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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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On 11/25/2021 at 6:33 PM, RCgothic said:

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.

I'll also point out that an airbreathing SSTO framework makes more sense in comparison to launch architectures like Soyuz, Delta IV, Atlas V, and Ariane 5, but less sense in comparison to architectures like New Glenn, Proton, Electron, and Falcon 9. With the former launch architectures, you have (relatively) briefly-burning boosters which are used to accelerate a large, high-efficiency core stage which provides the bulk of the dV for orbit, lofting a small, low-thrust upper stage which provides only a small amount of dV for circularization and whatever else is needed for BLEO. Recovering the core stage would be almost as challenging as recovering it from orbit altogether because of its high energy.

Since you have a very large, very high-energy core stage which fires all the way from the surface and gets within spitting distance of orbit, it sort of makes sense to imagine replacing the boosters with horizontal takeoff and jet engines and then just accepting the payload percentage loss. Then you have the challenge of recovering it, which is quite difficult but not significantly harder than if you had tried to recover the core stage from the original architecture. There are even some advantages, like being able to coast around the world before re-entry so you can land at your launch site instead of having to create a far-downrange recovery infrastructure and then try to figure out how to get back to your launch site.

But in comparison to a "balanced" TSTO like New Glenn, Electron, and Falcon 9 (or a balanced 3STO like Proton), an airbreathing SSTO makes much much less sense. These vehicles allocate the dV nearly equally among their stages, so there's no single high-energy core stage which you can imagine converting into an SSTO. Plus, recovering the first stage is much easier because it has lower energy and so it can either be recovered a short distance downrange or boostback to the launch site.

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  • 3 weeks later...

I don’t see Skylon (the spaceplane using SABRE) as an SSTO, but rather as the first stage in a two-stage launch system with a small second stage (reusable or not) to reach orbit. In that role it would be competitive with any VTVL two stage reusable rocket as the combination of using air-breathing engines and atmospheric lift saves a lot of fuel over a conventional rocket using brute force to counter gravity.

A spaceplane has no real need for a TWR over 1 so the engines can be lighter, using air as oxidiser means you need to carry less oxidiser so the fuel tanks are lighter and the inherent cross-range ability you get with an aircraft would offer greater flexibility- just think of all those backup Shuttle runways that would be ideal as either launch sites or landing sites with refuelling infrastructure. (Of course, setting up the infrastructure to refuel a plane that burns liquid hydrogen as fuel wouldn’t be a trivial task.)

 

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27 minutes ago, jimmymcgoochie said:

I don’t see Skylon (the spaceplane using SABRE) as an SSTO, but rather as the first stage in a two-stage launch system with a small second stage (reusable or not) to reach orbit. In that role it would be competitive with any VTVL two stage reusable rocket as the combination of using air-breathing engines and atmospheric lift saves a lot of fuel over a conventional rocket using brute force to counter gravity.

A spaceplane has no real need for a TWR over 1 so the engines can be lighter, using air as oxidiser means you need to carry less oxidiser so the fuel tanks are lighter and the inherent cross-range ability you get with an aircraft would offer greater flexibility- just think of all those backup Shuttle runways that would be ideal as either launch sites or landing sites with refuelling infrastructure. (Of course, setting up the infrastructure to refuel a plane that burns liquid hydrogen as fuel wouldn’t be a trivial task.)

 

Agreeing here, now the idea is not new, benefit of sabre is that it let you get up to an so high velocity you can release your payload in space, this let you avoid the problem with supersonic seperation and no need for an fairing, an mach 6 plane is much much easier to make than an SSTO
Make me wonder could sabre run on other fuels like methane or RP1 for cruise mode, that is going to launch site and returning after dropping payload? 

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20 minutes ago, magnemoe said:

Agreeing here, now the idea is not new, benefit of sabre is that it let you get up to an so high velocity you can release your payload in space, this let you avoid the problem with supersonic seperation and no need for an fairing, an mach 6 plane is much much easier to make than an SSTO
Make me wonder could sabre run on other fuels like methane or RP1 for cruise mode, that is going to launch site and returning after dropping payload? 

Edit- turns out I was wrong, SABRE doesn’t liquefy the incoming air before burning it, however the precooler’s helium cooling system dumps heat into the hydrogen fuel in air-breathing mode so other fuels wouldn’t really work for that. Hydrogen is also very light and has a higher specific impulse. It also turns out that Skylon is intended to be a full SSTO, not suborbital; would a suborbital TSTO version be better than the full SSTO?

Edited by jimmymcgoochie
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18 minutes ago, jimmymcgoochie said:

Edit- turns out I was wrong, SABRE doesn’t liquefy the incoming air before burning it, however the precooler’s helium cooling system dumps heat into the hydrogen fuel in air-breathing mode so other fuels wouldn’t really work for that. Hydrogen is also very light and has a higher specific impulse. It also turns out that Skylon is intended to be a full SSTO, not suborbital; would a suborbital TSTO version be better than the full SSTO?

As I understand an major problem with hypersonic flight is that the air in the engine get very hot then compressed, using the hydrogen to cool the air solves a lot of this. 
Then flying slower and more economical this cooling is not needed. 

Problem with Skylon is that it would be much more expensive and risky to design and build than an simpler mach 6 plane who still is an challenge.
 

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On 12/16/2021 at 7:04 AM, jimmymcgoochie said:

I don’t see Skylon (the spaceplane using SABRE) as an SSTO, but rather as the first stage in a two-stage launch system with a small second stage (reusable or not) to reach orbit. In that role it would be competitive with any VTVL two stage reusable rocket as the combination of using air-breathing engines and atmospheric lift saves a lot of fuel over a conventional rocket using brute force to counter gravity.

A spaceplane has no real need for a TWR over 1 so the engines can be lighter, using air as oxidiser means you need to carry less oxidiser so the fuel tanks are lighter and the inherent cross-range ability you get with an aircraft would offer greater flexibility- just think of all those backup Shuttle runways that would be ideal as either launch sites or landing sites with refuelling infrastructure. (Of course, setting up the infrastructure to refuel a plane that burns liquid hydrogen as fuel wouldn’t be a trivial task.)

A good comparison would be between Skylon as a suborbital first stage and something like RocketLab's new Neutron design.

One difference between the SABRE engine and other airbreathing engines, like the SR-71's Pratt & Whitney J58 (a combined-cycle turbojet and afterburning ramjet) and the engines on the X-43, X-51, and HAWB (scramjets) is that SABRE is not an inlet-based jet engine at all. It is, fundamentally, a rocket engine. In airbreathing mode, it uses a separate compressor and precooler to inject supercooled air into the rocket engine chamber at 141 bar. This is very different from a turbojet, where the pressure in the combustion chamber is less than 25 bar for jet fighter engines and no greater than 40 bar for the very highest-performing engines like the J58. Scramjet engines have even lower combustion pressures, since high compression ratios slow down the airflow due to Bernoulli's principle, and scramjet engines need supersonic combustion in order to function.

Because combustion pressure (to a first order) scales with T/W ratio, this means the SABRE produces much more thrust for its weight than something like a J58 or an F119, and vastly more thrust than a scramjet engine. It also means that operating in pure rocket mode is straightforward; simply close the inlet, stop injecting supercooled air, and start injecting liquid oxygen instead.

However, because it is a rocket engine, it produces a rocket plume. And because it is a rocket engine designed to operate at sea level, it is significantly underexpanded at altitude. This is a problem. A 2015 NASA study found that operating the Skylon design in pure rocket mode would result in what the study termed an "adverse thermal environment" for the back end of the vehicle:

skylon-problems.png

As you might imagine, "adverse thermal environment" is a clever euphemism for "barbecue" as it relates to the back end of the plane. 

So if you were going to compare Skylon to Neutron, you should limit it to its maximum airbreathing velocity of around Mach 5.4. This makes the design a lot simpler, because it doesn't have to carry any liquid oxygen at all. It takes off horizontally, accelerates at a moderate ascent angle, and then pitches up during the final acceleration phase in order to achieve a suborbital trajectory.

Let us suppose that once outside of most of the atmosphere, it opens its payload bay and releases the same upper stage that Neutron uses.

Thus, the question becomes: what's more cost-efficient? Should we fly that expendable upper stage with Neutron or should we fly it with Skyon?

I suspect (although I do not know for sure) that Neutron, if it works as advertised, is going to be substantially more cost-efficient than Skylon would.

On 12/16/2021 at 7:56 AM, jimmymcgoochie said:
On 12/16/2021 at 7:44 AM, magnemoe said:

Agreeing here, now the idea is not new, benefit of sabre is that it let you get up to an so high velocity you can release your payload in space, this let you avoid the problem with supersonic seperation and no need for an fairing, an mach 6 plane is much much easier to make than an SSTO
Make me wonder could sabre run on other fuels like methane or RP1 for cruise mode, that is going to launch site and returning after dropping payload? 

[T]he precooler’s helium cooling system dumps heat into the hydrogen fuel in air-breathing mode so other fuels wouldn’t really work for that. Hydrogen is also very light and has a higher specific impulse. 

Don't write methane off completely. Hydrogen has a much better heat capacity than methane, but methane is MUCH more dense. An airbreathing methane-based SABRE engine isn't entirely impossible, especially if the use of hydrogen is unnecessary since you never need that high specific impulse to push into orbit. Plus, liquid methane is more easily stored in wings.

On 12/16/2021 at 8:34 AM, magnemoe said:

As I understand an major problem with hypersonic flight is that the air in the engine get very hot then compressed, using the hydrogen to cool the air solves a lot of this. 
Then flying slower and more economical this cooling is not needed. 

If you're going to be getting to any sort of useful suborbital velocities, you will need the full cooling capacity of SABRE.

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On 12/16/2021 at 4:04 AM, jimmymcgoochie said:

I don’t see Skylon (the spaceplane using SABRE) as an SSTO, but rather as the first stage in a two-stage launch system with a small second stage (reusable or not) to reach orbit.

To be blunt, I don't see Skylon as anything other than a powerpoint deck designed to elicit funding.

As best I can tell, most the major interest that Reaction Engines has gotten is from companies trying to buy access to their heat exchanger technology. If you could stick a heat exchanger into the core flow of a jet engine without too much pressure loss, it would open up some recuperative cycle possibilities.

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