What do we know about SABRE / Skylon ?

[AeroGav]

There's quite a bit more info to be found on academic papers on the internet than is currently to be found on Reaction Engine's website.    The problem is, a lot of it goes over my head 

To keep things simple,  the SABRE engine  is like a jet engine, except it uses the  supercold hydrogen fuel to chill the incoming air before feeding it to the compressor.   This keeps the compressor light and compact and allows it to work to higher mach numbers than conventional jet engines.    The combustion chamber and nozzle look a lot more like that of a rocket engine than a jet, but ultimately have the same function.

This engine can run a little over mach 5,  a significant improvement over the pratt and whitney j-58's "a little bit" over mach 3.    What's the limiting factor? Is intake drag reducing net thrust to the point it'd be more efficient to go "closed cycle"?   Or is it the old enemy, heat?  

In conventional jets,  the incoming air stream gets compressed in the intake as it gets accelerated to the speed of the aircraft.   This happens before it meets the turbine's compressor stage, and since gases get hotter when compressed, there is a temperature limit.  You can make the compressor out of the same heat resistant materials as the turbine, but that adds a lot of weight (over aluminium alloys that would otherwise be used).  Also , since the air is hotter before fuel gets burned, it will be hotter still after combustion, and you could then run into turbine limits , which are already using the most heat resistant materials available.   The only options then are to reduce fuel added in the main combustor and burn more in the afterburner, or lower the engine compression ratio, both of which hurt performance and fuel consumption.      An SSTO engine needs high TWR or it's going to add too much dry mass to the orbiter.

Would it be correct to say that in the intake, as the air is sped up to the speed of the airplane, kinetic energy is being converted to compression and heat gain in the air.   Theoretically, that energy gets returned back to kinetic as the gas flow expands through the turbine stages and back out the nozzle.  But in practice there will be losses, which set the limits for air breathing propulsion of any kind.   Apparently,  at mach 8, the kinetic energy of the intake air is equal to the chemical energy that can be released by burning fuel in it to use up all the available oxygen of that intake air charge.    So unless your ram compression/expansion is perfectly efficient, you can see how hard it is even for scramjets to make max thrust.

Speaking of Ramjets, the SABRE has "bypass ramjets" surrounding the four combustors/bell nozzles.    Apparently, more liquid hydrogen is used needed to cool the incoming air , than the main engine can use.   So the excess hydrogen is burned in the bypass ducts.  They are sometimes known as "bypass ramjets" but apparently the goal was "negative drag" not thrust.       Even so, do they provide more power at higher speeds?

I'd be interested to know how the power characteristics of the proposed engine would look.   Certainly the Kerbal RAPIER has a mad 8x thrust multiplier at high speed.  The only info i could find on the SABRE, stated that  the core mass flow rate was pretty constant over the Skylon's flight profile,  varying only by a factor of 1.6 or so.     However it's possible these bypass ducts change the overall picture.

Even so, it cannot possibly replicate the crazy power rush of the RAPIER, since apparently the Skylon would need 2 hours to compete the air breathing phase of flight, reaching mach 5.2 or so at 22km.   One other little fact slipped out during a discussion about nozzle design - apparently combustion chamber pressure doubles when switching to close cycle.    I don't know enough about rocket engines, but i suspect power increases at least that much - sounds like a real kick in the pants...

 

[regex]

Quote

What do we know about SABRE / Skylon ?

It doesn't exist.

[Nuke]

its the heat, the precooler tricks the engine into thinking its operating in a cryogenic atmosphere, so it drastically raises the performance ceiling. the rest of the engine is some serious thermodynamic mastery. nothing is wasted, hence the bypass ramjets, which help to reduce intake drag. their performance likely goes up with speed but then again so does drag. none of this matters above mach 5 as thats where the intakes close and the engine goes into rocket mode. if you could improve performance of the engines to get it to mach 6 or 7 that means less time in rocket mode and less oxidizer to carry. a second gen design might use bypass scramjets or something instead, but again drag will also go up. i dont know what the max theoretical airspeed for an air breathing engine is, but theres probibly a long way to go still before a hard limit is reached. the doubling of chamber pressure is probibly due to more efficient burning of lh2/lox, since its not full of nitrogen to rob heat from the exhaust products.

Edited February 27, 2017 by Nuke

[ImmaStegosaurus!]

1 hour ago, regex said:

It doesn't exist.

Yet. Blackbird flew first time over 50 years ago. It's about time we break it's records.

[regex]

4 minutes ago, ImmaStegosaurus! said:

Yet. Blackbird flew first time over 50 years ago. It's about time we break it's records.

I look forward to an engine firing. Until then, just like the Falcon Heavy, it's a unicorn.

E: Well, that's unfair to the Falcon Heavy which actually has engines that work.

Edited February 28, 2017 by regex

[ImmaStegosaurus!]

3 minutes ago, regex said:

I look forward to an engine firing. Until then, just like the Falcon Heavy, it's a unicorn.

Falcon Heavy has it's components already tested. It's all about strapping them together and launching the thing.

[kerbiloid]

10 hours ago, AeroGav said:

To keep things simple,  the SABRE engine  is like a jet engine, except it uses the  supercold hydrogen fuel to chill the incoming air before feeding it to the compressor.   This keeps the compressor light and compact and allows it to work to higher mach numbers than conventional jet engines.    The combustion chamber and nozzle look a lot more like that of a rocket engine than a jet, but ultimately have the same function.

This is an accurate description of several dozens skyplane projects from 50s-60s being actively developed. _{(Can't list them right know without a book at my place).}
All of them were realistic, were being developed by experienced, respectable companies, and... And still nothing.

(A feature from the past I like very much: the spaceplane starts with no oxygen in tanks and condenses it from air while accelerating. So, its flight mass is greater than start mass.)

5 hours ago, ImmaStegosaurus! said:

Blackbird flew first time over 50 years ago

And that's what makes to think that things are not so simple. As 50 years after SR-71, presumed SR-91 is still secret.

[Streetwind]

@regex kind of has a point here, at least in regards to Skylon. You can give the SABRE engines the benefit of doubt, as components of them have been tested and all involved science has been vetted by third parties; all that's left now is engineering, basically. A whole damn lot of it, but it's not an insurmountable hurdle.

But Skylon? That's not a thing, and probably won't ever be a thing.

Look at the company name. "Reaction Engines Ltd." <--- this is an engine manufacturer. And if they had any other name, they would still be an engine manufacturer (it just would be less obvious). They build, or plan to build, jet engines for a living. It so happens that the SABRE engine they're proposing has completely unique capabilities, and it always helps the public and customers alike to visualize what that means when there's an example implementation. So they designed Skylon: a paper study that uses the SABRE engines, as an example.

Once SABRE is up and running, they'll find customers. The US military is already interested, as is ESA, and multiple British organizations. They will then manufacture engines and sell them to these customers, like the engine manufacturer they are. And most likely, the company will be quite happy with that, and spend several years doing nothing else in order to recoup initial investments made into R&D of SABRE.

And then, one day when their funds are well in the green, or they get bored, or if nobody wants to buy SABREs after all... maybe then they'll return to that old paper study they once did, to see if it makes sense for them to try. But they probably won't.

 

Mind you: that's not to say we won't ever see a SABRE-equipped orbital spaceplane. I just don't think it will be named Skylon, I don't think it will be developed by Reaction Engines themselves, and I don't think we'll see a prototype spooling up its turbines on a runway before 2030.

Edited February 28, 2017 by Streetwind

[magnemoe]

6 hours ago, kerbiloid said:

This is an accurate description of several dozens skyplane projects from 50s-60s being actively developed. _{(Can't list them right know without a book at my place).}
All of them were realistic, were being developed by experienced, respectable companies, and... And still nothing.

(A feature from the past I like very much: the spaceplane starts with no oxygen in tanks and condenses it from air while accelerating. So, its flight mass is greater than start mass.)

And that's what makes to think that things are not so simple. As 50 years after SR-71, presumed SR-91 is still secret.

Military stopped being interested in high speed planes as ground to air missiles could intercept planes going well over mach 3 and they was easy to pick up on radar. 
Solution was to go low and later stealth. 

In later years it has been an interest in fast air breathing missiles. 

[kerbiloid]

9 minutes ago, magnemoe said:

to go low

Not good for recon plane.

[p1t1o]

8 hours ago, kerbiloid said:

SR-91

Nonono. We dont need 1980's conspiracy theories, we have our own

(You cant *hide* hypersonic aircraft, their skin will be hundreds of degrees C in places and you can even detect them acoustically)

 

1 hour ago, magnemoe said:

Military stopped being interested in high speed planes as ground to air missiles could intercept planes going well over mach 3 and they was easy to pick up on radar. 
Solution was to go low and later stealth. 

In later years it has been an interest in fast air breathing missiles. 

SAMs were the reason, but the replacement was recon satellites, low level was for armed penetration.

Nowadays with hypersonics on the horizon ("horizon" - the proposed "SR-72" still does not have an engine) the idea of putting eyes over any location on the globe within 24 hours - something not always possible with satellites - is attracting attention again. Although if it actually comes to pass and isnt just an aerospace firm keeping up appearances is anybody's guess (The SR-72 for example, is a conceptual aircraft only at this point and has been thought up off Lockheed's own back. The US air force has "expressed interest" but they didn't ask for it or provide an operational requirement).

High-speed missiles are, of course, all the rage.

 

 

[Kryten]

1 hour ago, kerbiloid said:

Not good for recon plane.

We've got sats for the high altitude role now.

[kerbiloid]

1 minute ago, Kryten said:

We've got sats for the high altitude role now.

Sats cannot move.

[Codraroll]

1 hour ago, kerbiloid said:

Sats cannot move.

To a certain degree, they can. But that's not the problem. Satellites cannot loiter. Planes like the SR-71 are fine for taking a snapshot of an area, and possibly see what the enemy is up to at that one, exact moment in time, but if you want any more than a still image, you have to use something that can stick around for a while. A satellite can give you a few minutes of continuous coverage from an area, but a drone like the RQ-170 can linger for days, under the right circumstances.

[DerekL1963]

2 hours ago, p1t1o said:

Nowadays with hypersonics on the horizon ("horizon" - the proposed "SR-72" still does not have an engine) the idea of putting eyes over any location on the globe within 24 hours - something not always possible with satellites - is attracting attention again.

That's as much about streamlining the process of disseminating [tactical] intelligence information (I.E. the USAF wants assets that it controls rather than relying on three letter agencies or DoD bureaucracy) as anything else.  (So does the USN, the USMC, and the Army - but they're looking to smaller subsonic drones as they lack the business case for expensive hypersonic drones.)  The DoD itself is taking a different tack, their goal for about the last twenty years has been quick reaction launches sending a bird (or birds) to orbits chosen to concentrate on a given area.

Though really, as the SR-72 emerged from an "unsolicited" proposal, and thus smells less like any kind of widespread interest and more like pork or a pet project to me.  (With the caveat that it's often hard to discern what's going on from the outside...)

[Nuke]

10 hours ago, Streetwind said:

@regex kind of has a point here, at least in regards to Skylon. You can give the SABRE engines the benefit of doubt, as components of them have been tested and all involved science has been vetted by third parties; all that's left now is engineering, basically. A whole damn lot of it, but it's not an insurmountable hurdle.

But Skylon? That's not a thing, and probably won't ever be a thing.

Look at the compnay name. "Reaction Engines Ltd." <--- this is an engine manufacturer. And if they had any other name, they would still be an engine manufacturer (it just would be less obvious). They build, or plan to build, jet engines for a livng. It so happens that the SABRE engine they're proposing has completely unique capabilities, and it always helps the public and customers alike to visualize what that means when there's an example implementation. So they designed Skylon: a paper study that uses the SARE engines, as an example.

Once SABRE is up and running, they'll find customers. The US military is already interested, as is ESA, and multiple British organizations. They will then manufacture engines and sell them to these customers, like the engine manufacturer they are. And most likely, the company will be quite happy with that, and spend several years doing nothing else in order to recoup initial investments made into R&D of SABRE.

And then, one day when their funds are well in the green, or they get bored, or if nobody wants to buy SABREs after all... maybe then they'll return to that old paper study they once did, to see if it makes sense for them to try. But they probably won't.

 

Mind you: that's not to say we won't ever see a SABRE-equipped orbital spaceplane. I just don't think it will be named Skylon, I don't think it will be developed by Reaction Engines themselves, and I don't think we'll see a prototype spooling up its turbines on a runway before 2030.

i dont see rel making an airframe at all. they dont have the facilities for it. especially not an airframe like skylon, which is down right huge. the initial testbed maybe. if they were smart they would contract with someone like scaled composites for the airframe. they are fairly good at building one off research aircraft, and have some space experience. virgin galactic might use the engines for the space tourism market, so it puts them in there with a potential customer.

[Steel]

2 minutes ago, Nuke said:

i dont see rel making an airframe at all. they dont have the facilities for it. especially not an airframe like skylon, which is down right huge. the initial testbed maybe. if they were smart they would contract with someone like scaled composites for the airframe. they are fairly good at building one off research aircraft, and have some space experience. virgin galactic might use the engines for the space tourism market, so it puts them in there with a potential customer.

I agree, REL will never make an airframe. Although I'm not sure scaled composites have any of the right experience for a job like that, they only have experience with large, subsonic aircraft and Spaceship One. They've not got any real experience with hypersonic flight, especially not at a scale that you could fit a SABRE to (they will be pretty enormous engines if they ever get made). Also I have a feeling that if SABRE is as effective as is claimed it will disappear of into the defense sector not to be seen for a few decades (BAE already had dibs on that avenue with the funding program it's got)

[Nuke]

On 2/28/2017 at 6:33 AM, kerbiloid said:

Sats cannot move.

no, but you can saturate the sky with them so they dont need to.

[Nibb31]

On 2/28/2017 at 8:21 PM, Steel said:

I agree, REL will never make an airframe. Although I'm not sure scaled composites have any of the right experience for a job like that.

Nobody does. The entire Skylon airframe is nothing like any modern aerospace design. It requires new tooling, new construction techniques, new sourcing, and fundamentally different skillsets to those actually employed in the industry. The industrialization effort would be similar to Boeing having to completely reinvent how to build a composite 787, including going through all the pitfalls. For Boeing, it was worth it, because they plan to sell thousands of 787s in the future, and they'll probably extend those techniques to smaller jets too. For Skylon, it makes no sense at all. 

[Tyko]

Depends on who they plan on spying on too...Planes would have a hard time against a nation that has high-end SAMs. If they interested in spying on insurgents in a desert somewhere who have nothing bigger than a stinger missile, it's a different story.

[DerekL1963]

1 hour ago, Tyko said:

Depends on who they plan on spying on too...Planes would have a hard time against a nation that has high-end SAMs. If they interested in spying on insurgents in a desert somewhere who have nothing bigger than a stinger missile, it's a different story.

If they've nothing better than a Stinger, you don't need a hypersonic aircraft either...  Any one of a number of drones currently in service will more than suffice.   That's the big problem with manned recce aircraft in general nowadays, increasingly they're solutions in search of a problem.

REL has been shopping around air breathing SABRE derived engines for everything from hypersonic cruise missiles to hypersonic passenger aircraft for some years now, no takers.

[Tyko]

2 minutes ago, DerekL1963 said:

If they've nothing better than a Stinger, you don't need a hypersonic aircraft either...  Any one of a number of drones currently in service will more than suffice.   That's the big problem with manned recce aircraft in general nowadays, increasingly they're solutions in search of a problem.

REL has been shopping around air breathing SABRE derived engines for everything from hypersonic cruise missiles to hypersonic passenger aircraft for some years now, no takers.

I was thinking in terms of getting very rapidly to the area in question. being able to eyes on an area rapidly could be very valuable

[Steel]

2 minutes ago, DerekL1963 said:

If they've nothing better than a Stinger, you don't need a hypersonic aircraft either...  Any one of a number of drones currently in service will more than suffice.   That's the big problem with manned recce aircraft in general nowadays, increasingly they're solutions in search of a problem.

REL has been shopping around air breathing SABRE derived engines for everything from hypersonic cruise missiles to hypersonic passenger aircraft for some years now, no takers.

I'd argue that nobody has bitten because they haven't actually got an engine to sell yet. The engine would be hugely valuable so long as they can prove it works.

Alternatively it's already working in some form and BAE have got it classified so we won't hear any updates about it for decades. 

[DerekL1963]

2 minutes ago, Tyko said:

I was thinking in terms of getting very rapidly to the area in question. being able to eyes on an area rapidly could be very valuable

Rats in the desert with nothing more than a Stinger almost certainly don't pose sufficient threat that you have to get eyes on them that fast.  Even at hypersonic speeds, it still takes considerable time to get from their likely bases in the US or Europe, so there's still going to be significant delay.  Better to use a forward deployed drone.

Speed isn't always the solution.
 

11 minutes ago, Steel said:

I'd argue that nobody has bitten because they haven't actually got an engine to sell yet. The engine would be hugely valuable so long as they can prove it works.

That's certainly a valid argument...  But I'd phrase it slightly differently, they need an engine that works - and that can be operated economically.   The one they're proposing for passenger aircraft uses LH2, which is almost certainly a non-starter.  If I were them though, I'd be looking at the luxury bizjet market...  It's not a big market, but the folks buying in that range aren't as obsessively focused on the bottom as the airlines are.

[Nuke]

10 hours ago, Nibb31 said:

Nobody does. The entire Skylon airframe is nothing like any modern aerospace design. It requires new tooling, new construction techniques, new sourcing, and fundamentally different skillsets to those actually employed in the industry. The industrialization effort would be similar to Boeing having to completely reinvent how to build a composite 787, including going through all the pitfalls. For Boeing, it was worth it, because they plan to sell thousands of 787s in the future, and they'll probably extend those techniques to smaller jets too. For Skylon, it makes no sense at all. 

skylon takes it a step beyond that because to my knowledge nobody has ever skinned an aircraft in ceramics (unless you count the space shuttle tiles). i dont even know how you would go about that, having to fire large sections and then have them fit on the composite spaceframe. thats some serious stuff there. you can get ceramic parts to exacting specifications and im sure the tolerances are good enough for an aircraft.

scaled does have experience with composite air frames though, experience that goes back pretty far. so even if they dont put the skin on the plane they could handle all the structural stuff. i can imagine them building the space frame and shipping it to a location in the uk where the internals can be fitted and the ceramic skin installed. the skin itself would likely come from a ceramics manufacturer that has never delt with aircraft parts before. this isnt an unfamiliar practice in the aircraft manufacturing world, as you have factories all over the place delivering sub assemblies to the main plant that installs them in the aircraft. the skin and the engines are really the only things we haven't done before.

Edited March 3, 2017 by Nuke