The Reaction Engines Skylon Master Thread (1)

[flymetothemoon]

https://info.aiaa.org/tac/PEG/HSABPTC/Public%20Documents/Skylon%20Spaceplane.pdf

[deltaV]

The SABRE's helium goes through the following components in this circular sequence: cooling in heat exchanger with hydrogen (HX4), compressor (labeled He circulator), heating in heat exchanger with intake air (the precooler), heating in heat exchanger with preburner exhaust (HX3), turbine powering the oxidizer (air or LOX) compressor, and then back to HX4 again to start the cycle again. Why not add a second precooler as a final step after the turbine? The resulting higher He temperature wouldn't require much if any additional LH2 to cool if the LH2/He heat exchanger is counter-current and the flow rates are appropriately matched. This would potentially allow the LH2 consumption when airbreathing to be reduced closer to stoichiometric.

SABRE diagram: http://www.reactionengines.co.uk/images/sabre/library/sabre_cycle_m.jpg

[Robotbeat]

Are they going to be using slush hydrogen? It's got 16-20% higher density. Probably pretty expensive, though, which isn't terribly good for a SSTO RLV which wants to reduce cost (not too bad for an upper stage, though).

It may be something they're forced into if they run out of margin.

[john smith 19]

Are they going to be using slush hydrogen? It's got 16-20% higher density. Probably pretty expensive, though, which isn't terribly good for a SSTO RLV which wants to reduce cost (not too bad for an upper stage, though).

It may be something they're forced into if they run out of margin.

REL have always stated they would use sub-cooled propellants IE single phase. The drivers seems to be to run with zero boiloff and the complexity of vents, stand pipes, burners etc being permanently attached until launch (not something you see with other aircraft).

Having designed an engine to *eliminate* the phase change of incoming air seen in the LACE cycle it seems doubtful they'd re-insert the complexity of 2 phase flow handling *anywhere* in the system if they can avoid it.

Slush Hydrogen production methods made big improvements with the X33 programme (one of its more successful parts), especially using LH2 to regeneratively cool itself, eliminating Helium) so I'll guess sub-cooling is not viewed as a core technology challenge.

I've never understood the attraction of "slush" propellants. Deep pre-cooling allows smaller tanks *if* designed in from day 1 (which with the exception of the Kistler design it never seems to have been) and they are the bits you're going to build, whereas a retrofit means either shortening the tanks (It's not the mfg, it's the *certification* of the change that's going to be PITA) or uprating the engines (same issues plus questions of how much re-design is needed to increase the thrust).
Various LH2 turbo pump programmes hit snags when you get liquid/gas 2 phase flow, by accident. Designing in solid/liquid (and possibly a bit of gas if anything goes wrong) is just asking for trouble. I'll suggest sub-cooled gives the proverbial 80% of the benefits with 20% of the trouble.

Keep in mind that HTOL vehicles are *slightly* more forgiving than VTOL vehicles in this regard. The big one is if vehicle growth exceeded their landing gear design margin and they have to design a new one.

It also depends how much optimisation they design into the prototype. If you're already doing mixture ratio shifting (high O2/ low H2 burns off heavy LOX earlier in the flight at slightly lower Isp) that's one less trick you can use if the real vehicle does not quite measure up on the performance or weight front.

[Robotbeat]

What rockets have used subcooling of propellants, by the way?

[baldusi]

What rockets have used subcooling of propellants, by the way?

Energyia-Buran core stage used subcooled H2. It was more advanced than most western press gives them credit.

[john smith 19]

What rockets have used subcooling of propellants, by the way?

Kistler was designed to use it and think one of the russian launchers used sub cooled storables on at least one flight.

[cuddihy]

FWIW I just read a fiction book about a Skylon-type vehicle called "Perigee" ( http://www.amzn.com/B006PNL48I ) that fleshes out some of the issues you would see with a not-quite-orbital Skylon-type suborbital point-to-point plane. It never uses the word Skylon or SABRE but it seems to describe the same process, even to the point of talking about a Farnborough England company designing it.

I think the author's contention is that you would need conformal drop tanks for an actual orbital Skylon, although thats not the point of the book.

Anyway it got me interested in the concept although I'm not sure how accurate the contention is about suborbital transports.

*update* ok after reading a little more about Skylon I see that it does not have liquification of air (the planes in the book do), just compression and precooling. The book must have been based on a LACE-like predecessor.

Still a great story though.

[cuddihy]

I notice both of the articles from flightglobal linked earlier intimate that if the pre-cooler problem is solved, the rest is pretty simple. Meanwhile I think:
-aren't there anticipated injector issues in the rocket engines? You're going to have the same engine injecting compressed gaseous air at various different pressures as an oxidizer, and then later compressed liquid oxygen.

Different phases of oxidizer into the same compression chamber? likely to have some major injector development issues.
-if you use different injectors for air/LOX, you're going to have heat problems with whichever injector is shut down
-if you somehow work out using the same injector, the injector itself is going to be extremely tricky.

Just a thought.

[QuantumG]

Don't trust anything you read on Flight Global. If you saw the XCOR article the other day, and managed to catch the response from XCOR published over at Hobby Space, you shouldn't need to be told.

[Crispy]

A link or two would help!

[QuantumG]

http://hobbyspace.com/nucleus/?itemid=34962

[john smith 19]

I notice both of the articles from flightglobal linked earlier intimate that if the pre-cooler problem is solved, the rest is pretty simple. Meanwhile I think:
-aren't there anticipated injector issues in the rocket engines? You're going to have the same engine injecting compressed gaseous air at various different pressures as an oxidizer, and then later compressed liquid oxygen.

Different phases of oxidizer into the same compression chamber? likely to have some major injector development issues.
-if you use different injectors for air/LOX, you're going to have heat problems with whichever injector is shut down
-if you somehow work out using the same injector, the injector itself is going to be extremely tricky.

Just a thought.

It's actually worse than that. The chamber is air cooled during air breathing flight and switches to LOX cooling in rocket mode. 

LOX cooled combustion chambers were tested by both Rotary Rocket and NASA in the early 90s. NASA deliberately put leaks into the inside walls to see what a LOX leak into the chamber would do.

Nothing as it turned out. RR (Whose engine team form the core of Xcor Aerospace) also appeared to have no problems, although AFAIK they never published results. I'm sure Doug Jones knows but I doubt he'll say.

The REL chamber design work has been done as part of a joint project with DLR and EADS at Lampouldhousen. This tested both the air/LOX cooling and injector design. Another part of the jigsaw. See REL website for more details.

I suspect the fear of using LOX (and air) is greater than the actual problems arising. Historically it's *much* more likely to be the fuel that's changed rather than the oxidiser so an oxidizer cooled chamber should be a simpler re-design (potentially just changing the pump flow rate)

JPL demonstrated safe operation of an engine that ran from roughly 38 injectors down to 2 with no burning issues. I'd suggest recessing the injectors into the face makes quite a difference, although this work was done with storable propellants and an "impinging sheet" injector design I've seen nowhere else.

The J-2X (original 1960s) also ran down to IIRC 10%  with recessed injectors and maintaining full LH2 flow but shutting down the LOX flow.

 OTOH I'm not sure the *apparent* differences between LOX and air are that great since (AFAIK) they will both be super-critical fluids once pumped into the chamber. 

For fiction you might like to look at James Follets 1997 novel "Sabre" for comparison.

[Robotbeat]

I think either Masten or Armadillo also successfully ran one of their rockets with LOx cooling by accident without problems.

[93143]

I remember that.  It was a film-cooled engine as I recall...

[tnphysics]

I remember that.  It was a film-cooled engine as I recall...

Can anybody link to that story?

[tnphysics]

The SABRE's helium goes through the following components in this circular sequence: cooling in heat exchanger with hydrogen (HX4), compressor (labeled He circulator), heating in heat exchanger with intake air (the precooler), heating in heat exchanger with preburner exhaust (HX3), turbine powering the oxidizer (air or LOX) compressor, and then back to HX4 again to start the cycle again. Why not add a second precooler as a final step after the turbine? The resulting higher He temperature wouldn't require much if any additional LH2 to cool if the LH2/He heat exchanger is counter-current and the flow rates are appropriately matched. This would potentially allow the LH2 consumption when airbreathing to be reduced closer to stoichiometric.

SABRE diagram: http://www.reactionengines.co.uk/images/sabre/library/sabre_cycle_m.jpg

I had thought the same thing-could somebody explain?

One thought I had is that this heat exchanger would need to be at high temperature-so it would need to be heavier.

Also, could the RCS/OMS of the payload be used to insert a larger payload injected suborbitally?

[93143]

Can anybody link to that story?

http://www.armadilloaerospace.com/n.x/Armadillo/Home/News?news_id=369#EngineDevelopment

[HMXHMX]

I notice both of the articles from flightglobal linked earlier intimate that if the pre-cooler problem is solved, the rest is pretty simple. Meanwhile I think:
-aren't there anticipated injector issues in the rocket engines? You're going to have the same engine injecting compressed gaseous air at various different pressures as an oxidizer, and then later compressed liquid oxygen.

Different phases of oxidizer into the same compression chamber? likely to have some major injector development issues.
-if you use different injectors for air/LOX, you're going to have heat problems with whichever injector is shut down
-if you somehow work out using the same injector, the injector itself is going to be extremely tricky.

Just a thought.

It's actually worse than that. The chamber is air cooled during air breathing flight and switches to LOX cooling in rocket mode. 

LOX cooled combustion chambers were tested by both Rotary Rocket and NASA in the early 90s. NASA deliberately put leaks into the inside walls to see what a LOX leak into the chamber would do.

Nothing as it turned out. RR (Whose engine team form the core of Xcor Aerospace) also appeared to have no problems, although AFAIK they never published results. I'm sure Doug Jones knows but I doubt he'll say.

The REL chamber design work has been done as part of a joint project with DLR and EADS at Lampouldhousen. This tested both the air/LOX cooling and injector design. Another part of the jigsaw. See REL website for more details.

I suspect the fear of using LOX (and air) is greater than the actual problems arising. Historically it's *much* more likely to be the fuel that's changed rather than the oxidiser so an oxidizer cooled chamber should be a simpler re-design (potentially just changing the pump flow rate)

JPL demonstrated safe operation of an engine that ran from roughly 38 injectors down to 2 with no burning issues. I'd suggest recessing the injectors into the face makes quite a difference, although this work was done with storable propellants and an "impinging sheet" injector design I've seen nowhere else.

The J-2X (original 1960s) also ran down to IIRC 10%  with recessed injectors and maintaining full LH2 flow but shutting down the LOX flow.

 OTOH I'm not sure the *apparent* differences between LOX and air are that great since (AFAIK) they will both be super-critical fluids once pumped into the chamber. 

For fiction you might like to look at James Follets 1997 novel "Sabre" for comparison.

Yes, we ran with LOX cooling at Rotary and it worked just fine.  We went that route since we had LOX pressure to spare. On one of the chambers we had a crack that leaked LOX into the chamber, and there was no effect whatever (no combustion, discoloration, etc.)  It stands to reason, since the o/f would have gone LOX-rich at that point and been locally cooling, rather than increasing, wall temperature.

[john smith 19]

Yes, we ran with LOX cooling at Rotary and it worked just fine.  We went that route since we had LOX pressure to spare. On one of the chambers we had a crack that leaked LOX into the chamber, and there was no effect whatever (no combustion, discoloration, etc.)  It stands to reason, since the o/f would have gone LOX-rich at that point and been locally cooling, rather than increasing, wall temperature.

I suspect the *perception* of Oxygen has been "It's Oxygen! Anything that's warm and it touches will burn, including the metal of the combustion chamber." To the point where no one even bothered to *question* that is what would happen.

Like a lot of things in rocket engineering its one of those area where I suspect the *rate* of something matters as much as the overall numbers. Everyone likes a good disaster movie.

I suspect there *is* a leak level which is inadequate to cool the liner below its ignition temperature. People then extrapolate from there it will burn without *limit*, perhaps (unconsciously) likening it to combustion instability.

 In reality it seems to self limit, leaving you with excess LOX consumption and a loss of liner pressure (both of which should be detectable).

I don't know of *any* reference to the use of air cooling on a rocket combustion chamber. This might be *totally* new ground. Likewise the injector design to the chamber.

[removed OT link to NASA/DARPA HTO link]