable SSTO we're in the context of an expendable launch industry and a white elephant of a "partially reusable" STS. I've long maintained that the "advantages" of SSTO are actually just advantages of Economical Full Reusability, wrapped in the shuttle-era assumption that multistage designs can never be Economical.

The biggest problem facing  SSTO in the near future is the SpaceX Starship, and presumably RUS New Glen as well. By undercutting SSTOs assumed advantages of reusability, Commercial TSTO forces the comparisons back to "why staging exists," which on Earth is a trade TSTO will always win, given any drive system we'd actually be allowed to deploy in a biosphere.

Offline john smith 19

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Re: Problems of SSTO and technologies to solve these
« Reply #307 on: 04/14/2019 10:13 am »
On the topic, the viability of reusable SSTOs is very much an issue of economy.  If they can operate for less than the competition, then they can probably find funding and begin operations in today's environment. If they can't, then they'll have to wait.
Correct. And so far none has. Only one concept can deliver the equal of a TSTO VTO vehicle payload fraction.

Quote from: daveklingler
I had a long conversation with Max Hunter long ago outside a latrine at a DC-X launch. He did a nice job of convincing me that VTVL reusable SSTOs were absolutely economically viable, and I still haven't seen any data that says differently.
Aside from the Kudos of meeting him, do actually recall any of his argument? I'll note that the end game for the DC-X programme was a vehicle design for deployment of military constellations. "Economic viability" in that sense is a little different from a fully commercial approach.
Quote from: daveklingler
At the time he made the argument, the future looked bright for his school of thought, but then along came X-33 and Venturestar to (quite unjustly) discredit it. Humans are susceptible to group-think, and sometimes you just have to wait a generation or two for folks to say, "Hey, the numbers look fine to me," as some are doing on this forum.
Quite true. The X30 wasted another $1Bn+ when competent technical due diligence would have found basic errors in the model.

Quote from: daveklingler
Regarding the alleged issue of higher part quality for reusable rockets as opposed to expendables, it's my view that if your expendable's design isn't such that you could probably reuse it several times if you could just get it back, your failure rate is going to put you out of business. I never thought Rocketdyne's philosophy of building engines to just barely meet their mission requirements was at all a good idea.
I'd call it more the idea that the parts have to be heavier to have a longer (IE more than 1 launch) life.

DC-X suggested that's not not so (indeed that was the main point of the exercise). It can be argued the booster stage reuse of the F9 proves it, but without knowing what the refurbishment process is that cannot be confirmed.

Quote from: daveklingler
I think the industry will probably take a little bit more time to get accustomed to the idea that rockets can be truly reusable.  When that paradigm is fully accepted, then fully reusable SSTO's will again find fertile soil.
The problem that all VTVL SSTO's face is pretty simple.

You can't get a rocket powered SSTO to have the same payload fraction as a VTO TSTO.  It will have higher technical risk and is known to be extremely sensitive to weight growth. For "higher technical risk" read "need a bigger budget."

Note those qualifications, VTOL and (pure) rocket powered.  If you move to HTOL and away from the pure rocket system options open up, but so does the technical risk. Some systems can have most of the risk retired on the ground. Some need a flight vehicle to design a flight vehicle, as a SCramjet advocate stated in Aerospace America some months ago.

Unfortunately the only system the USG has funded in this area are SCramjets, which despite 6 decades of funding (something North of $15Bn IIRC) has delivered not a single operational system.
« Last Edit: 04/14/2019 10:16 am by john smith 19 »
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Offline john smith 19

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Re: Problems of SSTO and technologies to solve these
« Reply #308 on: 04/14/2019 10:24 am »
The biggest problem facing  SSTO in the near future is the SpaceX Starship, and presumably RUS New Glen as well. By undercutting SSTOs assumed advantages of reusability, Commercial TSTO forces the comparisons back to "why staging exists," which on Earth is a trade TSTO will always win, given any drive system we'd actually be allowed to deploy in a biosphere.
"Will always win" as long as you assume you're comparing rocket powered VTO SSTO with rocket powered TSTO.

When you forget the assumptions you're making when you make statements you brain wash yourself into thinking things that are simply wrong.

"given any drive system we'd actually be allowed to deploy in a biosphere." Presumably you're talking about Orion. Now what happens to your world view if you have an engine that has an Isp of 3000sec, not 282secs?

The message on this thread (put square brackets around it to go there. I don't have the time to figure out how to embed this as a  proper inter-thread link).

quote author=MikeAtkinson link=topic=47745.msg1932859#msg1932859 date=1554620900

is an excellent dissection of the assumptions that can lead to a particular conclusion. Once you know those assumptions you can see why the conclusion makes perfect sense within their context.

But if you're circumstances are different (and for most other companies they will be very different) they make no sense.
« Last Edit: 04/18/2019 07:13 am by john smith 19 »
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Online rakaydos

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Re: Problems of SSTO and technologies to solve these
« Reply #309 on: 04/14/2019 02:12 pm »
The biggest problem facing  SSTO in the near future is the SpaceX Starship, and presumably RUS New Glen as well. By undercutting SSTOs assumed advantages of reusability, Commercial TSTO forces the comparisons back to "why staging exists," which on Earth is a trade TSTO will always win, given any drive system we'd actually be allowed to deploy in a biosphere.
"Will always win" as long as you assume you're comparing rocket powered VTO SSTO with rocket powered TSTO.

When you forget the assumptions you're making when you make statements you brain wash yourself into thinking things that are simply wrong.

"given any drive system we'd actually be allowed to deploy in a biosphere." Presumably you're talking about Orion. Now what happens to your world view if you have an engine that has an Isp of 3000sec, not 282secs?
That argument didnt get any more convincing the last five times you used it. Perhaps you could find new material?

Online lamontagne

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Re: Problems of SSTO and technologies to solve these
« Reply #310 on: 04/14/2019 02:49 pm »
I've joined a spreadsheet with Skylon and ITS, older version of Starship for comparison purposes.

I think I've used the correct ISP for Skylon, actually 6000s ISP so perhaps too much.
I find ITS is significantly less expensive than Skylon.  I'dd love to discuss these numbers if anyone feels like slogging through the spreadsheet!

The calculations are on the second page
« Last Edit: 04/14/2019 02:50 pm by lamontagne »

Offline libra

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Re: Problems of SSTO and technologies to solve these
« Reply #311 on: 04/14/2019 03:43 pm »
TAN (as I mentionned earlier) has big potential for SSTO. Except Aerojet is doing nothing of it, and no startup seems to have grasped its potential (bangs my head against a wall).

Here are all the papers I could find on it (I already posted them on other threads).
All of them by Melvin Bulman, the spiritual heir to Rudi Beichel and Robert Salkeld, who pioneered tripropellant engines from 1970 onwards, Beichel bringing the subject to Aerojet, where Bulman come in 1988 (see his LinkedIn profile)
https://www.linkedin.com/in/mel-bulman-59439329/

On the Soviet side was the RD-701 of MAKS fame.  Look at the last two papers: seems Aerojet took a look at the RD-701.

As for me, I remain convinced that suborbital refueling of a rocketplane (jets-and-rockets) at 5 km/s and 350 000 ft in a suborbital parabola could have resulted in a SSTO (with a cheat / trick) in the Shuttle days, but we already dicussed the matter many times on this forum, and I know the pros and cons of the subject. It can go sour pretty fast.  ;)
« Last Edit: 04/14/2019 03:47 pm by libra »

Offline daveklingler

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Re: Problems of SSTO and technologies to solve these
« Reply #312 on: 04/14/2019 10:50 pm »


I think the industry will probably take a little bit more time to get accustomed to the idea that rockets can be truly reusable.  When that paradigm is fully accepted, then fully reusable SSTO's will again find fertile soil.
The problem is that your talks about the viability of reusable SSTO we're in the context of an expendable launch industry and a white elephant of a "partially reusable" STS. I've long maintained that the "advantages" of SSTO are actually just advantages of Economical Full Reusability, wrapped in the shuttle-era assumption that multistage designs can never be Economical.

The biggest problem facing  SSTO in the near future is the SpaceX Starship, and presumably RUS New Glen as well. By undercutting SSTOs assumed advantages of reusability, Commercial TSTO forces the comparisons back to "why staging exists," which on Earth is a trade TSTO will always win, given any drive system we'd actually be allowed to deploy in a biosphere.

Pretty much everybody agrees that staging works great as long as long turnaround and readiness times aren't a problem.

So...reusable SSTO can have slightly lower costs if (the cost of labor and hardware necessary for Reusable TSTO) > (the cost of larger rockets and extra propellant necessary for RSSTO), or (the labor and hardware necessary for higher TSTO readiness levels) > (the cost of larger rockets and extra propellant necessary for RSSTO).

It's likely that for several more years RTSTO systems will appear and grow mature as new markets (hopefully) develop, demand climbs and competition gets fiercer. More and more people will eye RSSTO systems as they review ways to compete and we'll start to see more attempts to make it work.

There's also the possibility of disruptive technological improvement past the present-day chemical rocket limitations that make staging so advantageous, probable but hard to discuss if it doesn't exist yet.  For example,  a sheared-flow stabilized Z-pinch rocket engine would have exhaust velocities in the 10^7 range (that number makes me giggle and shake my head every time I type it). If, or more likely when, anything like that ever happens the whole field might change very quickly, as it might anyway if air-breathing hybrids gain maturity.  And for that matter, had NERVA or Timberwind reached flight we might be there right now.

For now, IMO the limiting factor seems to be that nobody's really sure what to do with a RSSTO, so RTSTO is plenty good enough until another billionaire comes along.  But I think that's basically where we were before NewSpace came along - SpaceX has opened up new markets and we're all happy about it but there are still plenty of people denying that reusability is even needed.

In other words, if another billionaire DID come along and push RSSTOs into the market, we'd all be pretty jazzed about it and a sizable contingent of people would grumble that RTSTOs were just fine.  :)

Offline daveklingler

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Re: Problems of SSTO and technologies to solve these
« Reply #313 on: 04/14/2019 11:16 pm »
I had a long conversation with Max Hunter long ago outside a latrine at a DC-X launch. He did a nice job of convincing me that VTVL reusable SSTOs were absolutely economically viable, and I still haven't seen any data that says differently.
Aside from the Kudos of meeting him, do actually recall any of his argument? I'll note that the end game for the DC-X programme was a vehicle design for deployment of military constellations. "Economic viability" in that sense is a little different from a fully commercial approach.

I do remember the conversation pretty well. It was seminal for me.

The end game for DC-X, in Max's mind, was low-cost civilian space travel similar to civilian air travel. He made a few basic points - people were focused on propellant use instead of overall launch costs, one or two extra engines for redundancy were preferable to the mass of wings and a landing gear sized for a full takeoff, and big VTVL SSTO rockets traded well against smaller TSTOs in the context of frequent launches for a thriving space economy.

I'll just add that SSTO's can be fluffy, and a bit lower in TPS requirements, as an example of how the trades can be less intuitive than one might initially think.

Quote
Quote from: daveklingler
Regarding the alleged issue of higher part quality for reusable rockets as opposed to expendables, it's my view that if your expendable's design isn't such that you could probably reuse it several times if you could just get it back, your failure rate is going to put you out of business. I never thought Rocketdyne's philosophy of building engines to just barely meet their mission requirements was at all a good idea.
I'd call it more the idea that the parts have to be heavier to have a longer (IE more than 1 launch) life.

I don't think that's true. They just have to be well-designed and well-fabricated. Someone was telling me (or I was reading?) a while back that Rocketdyne used to hand-fabricate their turbines using a file, and if they "kind-of" balanced that was good enough not to tear themselves apart for a few minutes. I'm trying to remember whether that was a book or a living human being.

Quote
DC-X suggested that's not not so (indeed that was the main point of the exercise). It can be argued the booster stage reuse of the F9 proves it, but without knowing what the refurbishment process is that cannot be confirmed.

Well, actually the point of the exercise was to prove out the concept of rockets that could act like and be operated in the same way as passenger jets, with the same levels of safety.

We might be looking at this entire topic very differently in a year or two. The way I see it, Starship might buy down a lot of the technical risk for RSSTOs.
« Last Edit: 04/14/2019 11:21 pm by daveklingler »

Online tea monster

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Re: Problems of SSTO and technologies to solve these
« Reply #314 on: 04/15/2019 12:32 am »
If you don't have to buy a whole new spaceship each time you send something to orbit, then you can probably afford more robust materials for your launcher.  ;D

Offline Elmar Moelzer

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Re: Problems of SSTO and technologies to solve these
« Reply #315 on: 04/15/2019 03:00 am »
That is the thing though, with the large LEO constellations, there is a market for SSTO launchers, even if their payload is relatively low. Unfortunately, we do not know the details about the new Starship design. So I have no basis for calculating it, but I calculated that the old 2017 Star Ship could have made a decent reusable SSTO with more than enough payload to launch Starlink. You can gain operational experience with the Starship design much faster if you trade launching a large number of satellites on a single launch against launching an SSTO version more often. For a company like SpaceX, experience from launching often has a lot of value in itself. Launch often and quickly, fail quickly, improve, etc. Losing a whole SS/SH stack would be a lot more costly and it would take much longer to find out all the kinks. You can also implement improvements to the design much faster that way.

Online rakaydos

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Re: Problems of SSTO and technologies to solve these
« Reply #316 on: 04/15/2019 11:45 am »
That is the thing though, with the large LEO constellations, there is a market for SSTO launchers, even if their payload is relatively low. Unfortunately, we do not know the details about the new Starship design. So I have no basis for calculating it, but I calculated that the old 2017 Star Ship could have made a decent reusable SSTO with more than enough payload to launch Starlink. You can gain operational experience with the Starship design much faster if you trade launching a large number of satellites on a single launch against launching an SSTO version more often. For a company like SpaceX, experience from launching often has a lot of value in itself. Launch often and quickly, fail quickly, improve, etc. Losing a whole SS/SH stack would be a lot more costly and it would take much longer to find out all the kinks. You can also implement improvements to the design much faster that way.

The question becomes, is there ENOUGH LEO traffic to support a specialized payload launcher that cannot  effectively perform earth departure burns after refueling?

Right now, sure, LEO and GTO are practically all we CAN do anymore, but with Starship on the horizon, and something that might be New Armstrong visible past the horizon, with ISRU planned for mars, lunar ice mining, and even asteroid mining missions... how much of LEO service will, in 50-100 years, be served by shallower gravwells?

Offline daveklingler

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Re: Problems of SSTO and technologies to solve these
« Reply #317 on: 04/16/2019 02:29 am »
That is the thing though, with the large LEO constellations, there is a market for SSTO launchers, even if their payload is relatively low. Unfortunately, we do not know the details about the new Starship design. So I have no basis for calculating it, but I calculated that the old 2017 Star Ship could have made a decent reusable SSTO with more than enough payload to launch Starlink. You can gain operational experience with the Starship design much faster if you trade launching a large number of satellites on a single launch against launching an SSTO version more often. For a company like SpaceX, experience from launching often has a lot of value in itself. Launch often and quickly, fail quickly, improve, etc. Losing a whole SS/SH stack would be a lot more costly and it would take much longer to find out all the kinks. You can also implement improvements to the design much faster that way.

The question becomes, is there ENOUGH LEO traffic to support a specialized payload launcher that cannot  effectively perform earth departure burns after refueling?

Right now, sure, LEO and GTO are practically all we CAN do anymore, but with Starship on the horizon, and something that might be New Armstrong visible past the horizon, with ISRU planned for mars, lunar ice mining, and even asteroid mining missions... how much of LEO service will, in 50-100 years, be served by shallower gravwells?

I think in 50-100 years, well, heck, I was about to agree with you, and then I thought "torch ships inside of 20 years, baby!"  50-100 years is somewhere way past the Singularity - hard to talk about it.  :)

But I imagine that there will be a time in the relatively near future when healthy LEO infrastructure with hydrolox prop depots finally slips into existence.* As cargo flights climb, a long-awaited somewhat conventional space trucking business should finally appear, complete with load brokers and multiple flights per day. Combine that with multiple companies doing routine second stage reentries. At that point I think there will be a demand for RSSTO, plus low technological risk.

*Probably via Gateway depots and tugs developed to give SLS a reason to live - oh, the irony!

Offline Seamurda

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Re: Problems of SSTO and technologies to solve these
« Reply #318 on: 04/16/2019 11:52 pm »
TAN (as I mentionned earlier) has big potential for SSTO. Except Aerojet is doing nothing of it, and no startup seems to have grasped its potential (bangs my head against a wall).

Here are all the papers I could find on it (I already posted them on other threads).
All of them by Melvin Bulman, the spiritual heir to Rudi Beichel and Robert Salkeld, who pioneered tripropellant engines from 1970 onwards, Beichel bringing the subject to Aerojet, where Bulman come in 1988 (see his LinkedIn profile)
https://www.linkedin.com/in/mel-bulman-59439329/

On the Soviet side was the RD-701 of MAKS fame.  Look at the last two papers: seems Aerojet took a look at the RD-701.

As for me, I remain convinced that suborbital refueling of a rocketplane (jets-and-rockets) at 5 km/s and 350 000 ft in a suborbital parabola could have resulted in a SSTO (with a cheat / trick) in the Shuttle days, but we already dicussed the matter many times on this forum, and I know the pros and cons of the subject. It can go sour pretty fast.  ;)

Do you have any links to Salkeld's papers?

Offline john smith 19

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Re: Problems of SSTO and technologies to solve these
« Reply #319 on: 04/17/2019 07:22 am »

Here are all the papers I could find on it (I already posted them on other threads).
All of them by Melvin Bulman, the spiritual heir to Rudi Beichel and Robert Salkeld, who pioneered tripropellant engines from 1970 onwards, Beichel bringing the subject to Aerojet, where Bulman come in 1988 (see his LinkedIn profile)
SABRE, using air, then switching LOX to burn LH2, is also a tri-propellant engine.
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