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SABRE vs RAPIER


splashboom
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(which is better in a duel ..)

So the CR-7 R.A.P.I.E.R. engine is emulating the - ever in development - SABRE engine. Seeing that with it an SSTO is quite possible in KSP, you have to wonder some things

- What would be the $/Kg to orbit of a plane with SABRE? Is it so much better than SpaceX's Starship? (since neither exists let's use glossy brochures to make comparisons ..)

- Would a platform like Virgin Orbit benefit from SABRE technology - on either of the two stages

I need to know that my Kerbal SSTO is not for nothing!

 

 

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SSTO is easy in KSP because the low orbital velocity;  2400 m/s = 8640 km/h.
That is falcon 9 first stage velocity however an high one, but on Kerbin an falcon 9 would be an SSTO who could put an second stage into orbit and land. 

I say rapier is pretty much an KSP Sabre, I suspect it under perform, but the rapier flame out might be a bit faster. On the other hand I wonder if Sabre could use oxygen injection to run atmospheric mode above safe flame out?  Like all KSP engines the TWR is bad.
But space planes are so nice in KSP because the low orbital velocity, Laythe is even better as its lower, managed to raise Ap to high or higher than the orbital ship on atmospheric engines and is so used to having to switch to rocket mode long before Ap is above 70 Km. 

Edited by magnemoe
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1 hour ago, splashboom said:

(which is better in a duel ..)

So the CR-7 R.A.P.I.E.R. engine is emulating the - ever in development - SABRE engine. Seeing that with it an SSTO is quite possible in KSP, you have to wonder some things

- What would be the $/Kg to orbit of a plane with SABRE? Is it so much better than SpaceX's Starship? (since neither exists let's use glossy brochures to make comparisons ..)

- Would a platform like Virgin Orbit benefit from SABRE technology - on either of the two stages

I need to know that my Kerbal SSTO is not for nothing!

 

 

Any SSTO will always be outperformed by a TSTO.

Any fully reusable SSTO will always be outperformed by a fully reusable TSTO.

The margins of a TSTO are just bigger, so it has to use fewer compromises, doesn't have to shave as many grams, and so it will just be more robust and carry larger payloads than an equivalent SSTO.

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20 minutes ago, RCgothic said:

Any fully reusable SSTO will always be outperformed by a fully reusable TSTO

Okay, that rings true to me and may well be. However coming to SABRE(one or more stages), is there something to gain from aerodynamic lift combined with (vacuum)thrust as opposed to just thrust all the way. For sure the later is simpler ..

I see that I missed the target with the 'ssto' tag, but I still want to figure if hypersonic engines are viable :)

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2 hours ago, splashboom said:

Okay, that rings true to me and may well be. However coming to SABRE(one or more stages), is there something to gain from aerodynamic lift combined with (vacuum)thrust as opposed to just thrust all the way. For sure the later is simpler ..

I see that I missed the target with the 'ssto' tag, but I still want to figure if hypersonic engines are viable :)

Maybe a hypersonic version of Virgin Orbit's launcher might make sense for launching an upper stage.

However. Hypersonic air breathing is an extremely exotic regime. I suspect any hypersonic air breathing engine and airframe is always going to be much more stressed than a conventional rocket which does most of its work outside the atmosphere.

So what does a spaceplane first stage get you as opposed to a conventional first stage? You trade additional wear and stress, a dual axis loading of the payload as gravity's not in the same direction as thrust, more expensive engines, higher dry mass, and a lower takeoff and landing gross weight limit, all of that for reduced fuel consumption and lower required thrust.

With cheap rocket propellant and a very reusable conventional TSTO I suspect the trade offs are not really going to be particularly worth it.

With an extremely capable spaceplane launcher, if the drawbacks and wear become negligible, then perhaps it might be more worth it.

Perhaps Earth isn't the right planet. Somewhere with a denser, higher atmosphere and lower gravity. Maybe somewhere like Titan might be ideal. Except that would be more fuel-breathing than oxidiser breathing.

 

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24 minutes ago, RCgothic said:

Hypersonic air breathing is an extremely exotic regime.

Well I guess that's that - a lot of times in engineering its keep it simple or fail. As you point out, the hypersonic air breathing - exotic indeed - poses fundamental challenges. Go Elon!

Guess I will still finalize my KSP SSTO, so I may launch with 1/10 of the cost, and 10 times more piloting.

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One thing I've recalled about TSTO though - it only works if your industrial base and recovery infrastructure are on planet. If you're coming down from orbit and then need to return to orbit without servicing, SSTO becomes extremely desirable. It's not really workable with TSTO because you have to leave something behind and that makes it not fully reusable.

Landing sites for a nearly-fully laden spaceplane on unprepared surfaces would be pretty limited though. A conventional VTVL SSTO would have a lot fewer difficulties.

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On 11/25/2021 at 8:15 PM, RCgothic said:

Any SSTO will always be outperformed by a TSTO.

Any fully reusable SSTO will always be outperformed by a fully reusable TSTO.

Assuming similar tech/engines.

The question here is if the advantages of air breathing on a SSTO can outperform a non-airbreathing chemical TSTO.

Of course as I do in 3x KSP, an air breathing first stage of a TSTO is going to allow better performance than an air-breathing SSTO

On 11/26/2021 at 12:33 AM, RCgothic said:

One thing I've recalled about TSTO though - it only works if your industrial base and recovery infrastructure are on planet.

I don't know if you would consider ISRU mining as such, but reusable TSTO is workable in KSP without having to use the recovery function and launch new vessels via the SPH/VAB

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43 minutes ago, KerikBalm said:

I don't know if you would consider ISRU mining as such, but reusable TSTO is workable in KSP without having to use the recovery function and launch new vessels via the SPH/VAB

I'm saying Two Stage To Surface And Back (TSTAB) isn't desirable for orbital shuttles where the mothership is exploring unknown planets.

Despite SSTSAB having much lower payload than the two stage solution, it's preferable to get all the pieces of your infrastructure back. They may not be replaceable.

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On 11/25/2021 at 5:20 PM, RCgothic said:

Maybe a hypersonic version of Virgin Orbit's launcher might make sense for launching an upper stage.

However. Hypersonic air breathing is an extremely exotic regime. I suspect any hypersonic air breathing engine and airframe is always going to be much more stressed than a conventional rocket which does most of its work outside the atmosphere.

So what does a spaceplane first stage get you as opposed to a conventional first stage? You trade additional wear and stress, a dual axis loading of the payload as gravity's not in the same direction as thrust, more expensive engines, higher dry mass, and a lower takeoff and landing gross weight limit, all of that for reduced fuel consumption and lower required thrust.

With cheap rocket propellant and a very reusable conventional TSTO I suspect the trade offs are not really going to be particularly worth it.

With an extremely capable spaceplane launcher, if the drawbacks and wear become negligible, then perhaps it might be more worth it.

Perhaps Earth isn't the right planet. Somewhere with a denser, higher atmosphere and lower gravity. Maybe somewhere like Titan might be ideal. Except that would be more fuel-breathing than oxidiser breathing.

 

Virgin Orbit (and Orbital Pegasus) have the two key advantages you'd want on a SSTO:  low atmospheric pressure at launch (so less range to adjust output nozzles) and better scaling.  The scaling issue would also bite them, as any SSTO requires so much more mass than a TSTO.

The elephant in the room is that for Earth's delta-v requirements, finding any means of recovering an entire vehicle from orbit is next to impossible.  Just getting there requires high Isp and massive rockets.  A SABRE might work, as it presumably only has to add the air engine and fuel (not oxidizer) from the airbreathing regime.  Note that it is entirely possible such a craft might burn kerosene for 0-mach 4 or something, just to use a vastly smaller fuel tank (than a huge, if low mass, hydrogen tank).

I'd also like to remind everyone of the X-43 program.  Hit mach 6.4 during a test flight, not just a powerpoint presentation.  Managed to sustain mach 9.4 (although just barely, and the error bars definitely included purely negative acceleration).  I think the Air Force has it now, and is either canceled and/or classified.

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On 11/28/2021 at 4:40 PM, RCgothic said:

I'm saying Two Stage To Surface And Back (TSTAB) isn't desirable for orbital shuttles where the mothership is exploring unknown planets.

Despite SSTSAB having much lower payload than the two stage solution, it's preferable to get all the pieces of your infrastructure back. They may not be replaceable.

This is pretty obvious unless you discard the landing stage. SSTO here is get to orbit and back unless you can refuel at an base 

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On 11/28/2021 at 3:49 PM, KerikBalm said:

Assuming similar tech/engines.

The question here is if the advantages of air breathing on a SSTO can outperform a non-airbreathing chemical TSTO.

Of course as I do in 3x KSP, an air breathing first stage of a TSTO is going to allow better performance than an air-breathing SSTO

You could also use air breathing on an TSTO. The problem with an +mach 6 air-breathing engine is more of an issue.
It has been an series of space plane designs getting a bit past mach 3 using SR-71 style engines then switch to rocket engines to get to mach 6 and out of the atmosphere to drop the second stage. 

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On 11/28/2021 at 4:40 PM, RCgothic said:

I'm saying Two Stage To Surface And Back (TSTAB) isn't desirable for orbital shuttles where the mothership is exploring unknown planets.

Despite SSTSAB having much lower payload than the two stage solution, it's preferable to get all the pieces of your infrastructure back. They may not be replaceable.

Well, in that case, there is a third possible mission architecture:

A booster stage with ISRU that can make orbit with basically zero payload. It goes up carrying a 2nd stage as a payload. 2nd stage gets to orbit and meets with the mothership, transferring resources, crew, whatever. First stage falls back down, refuels itself, and then comes up to rendezvous with the mothership nearly empty.

This can make sense if SSTO margins are super thin, and using a 2nd stage gets you the payload that you need.

You can repeat your 2 stage flights as needed until the mothership's tanks are filled, and at the end, you don't deorbit the 2nd stage to link up with the lifter, instead the lifter comes up on its own, freed of the burden of a payload.

On 11/29/2021 at 8:01 PM, wumpus said:

A SABRE might work, as it presumably only has to add the air engine and fuel (not oxidizer) from the airbreathing regime.  Note that it is entirely possible such a craft might burn kerosene for 0-mach 4 or something, just to use a vastly smaller fuel tank (than a huge, if low mass, hydrogen tank).

Kerosene for 0-mach 4 wouldn't be a SABRE, kerosene wouldn't be suitable for use in a precooler, and making a sabre that can switch fuel types may be impractical given the massive difference in pipe diameter and temperature to deliver suitable H2/kerosene mass/sec

17 hours ago, magnemoe said:

You could also use air breathing on an TSTO. 

Like I said in the part you quoted:

"Of course as I do in 3x KSP, an air breathing first stage of a TSTO is going to allow better performance than an air-breathing SSTO"

But that ceases to be a Starship vs Skylon comparison, as the first post asked about.

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6 minutes ago, KerikBalm said:

Kerosene for 0-mach 4 wouldn't be a SABRE, kerosene wouldn't be suitable for use in a precooler, and making a sabre that can switch fuel types may be impractical given the massive difference in pipe diameter and temperature to deliver suitable H2/kerosene mass/sec

I didn't know they were more specific about the precooler (it is said to be the key to the SABRE).  I wonder if some air-breathing spaceship might have simple ramjets for machs 1-4ish, possibly recoverable boosters.  Or maybe stick the whole thing on some sort of catapult rail.

9 minutes ago, KerikBalm said:

But that ceases to be a Starship vs Skylon comparison, as the first post asked about.

Has there ever been any real work on a Skylon (outside the SABRE)?  It always seemed like "an example use of SABRE", and not really their line of business.  Presumably they'd work on that once SABRE was ready to move into production, but certainly expect anything to change (and I'm sure they are hardly certain of SSTO, that just looks really impressive when a SABRE can roughly make it happen).

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