156 posts in this topic

So I looked through 7 pages and all I saw were SSTO threads from the spacecraft exchange, none here, so forgive me if there was already a thread here :)

Anyway, SSTOs, I've got a lot of questions, and judging by the amount of SSTOs you guys build, I'm sure you can answer them, ask lots more questions, or take part quite easily in this thread.

  • How big (i.e. How much can one carry) before a SSTO gets to big and becomes ridiculously inefficient?
  • What would be the best engine setup for an SSTO, only rocket engines, jet and rocket engines, or SABRE derived engines?
  • What's the best design for a SSTO, rocket or plane?
  • What are all the different types types of SABRE derived engines? Maybe an aerospike type SABRE engine?
  • For a plane, What's the best wing setup?
  • Are they cheaper or more expensive to build than rockets, how much or little would one cost?
  • Are they easier or harder to build?
  • Are they more eco friendly or worse?
  • What are the pros and cons of SSTOs?
  • I have a very rough idea in my head for a SSTO that can carry 260 tonnes to LEO, is that good, or bad (In terms of power)?

Here's some links;

https://en.wikipedia.org/wiki/Single-stage-to-orbit

https://en.wikipedia.org/wiki/Skylon_%28spacecraft%29

http://spectech.bravepages.com/OT_Article_SSTO.htm


fig28.jpg

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Cue Nibb in 3....2....1....

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This is the Science subforum, lol. :D

Edited by Findthepin1

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Just now, Findthepin1 said:

This is the Science subforum, lol. :D

Erm... Yeah, what are you getting at?

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What he's getting at is that commercially viable, useful Earth-based SSTOs are essentially science fiction.

In very brief summary;

  • How big (i.e. How much can one carry) before a SSTO gets to big and becomes ridiculously inefficient? not very much - useful payload is essentially zero with chemical rockets
  • What would be the best engine setup for an SSTO, only rocket engines, jet and rocket engines, or SABRE derived engines? staging where used engines and tanks are dropped off at intervals
  • What's the best design for a SSTO, rocket or plane? rocket would be the best as it does not waste mass on wings, landing gear etc.
  • For a plane, What's the best wing setup? none - make it a rocket
  • Are they cheaper or more expensive to build than rockets, how much or little would one cost? extremely expensive as they're completely unproven technology
  • Are they easier or harder to build? useful ones are much harder, useless ones probably not so hard
  • What are the pros and cons of SSTOs? have a thorough read of the Skylon thread here - the pros and cons of SSTOs are discussed at length there
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2 minutes ago, Spaceception said:

Erm... Yeah, what are you getting at?

It's unclear whether you are talking in-game mechanics and building SSTOs for the game, or real-world applications.

Taking a stab at a few of your questions,  though (assuming you are talking real-world):

  • How big (i.e. How much can one carry) before a SSTO gets to big and becomes ridiculously inefficient?

Well, as it turns out, bigger is better. A larger SSTO is more efficient than a small SSTO, since it can hold larger tanks. Larger tanks have a better dry mass ratio than smaller tanks, due to the square-cube law. Thus, you want your SSTO to be as big as possible for highest efficiency. The more important question, then, is how large you can make your SSTO before it becomes too large to efficiently re-use. The larger your ship, the more man-hours it will take to refurbish it for reflight.

The Shuttle is a prime example of this -- it wasn't even fully reusable, and yet it took many many times longer than expected to refurbish between flights. So you probably want to aim for something smaller than that.

And what other people have said is correct; you need a really large SSTO to be able to have a useful payload at all.

  • What would be the best engine setup for an SSTO, only rocket engines, jet and rocket engines, or SABRE derived engines?

Adding jet engines is dry mass that will cut into your payload fraction horribly, so that's a non-starter.

The trouble is that when you're moving slowly, you need a LOT of thrust so you don't waste fuel fighting gravity. By the time you're moving quickly enough that you're no longer spending most of your fuel fighting gravity, you're pretty much moving too quickly to use airbreathing engines. SABRE is supposed to fix this by using wings to combat gravity and using precooling to extend the airbreathing regime, but that's still going to be tricky.

It's not enough to focus only on specific impulse; you have to focus on thrust-to-weight ratio as well. Because gravity drag is a killer. Basically, if you want to do an airbreather, you'll need to find a way to increase thrust faster than you increase launch mass. And that's tough.

  • What's the best design for a SSTO, rocket or plane?

Unless you are planning on landing with humans onboard, go with a VTVL rocket. Wings really only make sense if you need to glide to a landing. And honestly if you can repurpose your retropropulsion/RCS/OMS modules to land vertically while in a horizontal orientation, do so, because the fuel cost will still be lower than the dry mass cost of wings.

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Just now, Excalibur said:

What he's getting at is that commercially viable, useful Earth-based SSTOs are essentially science fiction.

Almost every future spacecraft design is science fiction until we build it, but if it works on paper (Or KSP RO ;) ) then you can bet it works in real life, with varying degrees of success.

But you are right, I'm just making a thread to discuss how practical SSTOs are.

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59 minutes ago, Spaceception said:

So I looked through 7 pages and all I saw were SSTO threads from the spacecraft exchange, none here, so forgive me if there was already a thread here :)

Anyway, SSTOs, I've got a lot of questions, and judging by the amount of SSTOs you guys build, I'm sure you can answer them, ask lots more questions, or take part quite easily in this thread.

  • How big (i.e. How much can one carry) before a SSTO gets to big and becomes ridiculously inefficient?

First of all, SSTO does not equal spaceplane. You could have pretty much any combination single-stage/multi-stage, vertical take-off/horizontal take-off, reusable/expendable launcher.

SSTOs are always less efficient in terms of payload fraction than a multi-stage vehicle, because physics. The Saturn V first stage or the Titan first stage could have been SSTO, but the payload fraction would have been so tiny that it would have been pointless. Musk says that the F9 first stage could fly to orbit too if you removed the upper stage and payload. But what good would that be?

If you have a small payload, then it's much cheaper to simply build a smaller multi-stage rocket than a huge single stage one. 

A multiple stage Falcon 9 weighs around 500mt for a 10mt payload. An equivalent SSTO would have to weigh something like SLS, but that doesn't factor in the extra hardware for getting it back, which would likely weigh more than 10mt.

So to be of any use, an SSTO needs to be as big as possible.

Quote
  • What would be the best engine setup for an SSTO, only rocket engines, jet and rocket engines, or SABRE derived engines?

SSTOs suck. The best setup will be a magical engine. SABRE is supposed to work on paper, but the economics behind it don't pan out and none of it is proven to work. It still has to be enormous (bigger than an A380) just to carry a 7mt payload to LEO.

Quote
  • What's the best design for a SSTO, rocket or plane?

Probably vertical takeoff. Wings are useless in space and dead weight that eat into your payload fraction. Your SLS sized SSTO with 10 mt of payload wouldn't even reach orbit if you added wings, TPS, hydraulics, and landing gear to it.

Quote
  • What are all the different types types of SABRE derived engines? Maybe an aerospike type SABRE engine?

SABRE is SABRE. There is only one SABRE design.

Quote
  • For a plane, What's the best wing setup?

None. Wings are useless in space. An SSTO with wings 

Quote
  • Are they cheaper or more expensive to build than rockets, how much or little would one cost?
  • Are they easier or harder to build?
  • Are they more eco friendly or worse?
  • What are the pros and cons of SSTOs?

Nobody knows because hasn't ever made sense to launch one. They simply are not practical.

Quote
  •  
  • I have a very rough idea in my head for a SSTO that can carry 260 tonnes to LEO, is that good, or bad (In terms of power)?

What SSTO ? Maybe one that is several hundred times the size of an A380. Good luck building a hangar to park it.

 

 

 

Edited by Nibb31
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Just now, Nibb31 said:

Nobody knows because hasn't ever made sense to launch one. They simply are not practical.

Not even on paper? I mean, I know going by paper isn't always completely correct, but it's a good ballpark.

Edited by Spaceception

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Just now, Spaceception said:

Not even on paper?

No-one has ever set out designs for an SSTO that was anything other than a demonstration that the idea might be possible.

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Just now, Steel said:

No-one has ever set out designs for an SSTO that was anything other than a demonstration that the idea might be possible.

Oh.

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2 hours ago, Spaceception said:

So I looked through 7 pages and all I saw were SSTO threads from the spacecraft exchange, none here, so forgive me if there was already a thread here :)

Anyway, SSTOs, I've got a lot of questions, and judging by the amount of SSTOs you guys build, I'm sure you can answer them, ask lots more questions, or take part quite easily in this thread.

  • How big (i.e. How much can one carry) before a SSTO gets to big and becomes ridiculously inefficient? Way too big (to carry *anything*, although a Falcon-9 booster could supposedly carry a few cubesats).  There was an earlier [air launched] SABRE discussion link showing that SABRE would "work with hydrogen peroxide" for a huge ship with 2 crewmen.
  • What would be the best engine setup for an SSTO, only rocket engines, jet and rocket engines, or SABRE derived engines? Anything that doesn't require using onboard fuel for chemical reactions (i.e. any known engine with anything close to >1g thrust).  This includes Orion [the real one from the 1950s] and Escape Dynamics' microwave powered rocket.   Note that this is only true for SABRE for speeds <mach 6 and inside the atmosphere, so that it isn't clear if that works better for staging than SSTO.
  • What's the best design for a SSTO, rocket or plane?  Have a working engine first, then build a spaceship around it.
  • What are all the different types types of SABRE derived engines? Maybe an aerospike type SABRE engine?  Maybe after a SABRE engine exists there might be another engine derived from it...
  • For a plane, What's the best wing setup?
  • Are they cheaper or more expensive to build than rockets, how much or little would one cost?  Mind-boggling more expensive.  Pay for a Saturn and get Mecury-Redstone performance.  Then you get to land and refurbish the Saturn.
  • Are they easier or harder to build?  Vastly harder.  All rockets are staged for a reason.
  • Are they more eco friendly or worse?  Bigger to make, use vastly more fuel.  Of course, they are also sufficiently far off in the future that some other limit will probably be key (any discussion in the 70s when the shuttle was being developed would assume that running out of oil would be the problem facing the Earth).
  • What are the pros and cons of SSTOs?  The only pro is easier integration: and this assumes that SSTOs can handwave tons of unsolved issues away while falcon9-style systems can't develop a KSP/Lego system of putting the rocket together (sure that's "impossible", but vastly easier than anything considered the "easy parts" of an SSTO design).  The cons are mass, mass, mass, and fuel use (fuel use is a trivial issue now, but presumably for any market that can afford SSTO craft, fuel use will have to be a significant chunk of the price, like plane tickets are now).
  • I have a very rough idea in my head for a SSTO that can carry 260 tonnes to LEO, is that good, or bad (In terms of power)?  Install realism overhaul and get a rough idea of how hard SSTO-Earth is (SSTO-Kerbin is pretty easy).

 

My comments are in red.

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3 hours ago, Spaceception said:
  •  
  • What are all the different types types of SABRE derived engines? Maybe an aerospike type SABRE engine?

I've only heard of the basic SABRE.

3 hours ago, Spaceception said:

What's the best design for a SSTO, rocket or plane?

Depends on what you want. If you want downmass, planes are probably better, since they have a cargo bay. Rockets are good for basic upmass.

3 hours ago, Spaceception said:
  •  
  • Are they cheaper or more expensive to build than rockets, how much or little would one cost?

More expensive, assuming reuseablility and X-33 style SSTO.

3 hours ago, Spaceception said:

Are they easier or harder to build?

Harder. Considering my KSP experiences, definitely harder.

3 hours ago, Spaceception said:
  •  
  • Are they more eco friendly or worse?

...Does that matter?

3 hours ago, Spaceception said:
  •  
  • I have a very rough idea in my head for a SSTO that can carry 260 tonnes to LEO, is that good, or bad (In terms of power)?

Impressively bad. I showed you the ROMBUS concepts earlier on your novel thread. That's pretty much the size of a 260 T to LEO SSTO. 2-stage or bust.

3 hours ago, Spaceception said:
  •  
  • What are the pros and cons of SSTOs?

I don't know, I don't make them in KSP, but I do know they have horrible mass fraction.

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@Spaceception

Actually, I read the first paragraph and thought you were talking about KSP. So I then thought you had mistakenly posted in the Science subforum. XD

Edited by Findthepin1

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2 hours ago, sevenperforce said:

 

The Shuttle is a prime example of this -- it wasn't even fully reusable, and yet it took many many times longer than expected to refurbish between flights. So you probably want to aim for something smaller than that.

 

2 hours ago, Steel said:

No-one has ever set out designs for an SSTO that was anything other than a demonstration that the idea might be possible.

Wait, not even X-33 and DC-X?

Oh wait:

 Here we go:

http://www.sciencedirect.com/science/article/pii/S0094576597001422

1 hour ago, wumpus said:

Way too big (to carry *anything*, although a Falcon-9 booster could supposedly carry a few cubesats).  There was an earlier [air launched] SABRE discussion link showing that SABRE would "work with hydrogen peroxide" for a huge ship with 2 crewmen.

You can use Tripropellant RP-1 and LH2 to increase efficiency, along with a stage combustion plug-nozzle, like the proposed Plug-nozzle SSME.

 

1 hour ago, wumpus said:

What's the best design for a SSTO, rocket or plane?  Have a working engine first, then build a spaceship around it.

Like Skylon? ;)

1 hour ago, wumpus said:

What are the pros and cons of SSTOs?  The only pro is easier integration: and this assumes that SSTOs can handwave tons of unsolved issues away while falcon9-style systems can't develop a KSP/Lego system of putting the rocket together (sure that's "impossible", but vastly easier than anything considered the "easy parts" of an SSTO design).  The cons are mass, mass, mass, and fuel use (fuel use is a trivial issue now, but presumably for any market that can afford SSTO craft, fuel use will have to be a significant chunk of the price, like plane tickets are now).

In theory, SSTOs are less complex, and can just refuel and launch. In practice...

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7 minutes ago, fredinno said:

In theory, SSTOs are less complex, and can just refuel and launch. In practice...

It's not so different from any other vehicle. The larger it is, the more opportunity there is for stuff to break, and the more refurbishing and repairing has to be done between flights. But since large rockets are more fuel-efficient than small rockets, a reusable SSTO must ride the knife-edge between "too small to deliver a meaningful payload" and "too large to allow reasonable turnaround time".

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14 minutes ago, fredinno said:

In theory, SSTOs are less complex, and can just refuel and launch. In practice...

Not true.

Well designed SSTOs that are designed from the ground up for reusability might have the capability to "just refuel and launch" but there's no reason that an SSTO would be any simpler than a conventional rocket.

16 minutes ago, fredinno said:

Wait, not even X-33 and DC-X?

Also, those are the very definition of what I was talking about, technology demonstrators, not SSTOs with a commercial purpose

21 minutes ago, fredinno said:

Finally, this is more a feasibility study than a genuine design.

Edited by Steel

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Just now, Findthepin1 said:

@Spaceception

Actually, I read the first paragraph and thought you were talking about KSP. So I then thought you had mistakenly posted in the Science subforum. XD

I don't have KSP yet, so posting there would be useless to me :)

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3 hours ago, Steel said:

Well designed SSTOs that are designed from the ground up for reusability might have the capability to "just refuel and launch" but there's no reason that an SSTO would be any simpler than a conventional rocket.

That's why I said, "IN PRACTICE..."

3 hours ago, Steel said:
3 hours ago, fredinno said:

 

Also, those are the very definition of what I was talking about, technology demonstrators, not SSTOs with a commercial purpose

They were intended to be scaled up to commercial usage, so yes, I think they are relevant.

3 hours ago, Steel said:
3 hours ago, fredinno said:

 

Finally, this is more a feasibility study than a genuine design.

No, but they still offer very good data.

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Staging simply offers a LOT of advantages, not all of which are readily apparent. You can use sea-level-optimized engines at sea level and vacuum-optimized engines in space.

Perhaps even more importantly, you can have a lot of thrust at launch and less thrust up high. Hydrolox offers by far the highest specific impulse of any reasonably fuel combination, and mass ratio is insanely important for an SSTO, so let's use hydrolox. You need about 7,500 m/s of dV to get into orbit, so that's a propellant mass fraction of 84%. Totally manageable, right?

Unfortunately, it's not that simple -- you need to account for the lower SL specific impulse; even if you have an altitude-compensating nozzle of some kind, you're only going to get around 400s max at sea level compared to hydrolox's 450+ second vacuum specific impulse. So that tacks on the equivalent of at least 500 m/s, bringing your dV up to 8 km/s and your propellant mass fraction of 86%. Still feasible, right?

But there's something else to account for: drag. Gravity drag and aerodynamic drag. And here's where the lack of staging really kills you. Hydrogen engines have high specific impulse but crappy thrust, which means they are going to have far worse susceptibility to gravity drag than a beefier fuel combination like kerolox. A kerolox engine might need an extra 1.5 km/s to fight drag going up, but a hydrolox engine will need closer to 2 or 2.5 km/s to fight drag, bringing the propellant mass fraction up above 90%. And your hydrogen engines will need to be heavier than a kerolox engine of equivalent thrust, which will probably end up cutting your payload in half or worse.

Couple that with hydrogen's low density and high tankage volume, and you'll REALLY be wishing you had a kerolox engine and a kerolox tank strung alongside to help lift you the first half of the way. But if you have a kerolox engine and a kerolox tank strapped alongside, then it really only makes sense to drop them at some point so they can RTLS for reuse while your hydrolox stage rockets into orbit.

1.5 stages to orbit is SO much simpler than single-stage-to-orbit.

Edited by sevenperforce
added

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Engine type? Orion drive, or gas core NTR... basically, nuclear or no

Sabre is interesting, airbreathing in general is good not just because of the O2 in the air, but the working mass that could roughly double effective Isp even in oxygenless atmospheres.

The problem with airbreathing designs is that they only get up to about 1,000 m/s... even being generous and allowing for 1,500 m/s designs like perhaps with the Sabre, you've still got another 6,500 m/s or more to go to get to orbit... in  KSP, its more like 650 m/s..... biiiiiiiiiiiiiiiiiiiiggggggggggggggg difference.

6,500 m/s is hard to get in a single stage with chemical rockets already... without adding in the deadweight of engines and wings and heat protection for the entire thing.

That is why I personally really like two stage designs like this early shuttle concept:

p333.jpg

If we had working sabre engines, then I'd put them in the lower booster. For the upper craft, I'd probably still get rid of the wings, or make it a lifting body like the X-24B, a shape which handles reentry heating much better:

x24a3.gif

The advantage of SSTOs is the hypothetical "Land refuel, drop in a new payload and launch again"... ignoring the massive maintenance that would need to be done, and the much higher fuel costs.

What space X is doing with recoverable boosters is great... you don't need to do SSTO to be able to just refeul something and use it again.

Recover booster, recover orbital craft, stick them together, refeul and launch again.... plus all that mainenance... which could be a lot easier because your sabre booster doesn't have to have much of a thermal protection system, and your engine cluster is a lot smaller because you take much much less dry mass to orbit.

Your orbital craft can have engines optimized for vaccum operation only, and so the nozzle design is simpler (no aerospikes here), and you can have lower chamber pressures in the rockets (you need really high chamber pressure to have your engines perform ok at sea level, good in a vacuum, like the space shuttles engines did). Fuel costs in this scenario are much lower because of both airbreathing and taking much less non payload dry mass to orbit.

As to your question of other Sabre "variants".. there was something similar for the canceled HOTOL, the RB545 https://en.wikipedia.org/wiki/HOTOL#Engine

Which was never actually made... there was also the LACE... for which a test engine was made https://en.wikipedia.org/wiki/Liquid_air_cycle_engine

Then there is the actual proposed Sabre variant, the Schimitar https://en.wikipedia.org/wiki/Reaction_Engines_Scimitar

Which doesn't have the closed cycle rocket feature, intended just for high speed atmospheric craft... in the 2 stage concept I outlined above, perhaps the booster could dispense with the rocket/closed cycle mode to make the engines simpler for easier maintenance... but I do like the concept of the booster going into closed cycle mode and accelerating another thousand m/s or so to increase the payload fraction of the orbital craft (its generally better to have 2 stages with 500 m/s each than 1 stage with 900 m/s and the other with 100 m/s - same concept here)... whatever speeds could be achieved before the booster needs more exotic and high maintenance thermal protection system.

Also... that means the booster will go much faster, higher, and farther... meaning it will have even farther to come back - thus I'm not sure what the optimum would be here.

 

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The Shuttle is actually not too far from the optimum layout for getting to orbit and back with some degree of fast and simple reusability. It was just way, way too big to be reused quickly, and the requirement of an internal cargo bay meant that an external disposable drop tank was unavoidable.

If your goal is 100% reuse with short turnaround time, then your best bet is to build a VERY downscaled version of the Shuttle. Replace the SRBs with Falcon-Heavy style kerolox boosters, but add auxiliary LH2 tanks. Replace the Shuttle with a core hydrolox booster and give it a truncated aerospike nozzle for altitude compensation. Put a Dragon V2 style capsule on top (or a payload with a fairing, if you like). At launch, crossfeed the core booster from the side-booster LOX tanks and the auxiliary LH2 tanks up to side booster separation. Side boosters RTLS with vertical landing; the core carries the capsule into orbit, circles once or twice, and then deorbits, using its truncated aerospike as a heat shield to aerobrake to a vertical landing RTLS. Capsule (if applicable) re-enters separately and lands propulsively.

fully_reusable_triple_core.png

On the one hand, that's four separate vehicles instead of just one. But any problems or extended repair time is now relegated to a single component, so that re-use of the other components is unhindered. With the extremely good specific impulse of hydrolox and the very high thrust of kerolox, the propellant mass fraction would be extremely low, meaning the launch vehicle could be quite small and thus easier to recertify and reuse.

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What about something like this?

_80896032_80896031.jpg

Landing with those boosters wouldn't be easier than falcon 9 style?

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Just now, Darnok said:

What about something like this?

_80896032_80896031.jpg

Landing with those boosters wouldn't be easier than falcon 9 style?

I believe sevenperforce posted the exact same rocket 12 minutes before you did :P 

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10 minutes ago, Spaceception said:

I believe sevenperforce posted the exact same rocket 12 minutes before you did :P 

Indeed. Though not quite the exact same. You don't need those useless fins, the center booster needs to be altitude-compensating with a truncated aerospike to act a heat shield, and you need to have LH2 and LOX crossfeed from the side boosters to the center booster (which themselves need to be kerolox). And everything needs landing legs.

You could do it with a single booster, I suppose, but then you do end up with some COM/COT issues.

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