Reaction engines Flight Test Vehicle speculation

[john smith 19]

I wasn't sure if this should go in "Advance Concepts" or "Commercial" with the main SABRESkylon thread, but it's not meant to be a product and it' would certainly be advanced. 

REL have talked about the idea of a "Flight Test Vehicle" on a couple of occasions. Earlier ideas were for a scaled down Skylon, running LOX/Methane rockets while the current design, resembles the D-21 M3 reconnaissance  drone designed to launch off the back of a couple of modified SR71s in hte late 60's. then modified with a booster longer than the drone, was tested off a modified B52, before the whole project was cancelled.

However looking through the archives I located this.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000004765.pdf
and this.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19990110312.pdf

Basically a late 90's plan to modify a couple of D-21 to demonstrate the "DRACO" Rock Based Combined Cycle engine some NASA centers were working on.  This system has multiple mode transitions in its flight trajectory and also featured a moving inlet spike to accommodate the speed changes.

In the process the second one discusses the D-21 in some detail, along with the LM Advanced Projects "Experimental Prototype" approach to cutting the development cost by about 1/3 (page 35 if you're interested).

Interestingly they reckoned you could  add a LOX tank between the fuel wing tanks and the engine duct, and they didn't think it would have thermal issues, although once you got above M3.5 the titanium skin and duct leading edge looked doubtful. Interestingly the issue around the LOX tank was not temperature related but pressurization. The D-21 was designed to operate its fuel tanks (there were 3, front, middle and back cross ways) at 1.5psi above the ambient pressure. Pressurizing the tanks to make the engine pressure fed was viewed as too dangerous.

On the upside the REL FTV is a clean sheet design with a thrust about 29x bigger than the D-21 ramjet. (Marquardt RJ43-MA-20S4) and recovery and reuse will be designed in from day one.

Beyond that we have a few data points.

The SABRE test engine is expected to have a thrust of 20 tonnes (44 000lbf) and according to the SEI study on the USAF TSTO design thrust needs to be at least 70% of GTOW, giving about 28500Kg of mass.

SABRE's T/W ratio is expected to be 14:1 so engine mass is say 1430Kg.
The D21 dry mass 5500lb vs fully loaded at 11200lb means it was 49% structure. It was stressed to withstand -2/+5g in both axial and directions normal to axial. So a lower peak acceleration should give a lighter structure.

28,500-1,430kg is a total mass of 27,070Kg. A 15% mass growth allowance means the maximum mass (structure and propellant) is 23,539Kg.

To put this in perspective 30tonnes is a small(ish) regional airliner or a large(ish) fighter aircraft.

My instinct is this layout is not a good fit to the issues around LH2 as a fuel, hence my interest in wheather it would be possible to make 2 engines and go with a "Mini Skylon" airframe.

Given the D-21 is going to be REL's FTV design to test inlet design and spike schedule what does that say about its capabilities?


My instinct is REL can go one of two ways on this.

A "bare bones" X-plane approach.
This is totally focused on flying SABRE through as much of its trajectory as possible and refining the inlet and spike performance to meet its goals. The vehicle structure is as simple as possible to do that.  Not intended to be anywhere close to deliver a substantial payload to orbit, or a 2nd stage to release.

A "Skylon risk retirement" approach.
This implements the FTV in technologies more like those planned for Skylon. IE a truss framework with appropriate levels of MLI and the SiC reinforced ceramic skin fabricated in corrugated panels.

Both options can help retire a number of operations features of a Skylon as well. EG fully automated fueling and de-fueling, although that would not be necessary for early tests. Likewise early tests could treat it as an RPV flown from a ground station, before moving to fully AGV status.

the big question would be what else  could it be used for ? What sort of things would 3rd parties like to test, and what sorts of instrumentation would they want to install to do so?

[Star One]

This will not be civilian related. BAE will make sure of that.

I don’t know why you cling to this idea that initially at least this flight demonstrator is going to be anything other than a vehicle that services military requirements in all it’s aspects. Especially as we now have three heavily military invested companies onboard. I don’t even expect the demonstrator to fly anywhere than the US. Military hypersonic research is where the money is and probably the main reason Boeing & RR invested in REL.

Skylon is pretty certainly off the table for the foreseeable future.

[john smith 19]

This will not be civilian related. BAE will make sure of that.

BAe have less than a 20% stake in the company.

I don’t know why you cling to this idea that initially at least this flight demonstrator is going to be anything other than a vehicle that services military requirements in all it’s aspects.

Because a demonstrator is what it is.
It will not be capable of "Servicing military requirements" until it's demonstrated it can actually work, just as none of the SCRamjet research vehicles have done.

But mostly because
a)There are no sensible hypersonic military applications that stand up to close scrutiny.
The truth is if you want fast long range strike nothing beats an ICBM already. The problem is to design a cheap 1 warhead ICBM to do so. The trouble is (and military contractors understand this verywell) once you do so the "floor price" for an actual ICBM drops through the floor

b) I have a long standing interest in substantially dropping the cost of launch IE by 10x. There are lots of people who want "on demand" launch to LEO. Some are military, others are not.
Not a ticket to ride with a 1-2% failure rate (on mature systems). I mean an actual asset they control.
And a lot more people would pay for P2P with rather more "aircraft like" operations than BFR will ever achieve.

Especially as we now have three heavily military invested companies onboard. I don’t even expect the demonstrator to fly anywhere than the US. Military hypersonic research is where the money is and probably the main reason Boeing & RR invested in REL.

No, you have 1 (BAe), one (RR) that's got mixed military and civilian interests and the Boeing connection is an investment fund, not actually the Boeing aircraft company.

Note that word research.
Hence my question about what features would the FTV need to appeal to other groups who want to do basic research in what is (AFAIK) the first reusable M5+ vehicle in half a century.
The continuing inability to build a viable SCramjet suggests there is still quite a need for basic data collection in this area.

Skylon is pretty certainly off the table for the foreseeable future.

As I noted there are quite a number of operational and structural features of Skylon that could be tested as part of the FTV programme.

Many of them have applications to whatever vehicle is ultimately used to house an operational SABRE engine.  Everyone of those tested by the FTV pushes Skylons TRL a notch further up the scale.

[Star One]

This will not be civilian related. BAE will make sure of that.

BAe have less than a 20% stake in the company.

I don’t know why you cling to this idea that initially at least this flight demonstrator is going to be anything other than a vehicle that services military requirements in all it’s aspects.

Because a demonstrator is what it is.
It will not be capable of "Servicing military requirements" until it's demonstrated it can actually work, just as none of the SCRamjet research vehicles have done.

But mostly because
a)There are no sensible hypersonic military applications that stand up to close scrutiny.
The truth is if you want fast long range strike nothing beats an ICBM already. The problem is to design a cheap 1 warhead ICBM to do so. The trouble is (and military contractors understand this verywell) once you do so the "floor price" for an actual ICBM drops through the floor

b) I have a long standing interest in substantially dropping the cost of launch IE by 10x. There are lots of people who want "on demand" launch to LEO. Some are military, others are not.
Not a ticket to ride with a 1-2% failure rate (on mature systems). I mean an actual asset they control.
And a lot more people would pay for P2P with rather more "aircraft like" operations than BFR will ever achieve.

Especially as we now have three heavily military invested companies onboard. I don’t even expect the demonstrator to fly anywhere than the US. Military hypersonic research is where the money is and probably the main reason Boeing & RR invested in REL.

No, you have 1 (BAe), one (RR) that's got mixed military and civilian interests and the Boeing connection is an investment fund, not actually the Boeing aircraft company.

Note that word research.
Hence my question about what features would the FTV need to appeal to other groups who want to do basic research in what is (AFAIK) the first reusable M5+ vehicle in half a century.
The continuing inability to build a viable SCramjet suggests there is still quite a need for basic data collection in this area.

Skylon is pretty certainly off the table for the foreseeable future.

As I noted there are quite a number of operational and structural features of Skylon that could be tested as part of the FTV programme.

Many of them have applications to whatever vehicle is ultimately used to house an operational SABRE engine.  Everyone of those tested by the FTV pushes Skylons TRL a notch further up the scale.

Every other discussion I’ve seen regarding this recent deal online, and other past developments leading up to this outside of this forum has been in terms of its military application.

BAE are the steering force now in this, their actual percentage is pretty irrelevant but it’s big enough to do what they want to do. Also why do you think they are working with DARPA in the US, they only have one purpose and it sure isn’t civilian.

BAE’s investments also highlight the potential defence applications, such as weapons capable of flying at hypersonic speeds.

https://www.telegraph.co.uk/business/2018/04/12/reaction-engines-secures-boeing-rolls-royce-backing-hypersonic/

Outside of that there’s plenty of applications for the technology fully outside of the aviation field and I imagine they’d be both easier and quicker to exploit than a space vehicle. After all a lot of investors these days are looking for the quick return or the shortest route to a return which this seems to offer here.

[john smith 19]

Every other discussion I’ve seen regarding this recent deal online, and other past developments leading up to this outside of this forum has been in terms of its military application.

Again this vehicle has no direct military applications.

I'd certainly agree that it could be used to gather the data to build a military vehicle. I'd also agree that many of the research groups who would like to use it are military funded.

You seem to be equating "Looks like a reconnaissance drone" with is a reconnaissance drone. IRL what you're talking about is about as sensible as sticking missiles on an X-15 (which is a plot device in a Stephen Baxter novel IIRC, but makes no real sense IRL).

Besides when was the last time you ever heard BAe spend their money on developing their own vehicle without an actual MoD or DoD requirements document or research budget?

BAE are the steering force now in this, their actual percentage is pretty irrelevant but it’s big enough to do what they want to do. Also why do you think they are working with DARPA in the US, they only have one purpose and it sure isn’t civilian.

If it comes to a shareholder vote you'll find the difference is between "We would very much like and" and "This is what you will do" to REL management. 

BAE’s investments also highlight the potential defence applications, such as weapons capable of flying at hypersonic speeds.

https://www.telegraph.co.uk/business/2018/04/12/reaction-engines-secures-boeing-rolls-royce-backing-hypersonic/

And if they'd made floor cleaners they'd no doubt stress its application to floor cleaning. It's what they do.

Outside of that there’s plenty of applications for the technology fully outside of the aviation field and I imagine they’d be both easier and quicker to exploit than a space vehicle.

Which won't need a Flight Test Vehicle. So not really relevant to this topic.

After all a lot of investors these days are looking for the quick return or the shortest route to a return which this seems to offer here.

The issue with REL is that it's not a get rich quick scheme.  It is (potentially) a get very rich scheme  (and lower the price of space access 10x).
This is a stage toward that process.

Could we discuss what this vehicle could do rather than why it would do it? I'd like to hear from people who have some idea what the outstanding questions in hypersonics are and what would be needed to resolve them if a reusable flight vehicle was available.

[Star One]

Every other discussion I’ve seen regarding this recent deal online, and other past developments leading up to this outside of this forum has been in terms of its military application.

Again this vehicle has no direct military applications.

I'd certainly agree that it could be used to gather the data to build a military vehicle. I'd also agree that many of the research groups who would like to use it are military funded.

You seem to be equating "Looks like a reconnaissance drone" with is a reconnaissance drone. IRL what you're talking about is about as sensible as sticking missiles on an X-15 (which is a plot device in a Stephen Baxter novel IIRC, but makes no real sense IRL).

Besides when was the last time you ever heard BAe spend their money on developing their own vehicle without an actual MoD or DoD requirements document or research budget?

BAE are the steering force now in this, their actual percentage is pretty irrelevant but it’s big enough to do what they want to do. Also why do you think they are working with DARPA in the US, they only have one purpose and it sure isn’t civilian.

If it comes to a shareholder vote you'll find the difference is between "We would very much like and" and "This is what you will do" to REL management. 

BAE’s investments also highlight the potential defence applications, such as weapons capable of flying at hypersonic speeds.

https://www.telegraph.co.uk/business/2018/04/12/reaction-engines-secures-boeing-rolls-royce-backing-hypersonic/

And if they'd made floor cleaners they'd no doubt stress its application to floor cleaning. It's what they do.

Outside of that there’s plenty of applications for the technology fully outside of the aviation field and I imagine they’d be both easier and quicker to exploit than a space vehicle.

Which won't need a Flight Test Vehicle. So not really relevant to this topic.

After all a lot of investors these days are looking for the quick return or the shortest route to a return which this seems to offer here.

The issue with REL is that it's not a get rich quick scheme.  It is (potentially) a get very rich scheme  (and lower the price of space access 10x).
This is a stage toward that process.

Could we discuss what this vehicle could do rather than why it would do it? I'd like to hear from people who have some idea what the outstanding questions in hypersonics are and what would be needed to resolve them if a reusable flight vehicle was available.

Anyway why did you feel the need to start a new thread on this when there is already a perfectly serviceable thread to discuss this. There is nothing in your OP that made it worthy of a separate thread. In fact it would quite easily have fitted in the main REL thread. Especially when any such demonstrator is still many years away and much could change before then.

[Phillip Clark]

Has anyone considered asking Reaction Engines about this?   I am sure that alan Bond would tell, if he's able to.

[john smith 19]

Anyway why did you feel the need to start a new thread on this when there is already a perfectly serviceable thread to discuss this. There is nothing in your OP that made it worthy of a separate thread. In fact it would quite easily have fitted in the main REL thread. Especially when any such demonstrator is still many years away and much could change before then.

Perhaps you'd like to try putting an "IMHO" in that first sentence?

It's certainly advanced and there is a pretty wide range of options for REL to consider. while not a top priority some of the initial planning should be started now.

For instance the outline conversion report for turning the D-21 into a test bed for the DRACO engine said there is no low speed wind tunnel data for this shape, because it was never designed to land in the first place.

That's fine for a 1 shot expendable system but a problem if you want to get it back.

[Star One]

Anyway why did you feel the need to start a new thread on this when there is already a perfectly serviceable thread to discuss this. There is nothing in your OP that made it worthy of a separate thread. In fact it would quite easily have fitted in the main REL thread. Especially when any such demonstrator is still many years away and much could change before then.

Perhaps you'd like to try putting an "IMHO" in that first sentence?

It's certainly advanced and there is a pretty wide range of options for REL to consider. while not a top priority some of the initial planning should be started now.

For instance the outline conversion report for turning the D-21 into a test bed for the DRACO engine said there is no low speed wind tunnel data for this shape, because it was never designed to land in the first place.

That's fine for a 1 shot expendable system but a problem if you want to get it back.

I’d thought post #7 probably suggests the best route to your OP.

[john smith 19]

I’d thought post #7 probably suggests the best route to your OP.

No.

[Asteroza]

Even if you were considering the D-21 planform, the mentioned LOX saddle tank by itself wouldn't be enough right? You would need to also colocate a methane or hydrogen tank in the same barrel area (concentric barrels with shared wall?). If, like some artist impressions, there is a secondary engine, then having wet wing tanks for kerosene makes some sense, but you aren't putting cryogens in the wings. Since this would be a test vehicle, you might have drop tanks for added propellant during takeoff/climbout, assuming you don't airdrop from something (Stratolaunch's Roc?)

[john smith 19]

Even if you were considering the D-21 planform, the mentioned LOX saddle tank by itself wouldn't be enough right? You would need to also colocate a methane or hydrogen tank in the same barrel area (concentric barrels with shared wall?). If, like some artist impressions, there is a secondary engine, then having wet wing tanks for kerosene makes some sense, but you aren't putting cryogens in the wings. Since this would be a test vehicle, you might have drop tanks for added propellant during takeoff/climbout, assuming you don't airdrop from something (Stratolaunch's Roc?)

This is where it gets tricky.  The 2nd file I linked to shows part of the actual D-21's internal structure. It's a series of frames running across the vehicle with a big hole punched through all of them. The original ramjet duct ran through the hole. The first, last and 2 middle frames ran right up to the duct. These were the original JP7 fuel tanks. NASA's planned DRACO installation would have put a (roughly elongated C section) tank in this gap and left the original tanks in place for JP7.

Note 2 things.
This is how Lockheed built it. REL have shown roughly the same shape. It's internal structure could (should?) be completely different.
DRACO's goal was to test up to M6 also but the feasibility study warned the D-21 was built out of Titanium and only really good up to M4, unless the skin was thicker and the leading edge TPS improved.

TBH I was pretty shocked when they planned to wrap the LOX tank around  the engine duct, given it's going to be a bit warm.  I can only presume that as long as it's not in actual contact  then the air (probably GN2 or GAr IRL) and some fibre insulation would be sufficient to keep it cool, given the LOX tank on the F9 booster seems to be protected by nothing much more than a thick coat of pain. 

But LH2 is not going to be so easy to deal with. AFAIK All LH2 proposals (including airliner designs from the mid 70's) wanted to keep it in one block, either in a big fuselage tank or a long fuselage "hump" running on top of the fuselage.
LH2 is not dense and it really needs volume.  The upside is this design is not pressure stabilized. It relies on the static strength of the materials. the D-21 DRACO conversion planned to run the LOX tank at about 1.5psi above the ambient pressure (which would not have been high at its launch altitude on a B52.

Moving to Methane helps the coldness issue a bit but then you lose LH2's great shc (4x that of water, which is excellent to begin with). OTOH REL plan on both cryogens to be sub cooled, which reduces burst issues due to heat leaks.

Basically monocoque construction does not work too well with temperature gradients this extreme. This is where you have to ask yourself "Do I really need minimum surface area cylinders or spheres inside the moldline, or could I go with a  low pressure more-or-less conformal tank with a layer of insulation between?"
BTW the SS301 used to make the Centaur tanks has 1/10 the thermal conductivity of Aluminium and very good weld properties.

For on orbit storage REL have done some work with a LH2 tank housing LO2 and LH2 for the on orbit RCS. The LHe vaporizes, cooling the internal LO2 and LH2 further, which is just what you want.  But LHe is an ever bigger PITA to keep cold on Earth.

Hmm. Perhaps separate LH2 tank inside LO2 tank either side of the duct? With internal partitions for CoG control.

This also raises the question of wheather they build it with the Skylon structural baseline of SiC reinforced Titanium trusses with SiC reinforced ceramic skins. It sounds exotic, but really, are there any options that are less exotic, that can handle the mass and thermal issues as well?

Incidentally the DRACO engine also uses an aerospike and the power to move it over the projected flight range was anticipated to peak at 5KVA, with average levels at 2.5KVA. That sounds a lot but was apparently expected to be within range of a ram air powered generator based APU.

It's also now clear from the presentation the engineers in charge of the test site that the test engine will not fly. It will generate the test data for that engine.

I was curious about this as once you've designed an engine the second copy of it is likely to be a lot less expensive, and something looking rather more Skylon like (whatever its made of internally) looks a simpler bet to solving the LH2 storage problem, along with various other fringe benefits (confirming the layout, aerodynamics, plume heating etc). That said whatever the FTV it will teach a lot of lessons about how to operate a HTOL reusable LO2/LH2 or Methane fueled vehicle.

[speedevil]

Incidentally the DRACO engine also uses an aerospike and the power to move it over the projected flight range was anticipated to peak at 5KVA, with average levels at 2.5KVA. That sounds a lot but was apparently expected to be within range of a ram air powered generator based APU.

Addressing only this part, 2.5KVA*10 minutes = a couple of kilos of batteries.

[john smith 19]

Incidentally the DRACO engine also uses an aerospike and the power to move it over the projected flight range was anticipated to peak at 5KVA, with average levels at 2.5KVA. That sounds a lot but was apparently expected to be within range of a ram air powered generator based APU.

Addressing only this part, 2.5KVA*10 minutes = a couple of kilos of batteries.

Noted.
Batteries and a ram air driven generator were the planned power source for the DRACO conversion back in 1999. Obviously the battery tech has moved on a bit since then.

[RanulfC]

JS19 wrote:

I wasn't sure if this should go in "Advance Concepts" or "Commercial" with the main SABRE/Skylon thread, but it's not meant to be a product and it' would certainly be advanced.

Works for me

REL have talked about the idea of a "Flight Test Vehicle" on a couple of occasions. Earlier ideas were for a scaled down Skylon, running LOX/Methane rockets while the current design, resembles the D-21 M3 reconnaissance drone designed to launch off the back of a couple of modified SR71s in the late 60's. then modified with a booster longer than the drone, was tested off a modified B52, before the whole project was cancelled.

I recall the rocket powered vehicles were actually more like aerodynamic test vehicles rather than an engine/cycle test vehicle which is what the current "FTV" seems to be aimed at. (Speaking of how about some links to past and present concepts, test goals, and other information?)

Nice find on the DRACO/D-21 having something to build on would help with the costing as even a subscale demonstrator is going to be expensive. (Despite what the cited studies actually "say" I'll note that they both include the verbiage "only vehicle designed to attain/maintain hypersonic speeds" which is wrong as the D-21 never came close to Mach-5 which is the actual boundary of "hypersonic" speed) I suppose the first question is what exactly are they trying to 'test'?

1) If they want to test the airframe design, (which I'm doubting as we've already seen the details differ from one company or group to another) then whatever is used will be designed around both the engines and the airframe and an integration of the two. Being's Boeing is onboard I highly doubt it will look like the Skylon we're used to from REL.

2) If they want to flight test the engine/cycle/system , (much more likely) then the airframe doesn't matter as much though the higher the testing speeds the more the design will want to "close" towards an approximation of "real" design. But initially you want to demonstrate a series of tests along a spectrum of speed if not to 'full' capability then at least to certain significant test points. For example such test points could include; Take off, climb, acceleration, ability to perform subsonic/transonic/supersonic and back transitions, accelerate to a 'maximum' speed, decelerate back through transonic and return to land, (or be recovered) and then do it all over again multiple times. (Note switching between air-breathing and pure rocket mode at various points will be a requirement so it will have to haul a LOX tank around as you suggest )

As noted in the reports the D21 doesn't have very good low speed handling qualities so it would probably need modification to the wings to provide such. You're also going to have to install landing and possibly take off gear.

If you are 'just' wanting to test the engine at various flight speeds then it might be better to pull a page from history and use a much simpler and more robust design type which while you'd have to 'build' can in fact be pretty straight forward AND cost effective. You can't really go wrong with something along the lines of the Lockheed X7 (https://en.wikipedia.org/wiki/Lockheed_X-7) ramjet test vehicle, which I will note CAN hit hypersonic speeds. (Late model drones topped out very near Mach-5 {and with a better engine could have done so) Given the SABRE T/W launch can be from a rail and landing kept 'simple' by keeping the parachute and "spike" from the original. Granted dimensions would have to be larger but it's certainly an option.

Lastly there is the "existing aircraft airframe" conversion option. Not that I can see a Learjet outfitted with a pair of 'mini-SABRE's' but conversions have been made of several supersonic aircraft by many nations with some of the US ones being "Q" series versions of the F-106, F4, and F-16 and of course there are 'sale' versions of the T-38, F-106, F-104 and others.

Finding and converting an Starfighter might be an good option as it's a bit bigger than the D-21 and if suitably braced you might be able to mount "mini-SABRE's" in place of the wingtip tanks and utilize the full capacity of the fuselage.

Of course having said all the above IF the various contractors can find someone to 'pay' for it most of them (Boeing, BAE, etc, frankly probably everyone BUT REL ) might prefer to 'build-from-scratch', especially if that 'customer' is a government. And while I DO agree the initial test vehicle won't have an obvious or "built-in" Military Operational Capability and disagree with Star One that there is any "obvious" bias towards having such in a "test" vehicle simple because who's interested and who's building it the fact is such a vehicle 'could' have a secondary purpose IF built to certain specifications. It's that last part that will be telling because I recall that the original (AF driven) specifications for the X-33 program were in fact quite interesting, especially given the launch and landing locations chosen for the program.

(Launch from Edwards AFB towards a facility in Utah, Speed in excess of Mach-12 and 'several hundred pounds' of "test instruments" in a long narrow bay... At the time it was noted by several people the 'test instrument bay' could hold a STAR solid motor and a microsatellite as 'payload' after all )

And while Star One is kind of obsessed with "hypersonic strike" missions the ACTUAL most likely mission is frankly as an advanced D-21 system for a reconnaissance drone. (Any 'weapon' has to be deployed from the inside of the vehicle mind you and THEN transition through a Mach-5+ shockwave AND still remain aimed at the target all of which is VERY difficult. And since it can't mass more than 2,000lbs at most it has to be highly accurate so obviously guided and everything has to be able to stand up to hypersonic speeds since that's when it's launched. As we've not developed any that work yet...) And for that you'd need specialized sensors, environmental conditioning equipment, (hypersonic speeds remember) power and others which will amount to something on the order of several hundred to maybe a thousand pounds. (Remember also you're flying at hypersonic speeds at almost 100,000ft so "OTS" sensor won't work) And then there's the 'range' question. The D-21 had a range of over 3,000 miles while modern UAV's have ranges from under 200 miles to over 14,000 miles but using LH2 or Liquid Methane there would be no opportunity for air-to-air refueling, (and transferring cryogenic fluids has been shown to have issue and that's before the operational problems with working with the stuff in bulk) and internal storage and insulation, (hypersonic again) issues abound for a smaller airframe.

Still the 'customer' has to be very upfront about such and willing to pay for it. Sensors, weapons bays, internal fuel storage will all have to specified UP FRONT so they can be included in the design as there won't be any way to 'retrofit' them once the vehicle is built. And all this has a very real possibility of not only the actual vehicle or engine not performing to specifications but that the 'added' requirements don't themselves cause the vehicle to fall short of requirements.

Since the main 'question' is (obviously) does the SABRE live up to expectations AND if so what are its actual performance metrics IN FLIGHT I have very high confidence that no one will be willing to pay for anything likely to be 'useful' till after all that data is in. Now something 'based' on the FTV could eventually be pitched but keep in mind there will be certain and strict requirements that have to be met along the way.

The most likely outcome is the FTV will be (as suggested by the "usual" aerospace contractors such as Boeing, BAE, etc) an expendable "test" vehicle on the line of the X-45/47 with each vehicle pushing the performance envelope along a series of 'goals' over the program. It's typical of such test programs today so it won't be either unexpected nor vastly difficult to pitch. On the other hand JS19 has a point that making it 'reusable' may in fact be both the better and 'simpler' option given the minimum size needed. But even a conversion of an existing airframe is going to be expensive and a 'scratch built' one probably out of the question. But you really DO want reusable despite the 'cost analysis' tending towards expendable since what you REALLY want to do is get data from the full spectrum of flight operations rather than just selected 'segments' which might induce errors or miss issues.

Star One wrote:

Every other discussion I've seen regarding this recent deal online, and other past developments leading up to this outside of this forum has been in terms of its military application.

Actually every time "hypersonics" is mentioned or written about the 'subject' turns to military applications which is different than what you're implying. Simply put, inserting "military applications" is a standard way to pad a subject that "might" actually have "military applications" whether the actual work being done IS directed towards that goal or not. Hypersonic flight has been specifically 'tied' to proposed "military applications" since the 1950s but actual versus assumed applications have been severely lacking and this is no different. SABRE, (which is the whole point of any test vehicle) is another, albeit rather better thought out, propulsion system that can possibly be used to push a vehicle to hypersonic speeds. Since it is possible for any vehicle to be used for 'military purposes', (and therefor used to pitch money from either the government or the military) such 'padding' is always inserted no matter if the actual application doesn't fit the suggested concept.

Let's take a look at the ACTUAL "military applications" of the SABRE engine:
1) It can be used to power a booster vehicle to launch expendable or reusable upper stages for vastly cheaper than current launch costs.

Now 'suggested' applications tend to be:
a) It can be used to power a bomber/fighter/recon aircraft flying at the edge of space and hypersonic, (Mach-6 to -10) speeds!

Actually no since a 'fighter' by definition needs a propulsion system that can allow it to do its job which is engage and destroy enemy aircraft and flying at 100,000ft and Mach-6 to Mach-10 you can't see, identify, lock-onto and engage a target with any reasonable chance of success. Similarly a 'bomber' needs to find, identify and engage its target which while not moving, (generally in fact the main 'purpose' of a hypersonic bomber is supposed to be the ability to reach a target area before a MOVING target can move outside its engagement area) and destroy it. Anyone that thinks that a platform moving as hypersonic speed at an altitude of 100,000ft plus can do this 'easily' is sadly out of touch with the reality of weapons technology. (Or trying to get money which pretty much covers the majority of sources for such suggestions) So that leaves the reconnaissance role which actually has possibilities as long as you ignore the rather obvious problems with a super-fast, super high altitude very "visible" (both to radar and basic IR sensors) target that while it might spot targets that would normally avoid predictable satellite passes or low and relatively slow "normal" aircraft is both vulnerable and restricted on what information it can gather. How can something flying so high and fast be 'vulnerable'? It is flying 'high' so again it's a LOT more visible than something flying very low and very slow so its chances of being spotted are vastly higher and unfortunately even 100,000ft is not 'low' enough to "hide" behind the curvature of the Earth as has often erroneously been suggested. Barring flying against someone with only the "Mark-One Eyeball" you WILL be spotted and tracked. You're also vulnerable in that neither the speed or altitude are immune from aggressive interception.

Further the SABRE cycle due to the inclusion of a rocket motor is vastly inferior for ANY hypersonic mission of any of the type given except launch vehicle when compared to vastly better cycles such as the Scimitar or other 'turbine' rather than 'rocket' based cycles. The 800-pound gorilla in the room everyone who focuses on 'military applications' for the SABRE is the fact it has ONE possible application and ONLY one: Launch Vehicle.

So therefor, (it should be quite obvious) if the FTV is planned to use SABRE cycle engines then it actually has one 'possible' application and most likely it will therefore be used to PROVE the SABRE cycle itself and not some way to 'sneak' a military drone into production.

BAE are the steering force now in this, their actual percentage is pretty irrelevant but it's big enough to do what they want to do. Also why do you think they are working with DARPA in the US, they only have one purpose and it sure isn't civilian.

Actually DARPA does in fact do a lot of projects that while they 'may' have military applications in the future can and have found civilian applications in more near term time frames. Self-driving vehicles is one good example as that started as a DARPA sponsored program but was rapidly embraced and improved upon by civilian agencies. Also while DARPA is sponsoring some of the work the actual main interest is from the Air Force Research Laboratory which is specifically tasked with 'long term' research and not procurement or operations. This is on purpose because the last time DARPA tried to 'shortcut' a research program it not only failed to get to flight testing it failed to reach the level of 50 year old research and development that the DARPA researchers didn't know had already been done! (RASCAL and MIPCC) AFRL was one of the agencies that pointed out the cost models for the program were significantly lacking in basic data while the proposed 'research' areas had already been done and the suggested 'vehicle' was in no way a 'research' or 'test' model but a clear 'operational' vehicle with which DARPA was attempting to bypass standard procurement and contracting procedures. (Which in fact they were doing)

https://www.telegraph.co.uk/business/2018/04/12/reaction-engines-secures-boeing-rolls-royce-backing-hypersonic/

Outside of that there's plenty of applications for the technology fully outside of the aviation field and I imagine they'd be both easier and quicker to exploit than a space vehicle. After all a lot of investors these days are looking for the quick return or the shortest route to a return which this seems to offer here.

Nice of you to point out another article that fully and totally misses the main 'point' of its own information in order to pad the word count with nonsensical and non-relevant subjects. Point of fact where in that article can you find ONE "application" for the SABRE OTHER than as a launch vehicle? That is after all the ONLY application that is suggested or implied by those quoted in the article. Supersonic and hypersonic flight is 'suggested' as something that can be 'derived' from the SABRE cycle in the future a number of times but any connection with the SABRE FTV is inferred and not explicit by anyone quoted in the article. Wonder why that is?

It's because the SABRE is not suitable for either supersonic or hypersonic "flight" and those being quoted are WELL aware of this fact. The engine that would power supersonic or hypersonic aircraft, (note not "spacecraft" which is what the SABRE is stated to be used for in the article) is the Scimitar which is optimized and designed for just such applications and does the job VASTLY better than the SABRE.

I think one thing that people seem to ignore is due to its nature SABRE powered vehicles simply can NOT fly from ANY 'standard' airport anywhere in the world. Not even 'lightly loaded' or 'partially fueled' and REL has pointed this out several times. It has nothing to do with runway length or loading or any of the other "operational" issue that have been discussed but directly due to the fact it uses a rocket engine in its design. It can't fly from an standard airport because it is impossible for it to meet the noise regulations of any standard airport. Period.
Scimitar CAN do so and is specifically designed to do so and meet ALL regulations and guidelines.

And lets discuss the applications outside the aviation industry which can be so lucrative...

Point of fact REL has a nice heat exchanger technology but as was pointed out back when they were trying to hype it, (and SABRE) up to get investment it is highly specialized and has few if any applications outside the ones REL has in mind. Granted if they exist you can be sure BAE will exploit them but really that has nothing to do with the thread. Still if you'd like to list them we can disassemble them again I suppose.

Phillip Clark wrote:

Has anyone considered asking Reaction Engines about this? I am sure that Alan Bond would tell, if he's able to.

And deprive ourselves of the possibly of vastly speculative and argumentative postings? What are you? Mad? I mean look how boring and mundane BFS turned out to be compared to our ideas and concepts, come on loosen up some

Some additional comments on the D21 design:
Note the LOX tank was wrapped around the 'duct' not the actual engine. Even at Mach-6 the heating wasn't going to be really 'bad' and there was insulation. But in any case, (Liquid Methane or LH2) is going to require a larger tankage and not be compatible with 'wet-wings' normally. (You have to admire the way they got 'away' with "wet-wings" cyro-LOX in the Star-Raker design though: http://www.alternatewars.com/SpaceRace/Star_Raker/Star_Raker.htm
https://motherboard.vice.com/en_us/article/ezvj4j/the-747-to-space-that-never-was
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790010900.pdf (page 73)

Having said that since you don't actually NEED to put propellant in the wings, and at hypersonic speeds "wings" aren't really needed there would be tendency to move towards a lifting fuselage body similar to the "waverider" and advanced aircraft we've been working on the last 50 years. Now with that as pointed out with the D-21 you DO need wings at lower speeds so unlike NASP and some of the more extreme designs you want good low speed landing wings. On the other hand if you don't assume that you have to squeeze every ounce of efficiency out of the propulsion, (Skylon doesn't, whereas most of the others DO and therefore the engines and fuselage are designed to synergistically support each other) then you can consider other options for engine placement. (I should note the ENTIRE fuselage is synergistically used so that the forward body helps compress and ingest the air while the after body helps align and expand the exhaust for more efficient operation)

Again this isn't 'lazy' on REL's part but it greatly simplifies the figures you need to play with since with the engines on the wingtips reduces some of the aerodynamic and weight-and-balance problems. (Of course you get a similar effect without the "engine-out" issues by putting the engine near the center of the airframe, again which is common on engine/body designs) I suppose the main question is how small can they make a SABRE engine?

Randy

[Star One]

JS19 wrote:

I wasn't sure if this should go in "Advance Concepts" or "Commercial" with the main SABRE/Skylon thread, but it's not meant to be a product and it' would certainly be advanced.

Works for me

REL have talked about the idea of a "Flight Test Vehicle" on a couple of occasions. Earlier ideas were for a scaled down Skylon, running LOX/Methane rockets while the current design, resembles the D-21 M3 reconnaissance drone designed to launch off the back of a couple of modified SR71s in the late 60's. then modified with a booster longer than the drone, was tested off a modified B52, before the whole project was cancelled.

I recall the rocket powered vehicles were actually more like aerodynamic test vehicles rather than an engine/cycle test vehicle which is what the current "FTV" seems to be aimed at. (Speaking of how about some links to past and present concepts, test goals, and other information?)

Nice find on the DRACO/D-21 having something to build on would help with the costing as even a subscale demonstrator is going to be expensive. (Despite what the cited studies actually "say" I'll note that they both include the verbiage "only vehicle designed to attain/maintain hypersonic speeds" which is wrong as the D-21 never came close to Mach-5 which is the actual boundary of "hypersonic" speed) I suppose the first question is what exactly are they trying to 'test'?

1) If they want to test the airframe design, (which I'm doubting as we've already seen the details differ from one company or group to another) then whatever is used will be designed around both the engines and the airframe and an integration of the two. Being's Boeing is onboard I highly doubt it will look like the Skylon we're used to from REL.

2) If they want to flight test the engine/cycle/system , (much more likely) then the airframe doesn't matter as much though the higher the testing speeds the more the design will want to "close" towards an approximation of "real" design. But initially you want to demonstrate a series of tests along a spectrum of speed if not to 'full' capability then at least to certain significant test points. For example such test points could include; Take off, climb, acceleration, ability to perform subsonic/transonic/supersonic and back transitions, accelerate to a 'maximum' speed, decelerate back through transonic and return to land, (or be recovered) and then do it all over again multiple times. (Note switching between air-breathing and pure rocket mode at various points will be a requirement so it will have to haul a LOX tank around as you suggest )

As noted in the reports the D21 doesn't have very good low speed handling qualities so it would probably need modification to the wings to provide such. You're also going to have to install landing and possibly take off gear.

If you are 'just' wanting to test the engine at various flight speeds then it might be better to pull a page from history and use a much simpler and more robust design type which while you'd have to 'build' can in fact be pretty straight forward AND cost effective. You can't really go wrong with something along the lines of the Lockheed X7 (https://en.wikipedia.org/wiki/Lockheed_X-7) ramjet test vehicle, which I will note CAN hit hypersonic speeds. (Late model drones topped out very near Mach-5 {and with a better engine could have done so) Given the SABRE T/W launch can be from a rail and landing kept 'simple' by keeping the parachute and "spike" from the original. Granted dimensions would have to be larger but it's certainly an option.

Lastly there is the "existing aircraft airframe" conversion option. Not that I can see a Learjet outfitted with a pair of 'mini-SABRE's' but conversions have been made of several supersonic aircraft by many nations with some of the US ones being "Q" series versions of the F-106, F4, and F-16 and of course there are 'sale' versions of the T-38, F-106, F-104 and others.

Finding and converting an Starfighter might be an good option as it's a bit bigger than the D-21 and if suitably braced you might be able to mount "mini-SABRE's" in place of the wingtip tanks and utilize the full capacity of the fuselage.

Of course having said all the above IF the various contractors can find someone to 'pay' for it most of them (Boeing, BAE, etc, frankly probably everyone BUT REL ) might prefer to 'build-from-scratch', especially if that 'customer' is a government. And while I DO agree the initial test vehicle won't have an obvious or "built-in" Military Operational Capability and disagree with Star One that there is any "obvious" bias towards having such in a "test" vehicle simple because who's interested and who's building it the fact is such a vehicle 'could' have a secondary purpose IF built to certain specifications. It's that last part that will be telling because I recall that the original (AF driven) specifications for the X-33 program were in fact quite interesting, especially given the launch and landing locations chosen for the program.

(Launch from Edwards AFB towards a facility in Utah, Speed in excess of Mach-12 and 'several hundred pounds' of "test instruments" in a long narrow bay... At the time it was noted by several people the 'test instrument bay' could hold a STAR solid motor and a microsatellite as 'payload' after all )

And while Star One is kind of obsessed with "hypersonic strike" missions the ACTUAL most likely mission is frankly as an advanced D-21 system for a reconnaissance drone. (Any 'weapon' has to be deployed from the inside of the vehicle mind you and THEN transition through a Mach-5+ shockwave AND still remain aimed at the target all of which is VERY difficult. And since it can't mass more than 2,000lbs at most it has to be highly accurate so obviously guided and everything has to be able to stand up to hypersonic speeds since that's when it's launched. As we've not developed any that work yet...) And for that you'd need specialized sensors, environmental conditioning equipment, (hypersonic speeds remember) power and others which will amount to something on the order of several hundred to maybe a thousand pounds. (Remember also you're flying at hypersonic speeds at almost 100,000ft so "OTS" sensor won't work) And then there's the 'range' question. The D-21 had a range of over 3,000 miles while modern UAV's have ranges from under 200 miles to over 14,000 miles but using LH2 or Liquid Methane there would be no opportunity for air-to-air refueling, (and transferring cryogenic fluids has been shown to have issue and that's before the operational problems with working with the stuff in bulk) and internal storage and insulation, (hypersonic again) issues abound for a smaller airframe.

Still the 'customer' has to be very upfront about such and willing to pay for it. Sensors, weapons bays, internal fuel storage will all have to specified UP FRONT so they can be included in the design as there won't be any way to 'retrofit' them once the vehicle is built. And all this has a very real possibility of not only the actual vehicle or engine not performing to specifications but that the 'added' requirements don't themselves cause the vehicle to fall short of requirements.

Since the main 'question' is (obviously) does the SABRE live up to expectations AND if so what are its actual performance metrics IN FLIGHT I have very high confidence that no one will be willing to pay for anything likely to be 'useful' till after all that data is in. Now something 'based' on the FTV could eventually be pitched but keep in mind there will be certain and strict requirements that have to be met along the way.

The most likely outcome is the FTV will be (as suggested by the "usual" aerospace contractors such as Boeing, BAE, etc) an expendable "test" vehicle on the line of the X-45/47 with each vehicle pushing the performance envelope along a series of 'goals' over the program. It's typical of such test programs today so it won't be either unexpected nor vastly difficult to pitch. On the other hand JS19 has a point that making it 'reusable' may in fact be both the better and 'simpler' option given the minimum size needed. But even a conversion of an existing airframe is going to be expensive and a 'scratch built' one probably out of the question. But you really DO want reusable despite the 'cost analysis' tending towards expendable since what you REALLY want to do is get data from the full spectrum of flight operations rather than just selected 'segments' which might induce errors or miss issues.

Star One wrote:

Every other discussion I've seen regarding this recent deal online, and other past developments leading up to this outside of this forum has been in terms of its military application.

Actually every time "hypersonics" is mentioned or written about the 'subject' turns to military applications which is different than what you're implying. Simply put, inserting "military applications" is a standard way to pad a subject that "might" actually have "military applications" whether the actual work being done IS directed towards that goal or not. Hypersonic flight has been specifically 'tied' to proposed "military applications" since the 1950s but actual versus assumed applications have been severely lacking and this is no different. SABRE, (which is the whole point of any test vehicle) is another, albeit rather better thought out, propulsion system that can possibly be used to push a vehicle to hypersonic speeds. Since it is possible for any vehicle to be used for 'military purposes', (and therefor used to pitch money from either the government or the military) such 'padding' is always inserted no matter if the actual application doesn't fit the suggested concept.

Let's take a look at the ACTUAL "military applications" of the SABRE engine:
1) It can be used to power a booster vehicle to launch expendable or reusable upper stages for vastly cheaper than current launch costs.

Now 'suggested' applications tend to be:
a) It can be used to power a bomber/fighter/recon aircraft flying at the edge of space and hypersonic, (Mach-6 to -10) speeds!

Actually no since a 'fighter' by definition needs a propulsion system that can allow it to do its job which is engage and destroy enemy aircraft and flying at 100,000ft and Mach-6 to Mach-10 you can't see, identify, lock-onto and engage a target with any reasonable chance of success. Similarly a 'bomber' needs to find, identify and engage its target which while not moving, (generally in fact the main 'purpose' of a hypersonic bomber is supposed to be the ability to reach a target area before a MOVING target can move outside its engagement area) and destroy it. Anyone that thinks that a platform moving as hypersonic speed at an altitude of 100,000ft plus can do this 'easily' is sadly out of touch with the reality of weapons technology. (Or trying to get money which pretty much covers the majority of sources for such suggestions) So that leaves the reconnaissance role which actually has possibilities as long as you ignore the rather obvious problems with a super-fast, super high altitude very "visible" (both to radar and basic IR sensors) target that while it might spot targets that would normally avoid predictable satellite passes or low and relatively slow "normal" aircraft is both vulnerable and restricted on what information it can gather. How can something flying so high and fast be 'vulnerable'? It is flying 'high' so again it's a LOT more visible than something flying very low and very slow so its chances of being spotted are vastly higher and unfortunately even 100,000ft is not 'low' enough to "hide" behind the curvature of the Earth as has often erroneously been suggested. Barring flying against someone with only the "Mark-One Eyeball" you WILL be spotted and tracked. You're also vulnerable in that neither the speed or altitude are immune from aggressive interception.

Further the SABRE cycle due to the inclusion of a rocket motor is vastly inferior for ANY hypersonic mission of any of the type given except launch vehicle when compared to vastly better cycles such as the Scimitar or other 'turbine' rather than 'rocket' based cycles. The 800-pound gorilla in the room everyone who focuses on 'military applications' for the SABRE is the fact it has ONE possible application and ONLY one: Launch Vehicle.

So therefor, (it should be quite obvious) if the FTV is planned to use SABRE cycle engines then it actually has one 'possible' application and most likely it will therefore be used to PROVE the SABRE cycle itself and not some way to 'sneak' a military drone into production.

BAE are the steering force now in this, their actual percentage is pretty irrelevant but it's big enough to do what they want to do. Also why do you think they are working with DARPA in the US, they only have one purpose and it sure isn't civilian.

Actually DARPA does in fact do a lot of projects that while they 'may' have military applications in the future can and have found civilian applications in more near term time frames. Self-driving vehicles is one good example as that started as a DARPA sponsored program but was rapidly embraced and improved upon by civilian agencies. Also while DARPA is sponsoring some of the work the actual main interest is from the Air Force Research Laboratory which is specifically tasked with 'long term' research and not procurement or operations. This is on purpose because the last time DARPA tried to 'shortcut' a research program it not only failed to get to flight testing it failed to reach the level of 50 year old research and development that the DARPA researchers didn't know had already been done! (RASCAL and MIPCC) AFRL was one of the agencies that pointed out the cost models for the program were significantly lacking in basic data while the proposed 'research' areas had already been done and the suggested 'vehicle' was in no way a 'research' or 'test' model but a clear 'operational' vehicle with which DARPA was attempting to bypass standard procurement and contracting procedures. (Which in fact they were doing)

https://www.telegraph.co.uk/business/2018/04/12/reaction-engines-secures-boeing-rolls-royce-backing-hypersonic/

Outside of that there's plenty of applications for the technology fully outside of the aviation field and I imagine they'd be both easier and quicker to exploit than a space vehicle. After all a lot of investors these days are looking for the quick return or the shortest route to a return which this seems to offer here.

Nice of you to point out another article that fully and totally misses the main 'point' of its own information in order to pad the word count with nonsensical and non-relevant subjects. Point of fact where in that article can you find ONE "application" for the SABRE OTHER than as a launch vehicle? That is after all the ONLY application that is suggested or implied by those quoted in the article. Supersonic and hypersonic flight is 'suggested' as something that can be 'derived' from the SABRE cycle in the future a number of times but any connection with the SABRE FTV is inferred and not explicit by anyone quoted in the article. Wonder why that is?

It's because the SABRE is not suitable for either supersonic or hypersonic "flight" and those being quoted are WELL aware of this fact. The engine that would power supersonic or hypersonic aircraft, (note not "spacecraft" which is what the SABRE is stated to be used for in the article) is the Scimitar which is optimized and designed for just such applications and does the job VASTLY better than the SABRE.

I think one thing that people seem to ignore is due to its nature SABRE powered vehicles simply can NOT fly from ANY 'standard' airport anywhere in the world. Not even 'lightly loaded' or 'partially fueled' and REL has pointed this out several times. It has nothing to do with runway length or loading or any of the other "operational" issue that have been discussed but directly due to the fact it uses a rocket engine in its design. It can't fly from an standard airport because it is impossible for it to meet the noise regulations of any standard airport. Period.
Scimitar CAN do so and is specifically designed to do so and meet ALL regulations and guidelines.

And lets discuss the applications outside the aviation industry which can be so lucrative...

Point of fact REL has a nice heat exchanger technology but as was pointed out back when they were trying to hype it, (and SABRE) up to get investment it is highly specialized and has few if any applications outside the ones REL has in mind. Granted if they exist you can be sure BAE will exploit them but really that has nothing to do with the thread. Still if you'd like to list them we can disassemble them again I suppose.

Phillip Clark wrote:

Has anyone considered asking Reaction Engines about this? I am sure that Alan Bond would tell, if he's able to.

And deprive ourselves of the possibly of vastly speculative and argumentative postings? What are you? Mad? I mean look how boring and mundane BFS turned out to be compared to our ideas and concepts, come on loosen up some

Some additional comments on the D21 design:
Note the LOX tank was wrapped around the 'duct' not the actual engine. Even at Mach-6 the heating wasn't going to be really 'bad' and there was insulation. But in any case, (Liquid Methane or LH2) is going to require a larger tankage and not be compatible with 'wet-wings' normally. (You have to admire the way they got 'away' with "wet-wings" cyro-LOX in the Star-Raker design though: http://www.alternatewars.com/SpaceRace/Star_Raker/Star_Raker.htm
https://motherboard.vice.com/en_us/article/ezvj4j/the-747-to-space-that-never-was
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790010900.pdf (page 73)

Having said that since you don't actually NEED to put propellant in the wings, and at hypersonic speeds "wings" aren't really needed there would be tendency to move towards a lifting fuselage body similar to the "waverider" and advanced aircraft we've been working on the last 50 years. Now with that as pointed out with the D-21 you DO need wings at lower speeds so unlike NASP and some of the more extreme designs you want good low speed landing wings. On the other hand if you don't assume that you have to squeeze every ounce of efficiency out of the propulsion, (Skylon doesn't, whereas most of the others DO and therefore the engines and fuselage are designed to synergistically support each other) then you can consider other options for engine placement. (I should note the ENTIRE fuselage is synergistically used so that the forward body helps compress and ingest the air while the after body helps align and expand the exhaust for more efficient operation)

Again this isn't 'lazy' on REL's part but it greatly simplifies the figures you need to play with since with the engines on the wingtips reduces some of the aerodynamic and weight-and-balance problems. (Of course you get a similar effect without the "engine-out" issues by putting the engine near the center of the airframe, again which is common on engine/body designs) I suppose the main question is how small can they make a SABRE engine?

Randy

I suggest you read any history of DARPA before suggesting they do anything civilian, if they do it’s accidental rather intentional.

[john smith 19]

I recall the rocket powered vehicles were actually more like aerodynamic test vehicles rather than an engine/cycle test vehicle which is what the current "FTV" seems to be aimed at. (Speaking of how about some links to past and present concepts, test goals, and other information?)

I think the key thing for the Methane rocket version was inlet development. The "Peace Jack" programme showed what a big difference you could get with a better (and bigger) inlet on a conventional turbojet.

Links? Hmm.  Only a few REL presentations mention the idea in any detail. They were also looking at a kind of sounding rocket using IIRC LO2/LNH3/LN2 to simulate the combustion and trajectory.

Nice find on the DRACO/D-21 having something to build on would help with the costing as even a subscale demonstrator is going to be expensive. (Despite what the cited studies actually "say" I'll note that they both include the verbiage "only vehicle designed to attain/maintain hypersonic speeds" which is wrong as the D-21 never came close to Mach-5 which is the actual boundary of "hypersonic" speed) I suppose the first question is what exactly are they trying to 'test'?

Indeed.  I think the DRACO programme was relatively small scale and NASA had looked at doing LASRE on the back of one of their SR71's. Basically a D-21 was the closest thing they could get OTS. The study made no bones that M3 was OK, they were probably good to M4.5, but after that....

1) If they want to test the airframe design, (which I'm doubting as we've already seen the details differ from one company or group to another) then whatever is used will be designed around both the engines and the airframe and an integration of the two. Being's Boeing is onboard I highly doubt it will look like the Skylon we're used to from REL.

I think people are making a lot of the Boeing involvement but this is the VC arm of Boeing. I'm not sure how much 2 way interaction happens between them and the actual aircraft building parts (either military or civilian). Of course as a potential customer it would good to get their input (provided  it can be done in a non ITAR contaminating way of course).

2) If they want to flight test the engine/cycle/system , (much more likely) then the airframe doesn't matter as much though the higher the testing speeds the more the design will want to "close" towards an approximation of "real" design. But initially you want to demonstrate a series of tests along a spectrum of speed if not to 'full' capability then at least to certain significant test points. For example such test points could include; Take off, climb, acceleration, ability to perform subsonic/transonic/supersonic and back transitions, accelerate to a 'maximum' speed, decelerate back through transonic and return to land, (or be recovered) and then do it all over again multiple times. (Note switching between air-breathing and pure rocket mode at various points will be a requirement so it will have to haul a LOX tank around as you suggest )

Exactly. It's pretty clear that a lot of people simply won't believe a vehicle and engine can fly this trajectory until one does (although they seem to have no trouble believing a SCramjet will do whatever its promoters claim it will  ) Therefor that needs to be the key goal.

As noted in the reports the D21 doesn't have very good low speed handling qualities so it would probably need modification to the wings to provide such. You're also going to have to install landing and possibly take off gear.

Actually they dug up the top aerodynamics guy at the time and he said they never tested the shape for low speed handling. No point. It's never going to land. TBH that's probably the case for any existing shape.
Looking at the D-21's internal structure what struck was how SR71 it was. Basically, take an SR71, chop off the wings past the engine nacelles (and the nacelles themselves), then core out the fuselage and stick the ramjet in there. No doubt it was considerably more subtle than that but that's the high level view I got.

If you are 'just' wanting to test the engine at various flight speeds then it might be better to pull a page from history and use a much simpler and more robust design type which while you'd have to 'build' can in fact be pretty straight forward AND cost effective. You can't really go wrong with something along the lines of the Lockheed X7 (https://en.wikipedia.org/wiki/Lockheed_X-7) ramjet test vehicle, which I will note CAN hit hypersonic speeds. (Late model drones topped out very near Mach-5 {and with a better engine could have done so) Given the SABRE T/W launch can be from a rail and landing kept 'simple' by keeping the parachute and "spike" from the original. Granted dimensions would have to be larger but it's certainly an option.

Now this works better. When this Idea came up I was thinking more of the V-1 but with proven high Mach flight to go on...
My instinct is LH2 storage is going to be the big issue for any test design. A design that gives you a solid centre body to put an LH2 tank inside seems  easier.  Of course if you goal is to demonstrate "mad design skillz" then the LH2 wet wing is definitely the way to go. But there's a very fine line between mad skillz, and just plain mad. 

Lastly there is the "existing aircraft airframe" conversion option. Not that I can see a Learjet outfitted with a pair of 'mini-SABRE's' but conversions have been made of several supersonic aircraft by many nations with some of the US ones being "Q" series versions of the F-106, F4, and F-16 and of course there are 'sale' versions of the T-38, F-106, F-104 and others.

Finding and converting an Starfighter might be an good option as it's a bit bigger than the D-21 and if suitably braced you might be able to mount "mini-SABRE's" in place of the wingtip tanks and utilize the full capacity of the fuselage.

IIRC HMX was involved with MIPCC and said they could get their hands on a couple of F106's, which had a pretty big weapons bay. They reckoned with upgraded leading edges it could  hit M5. Junking all that 1950's era SAGE computer hardware should've lightened it up considerably.

I think this is the issue with adapting existing designs. No existing design (except the X-15, X37b and Shuttle) were designed to fly above roughly M3.5.  True they wouldn't have to last long (for engine and trajectory test get up to the right altitude/speed, switch over and get stable combustion in rocket mode. 10s of seconds after transition?) but it's a gamble. And you'd probably need a pilot as well.

Of course having said all the above IF the various contractors can find someone to 'pay' for it most of them (Boeing, BAE, etc, frankly probably everyone BUT REL ) might prefer to 'build-from-scratch', especially if that 'customer' is a government. And while I DO agree the initial test vehicle won't have an obvious or "built-in" Military Operational Capability and disagree with Star One that there is any "obvious" bias towards having such in a "test" vehicle simple because who's interested and who's building it the fact is such a vehicle 'could' have a secondary purpose IF built to certain specifications. It's that last part that will be telling because I recall that the original (AF driven) specifications for the X-33 program were in fact quite interesting, especially given the launch and landing locations chosen for the program.

Agreed.  But now you have 2 goals.
1) Build a vehicle that can test the engine/inlet over its planned Mach and altitude range
2) Design it as a pre-production prototype for an actual vehicle with a specific purpose.

One of the ways APD said they cut development costs by 1/3 was not to include support for things like repairability or maintainability.  The language is quite important. It's a demonstrator, not a prototype.
The X-33 programme should have taught people what happens when you make one vehicle do both. You end up with no vehicle and no improvement in your ability to design one, unless you count "Designing multi lobed conformal composite LH2 tanks is hard." Did it really need a $1.5Bn programme to "discover" this?

OTOH setting aside some resources in the design (details TBD in discussion with interested parties) would give a resource other groups could use as a "flying laboratory" once it'd proved SABRE can do what they claim it can do.
REL recovers some costs, the groups get access to the first reusable hypersonic test vehicle in 50 years. Everyone wins.

(Launch from Edwards AFB towards a facility in Utah, Speed in excess of Mach-12 and 'several hundred pounds' of "test instruments" in a long narrow bay... At the time it was noted by several people the 'test instrument bay' could hold a STAR solid motor and a microsatellite as 'payload' after all )

And while Star One is kind of obsessed with "hypersonic strike" missions the ACTUAL most likely mission is frankly as an advanced D-21 system for a reconnaissance drone. (Any 'weapon' has to be deployed from the inside of the vehicle mind you and THEN transition through a Mach-5+ shockwave AND still remain aimed at the target all of which is VERY difficult. And since it can't mass more than 2,000lbs at most it has to be highly accurate so obviously guided and everything has to be able to stand up to hypersonic speeds since that's when it's launched. As we've not developed any that work yet...) And for that you'd need specialized sensors, environmental conditioning equipment, (hypersonic speeds remember) power and others which will amount to something on the order of several hundred to maybe a thousand pounds. (Remember also you're flying at hypersonic speeds at almost 100,000ft so "OTS" sensor won't work) And then there's the 'range' question. The D-21 had a range of over 3,000 miles while modern UAV's have ranges from under 200 miles to over 14,000 miles but using LH2 or Liquid Methane there would be no opportunity for air-to-air refueling, (and transferring cryogenic fluids has been shown to have issue and that's before the operational problems with working with the stuff in bulk) and internal storage and insulation, (hypersonic again) issues abound for a smaller airframe.

Of those the sensors thing might be got round by data transfer from reconnaissance satellites but that doesn't crack any of the other issues. 14 000 miles but at what speed? I know weather survey drones have been designed for at least 12-24 hour endurance but those things have propellers. The D-21 was not in flight refueled either and a 20tonne thrust engine (44 000lbf) would be quite a lot larger vehicle. Protecting any payload is going to be kind of tough (you know some of one of those propellants is going to a cooling package for the the equipment at some point). Making it deploy something in mid flight is going to be a royal PITA.

Still the 'customer' has to be very upfront about such and willing to pay for it. Sensors, weapons bays, internal fuel storage will all have to specified UP FRONT so they can be included in the design as there won't be any way to 'retrofit' them once the vehicle is built. And all this has a very real possibility of not only the actual vehicle or engine not performing to specifications but that the 'added' requirements don't themselves cause the vehicle to fall short of requirements.

And it wouldn't be the first time that's happened either. 

The "demonstrator" becomes essentially a "prototype" and the costs go up 10x (or more)?
I really hope REL management don't go down this road.  It ties so much that is completely irrelevant to the engine/inlet demonstration.  Fly the engine over as much of the trajectory as possible. Show it works with the inlet design. Show it can do AB/rocket transition to stable combustion. All else is nice to have.

Since the main 'question' is (obviously) does the SABRE live up to expectations AND if so what are its actual performance metrics IN FLIGHT I have very high confidence that no one will be willing to pay for anything likely to be 'useful' till after all that data is in. Now something 'based' on the FTV could eventually be pitched but keep in mind there will be certain and strict requirements that have to be met along the way.

I hope not, but aerospace contractors can be very persuasive when they have something that could (sort of) plausibly meet a long held desire. That's basically how NASP sank close to $3Bn. The PI told a really  good story. Which in the end was exactly what it turned out to be. 

The most likely outcome is the FTV will be (as suggested by the "usual" aerospace contractors such as Boeing, BAE, etc) an expendable "test" vehicle on the line of the X-45/47 with each vehicle pushing the performance envelope along a series of 'goals' over the program. It's typical of such test programs today so it won't be either unexpected nor vastly difficult to pitch. On the other hand JS19 has a point that making it 'reusable' may in fact be both the better and 'simpler' option given the minimum size needed. But even a conversion of an existing airframe is going to be expensive and a 'scratch built' one probably out of the question. But you really DO want reusable despite the 'cost analysis' tending towards expendable since what you REALLY want to do is get data from the full spectrum of flight operations rather than just selected 'segments' which might induce errors or miss issues.

I wonder if anyone's counted up how many of those hypersonic programmes demonstrators there have been, along with
a)How many of them failed in flight with little or no data gathered?
b)How many of the programmes delivered all the data they promised by the end?

Expendability means everything on every flight is a one-shot deal. That sounds a great deal for the contractors, not so good for the customers.

One of SABRE's goals is T/W ratio of about 14:1, about 7x (or at least 3.5x)better than any SCramjets I'm aware of. How does that affect the reusability/expendabilty calculation?

I came across a NASA study (didn't copy it) that had a diagram of a M8 passenger aircraft (SCramjet naturally) and its temperatures. Most of it was 800-1000c with the nose at 2200c.
Now where can REL find a jobbing shop that does Titanium, superalloys or ceramics?

Star One wrote:

Every other discussion I've seen regarding this recent deal online, and other past developments leading up to this outside of this forum has been in terms of its military application.

Actually every time "hypersonics" is mentioned or written about the 'subject' turns to military applications which is different than what you're implying. Simply put, inserting "military applications" is a standard way to pad a subject that "might" actually have "military applications" whether the actual work being done IS directed towards that goal or not. Hypersonic flight has been specifically 'tied' to proposed "military applications" since the 1950s but actual versus assumed applications have been severely lacking and this is no different. SABRE, (which is the whole point of any test vehicle) is another, albeit rather better thought out, propulsion system that can possibly be used to push a vehicle to hypersonic speeds. Since it is possible for any vehicle to be used for 'military purposes', (and therefor used to pitch money from either the government or the military) such 'padding' is always inserted no matter if the actual application doesn't fit the suggested concept.

Let's take a look at the ACTUAL "military applications" of the SABRE engine:
1) It can be used to power a booster vehicle to launch expendable or reusable upper stages for vastly cheaper than current launch costs.

Now 'suggested' applications tend to be:
a) It can be used to power a bomber/fighter/recon aircraft flying at the edge of space and hypersonic, (Mach-6 to -10) speeds!

Actually no since a 'fighter' by definition needs a propulsion system that can allow it to do its job which is engage and destroy enemy aircraft and flying at 100,000ft and Mach-6 to Mach-10 you can't see, identify, lock-onto and engage a target with any reasonable chance of success. Similarly a 'bomber' needs to find, identify and engage its target which while not moving, (generally in fact the main 'purpose' of a hypersonic bomber is supposed to be the ability to reach a target area before a MOVING target can move outside its engagement area) and destroy it. Anyone that thinks that a platform moving as hypersonic speed at an altitude of 100,000ft plus can do this 'easily' is sadly out of touch with the reality of weapons technology. (Or trying to get money which pretty much covers the majority of sources for such suggestions) So that leaves the reconnaissance role which actually has possibilities as long as you ignore the rather obvious problems with a super-fast, super high altitude very "visible" (both to radar and basic IR sensors) target that while it might spot targets that would normally avoid predictable satellite passes or low and relatively slow "normal" aircraft is both vulnerable and restricted on what information it can gather. How can something flying so high and fast be 'vulnerable'? It is flying 'high' so again it's a LOT more visible than something flying very low and very slow so its chances of being spotted are vastly higher and unfortunately even 100,000ft is not 'low' enough to "hide" behind the curvature of the Earth as has often erroneously been suggested. Barring flying against someone with only the "Mark-One Eyeball" you WILL be spotted and tracked. You're also vulnerable in that neither the speed or altitude are immune from aggressive interception.

Further the SABRE cycle due to the inclusion of a rocket motor is vastly inferior for ANY hypersonic mission of any of the type given except launch vehicle when compared to vastly better cycles such as the Scimitar or other 'turbine' rather than 'rocket' based cycles. The 800-pound gorilla in the room everyone who focuses on 'military applications' for the SABRE is the fact it has ONE possible application and ONLY one: Launch Vehicle.

So therefor, (it should be quite obvious) if the FTV is planned to use SABRE cycle engines then it actually has one 'possible' application and most likely it will therefore be used to PROVE the SABRE cycle itself and not some way to 'sneak' a military drone into production.

I do note that SABRE 4 lets you run air in the pre burner and has separate air breathing and rocket combustion chambers. In principle you can do air breathing flight exclusively, but I always thought Scimitar was more "tuned" to the air breathing role.

BAE are the steering force now in this, their actual percentage is pretty irrelevant but it's big enough to do what they want to do. Also why do you think they are working with DARPA in the US, they only have one purpose and it sure isn't civilian.

Actually DARPA does in fact do a lot of projects that while they 'may' have military applications in the future can and have found civilian applications in more near term time frames. Self-driving vehicles is one good example as that started as a DARPA sponsored program but was rapidly embraced and improved upon by civilian agencies. Also while DARPA is sponsoring some of the work the actual main interest is from the Air Force Research Laboratory which is specifically tasked with 'long term' research and not procurement or operations. This is on purpose because the last time DARPA tried to 'shortcut' a research program it not only failed to get to flight testing it failed to reach the level of 50 year old research and development that the DARPA researchers didn't know had already been done! (RASCAL and MIPCC) AFRL was one of the agencies that pointed out the cost models for the program were significantly lacking in basic data while the proposed 'research' areas had already been done and the suggested 'vehicle' was in no way a 'research' or 'test' model but a clear 'operational' vehicle with which DARPA was attempting to bypass standard procurement and contracting procedures. (Which in fact they were doing)

Good point. I'd never considered RASCAL from that PoV.

https://www.telegraph.co.uk/business/2018/04/12/reaction-engines-secures-boeing-rolls-royce-backing-hypersonic/

Outside of that there's plenty of applications for the technology fully outside of the aviation field and I imagine they'd be both easier and quicker to exploit than a space vehicle. After all a lot of investors these days are looking for the quick return or the shortest route to a return which this seems to offer here.

Nice of you to point out another article that fully and totally misses the main 'point' of its own information in order to pad the word count with nonsensical and non-relevant subjects. Point of fact where in that article can you find ONE "application" for the SABRE OTHER than as a launch vehicle? That is after all the ONLY application that is suggested or implied by those quoted in the article. Supersonic and hypersonic flight is 'suggested' as something that can be 'derived' from the SABRE cycle in the future a number of times but any connection with the SABRE FTV is inferred and not explicit by anyone quoted in the article. Wonder why that is?

It's because the SABRE is not suitable for either supersonic or hypersonic "flight" and those being quoted are WELL aware of this fact. The engine that would power supersonic or hypersonic aircraft, (note not "spacecraft" which is what the SABRE is stated to be used for in the article) is the Scimitar which is optimized and designed for just such applications and does the job VASTLY better than the SABRE.

I think one thing that people seem to ignore is due to its nature SABRE powered vehicles simply can NOT fly from ANY 'standard' airport anywhere in the world. Not even 'lightly loaded' or 'partially fueled' and REL has pointed this out several times. It has nothing to do with runway length or loading or any of the other "operational" issue that have been discussed but directly due to the fact it uses a rocket engine in its design. It can't fly from an standard airport because it is impossible for it to meet the noise regulations of any standard airport. Period.
Scimitar CAN do so and is specifically designed to do so and meet ALL regulations and guidelines.

A lesson the developers of Concorde learned the hard way. Only air bases might tolerate it, and that's got to be doubtful. 

And lets discuss the applications outside the aviation industry which can be so lucrative...

Point of fact REL has a nice heat exchanger technology but as was pointed out back when they were trying to hype it, (and SABRE) up to get investment it is highly specialized and has few if any applications outside the ones REL has in mind. Granted if they exist you can be sure BAE will exploit them but really that has nothing to do with the thread. Still if you'd like to list them we can disassemble them again I suppose.

Phillip Clark wrote:

Has anyone considered asking Reaction Engines about this? I am sure that Alan Bond would tell, if he's able to.

And deprive ourselves of the possibly of vastly speculative and argumentative postings? What are you? Mad? I mean look how boring and mundane BFS turned out to be compared to our ideas and concepts, come on loosen up some

Ho, ho.   

Some additional comments on the D21 design:
Note the LOX tank was wrapped around the 'duct' not the actual engine. Even at Mach-6 the heating wasn't going to be really 'bad' and there was insulation. But in any case, (Liquid Methane or LH2) is going to require a larger tankage and not be compatible with 'wet-wings' normally. (You have to admire the way they got 'away' with "wet-wings" cyro-LOX in the Star-Raker design though: http://www.alternatewars.com/SpaceRace/Star_Raker/Star_Raker.htm
https://motherboard.vice.com/en_us/article/ezvj4j/the-747-to-space-that-never-was
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790010900.pdf (page 73)

What a beast. 

Unfortunately that's the sort of vehicle that makes REL's claims and goals that much harder for skeptics to believe ("doubters" won't believe them anyway). 

Having said that since you don't actually NEED to put propellant in the wings, and at hypersonic speeds "wings" aren't really needed there would be tendency to move towards a lifting fuselage body similar to the "waverider" and advanced aircraft we've been working on the last 50 years. Now with that as pointed out with the D-21 you DO need wings at lower speeds so unlike NASP and some of the more extreme designs you want good low speed landing wings. On the other hand if you don't assume that you have to squeeze every ounce of efficiency out of the propulsion, (Skylon doesn't, whereas most of the others DO and therefore the engines and fuselage are designed to synergistically support each other) then you can consider other options for engine placement. (I should note the ENTIRE fuselage is synergistically used so that the forward body helps compress and ingest the air while the after body helps align and expand the exhaust for more efficient operation)

As NASA noted, plume heating is one of the "known unknowns."
Yes, it would be nice to get a jump on scoping how serious an issue (is it an issue?) this is.
But for now just getting something into the air seems adequately ambitious enough to me.

Again this isn't 'lazy' on REL's part but it greatly simplifies the figures you need to play with since with the engines on the wingtips reduces some of the aerodynamic and weight-and-balance problems. (Of course you get a similar effect without the "engine-out" issues by putting the engine near the center of the airframe, again which is common on engine/body designs) I suppose the main question is how small can they make a SABRE engine?

This being a UK project I think the question is "how small can they make a SABRE engine affordably?" 
The recording of the BIS meeting from the head of the TF1 test stand project said the test engine is basically a SABRE 4 engine but with "1 of everything."
previous posters on that thread mentioned that SABRE is roughly a 4 segment pre-cooler, dual LOX pump, dual LH2 pump engine.

So logically the test engine will be 1 pre cooler segment, 1 LOX, 1LH2 pump. The LH2 pump is from the Ariane programme, but REL remain coy about wheather it's from a Vulcain or a Vinci engine. The former is way too big for the test stand (and still too small for the full Skylon sized SABRE) and the latter is a little undersized, but probably has enough margin to cope.

The implication is you just need to make multiple copies of those parts to upscale to a full Skylon sized SABRE. The joker is the LHe circulator which they describe as OTS, so probably too heavy for flight.   There aren't too many uses for these outside of a)Air separation plants and b) Some high temperature nuclear reactors. By the end of the test programme I expect they will have a  much better idea of what the issues of building one in house will be to meet the Skylon weight budget.

[Katana]

The X7 airframe is great to begin with: simple and straightforward to mount a test engine, optimized for top speed at M4+ (even with aerodynamics of 1950s).

X7B have dual engines below wigs, resembling subsequent operational BOMARC missile with the same Marquardt RJ43 ramjets. BOMARC is optimized for balance between speed, range, and climbing ability after ground launch.

Eventually came the D21 with a special version of RJ43 optimized for long range cruise. The reasons for D21 to have a SR71 style flat lifting body are basically range and radar stealth, not top speed.

Having LH2 tanks around the engine is seeking trouble for a test vehicle.

[john smith 19]

The X7 airframe is great to begin with: simple and straightforward to mount a test engine, optimized for top speed at M4+ (even with aerodynamics of 1950s).

More importantly a place to put a big LH2 tank which is not a funny shape.

X7B have dual engines below wigs, resembling subsequent operational BOMARC missile with the same Marquardt RJ43 ramjets. BOMARC is optimized for balance between speed, range, and climbing ability after ground launch.

No. Those are the booster rockets to get it to ramjet operating speed.

Eventually came the D21 with a special version of RJ43 optimized for long range cruise. The reasons for D21 to have a SR71 style flat lifting body are basically range and radar stealth, not top speed.

True. Also the JP7 was fairly easy to store in any shape of tank.  LH2 is less forgiving.

Having LH2 tanks around the engine is seeking trouble for a test vehicle.

It seems an awful lot of work for the ability to inherit what is at best only a partial aerodynamic database (especially if you want to reuse the vehicle).

[john smith 19]

To give some idea of the sort of information people might want to get from a FTV this is a description of the 
X-15's telemetry setup.

It would be interesting to find out how many of the questions the X-15 collected data on are still relevant today. 

Obviously the FTV's goal is primarily to flight test the SABRE engine and its inlet, so I wouldn't expect it to have anything like the 590Kg (1300lb) allocated to flight test instrumentation the X-15 had. Most of this was prepared away from the aircraft and mounted onto an "elevator" pallet for installation into the instrument bay.

OTOH it is true that such hardware has become much smaller and lighter in the half century since the X-15 last flew. Although I'm not sure if a combined pressure and temperature sensor operating over the needed range exists. For this environment I think Low or High Temperature Cofired Ceramic would be a better technology than Silicon based MEMS. No one is going to be doing multi channel recording by deflecting light beams with mirrors onto film stock, when you can record onto a 32256GB micro SD card today.

As for its construction one technique I'd not seen to help construct leading edges is the notion of prestressing them,  although it never seems to have been tried in practice it looks like quite a good way to save mass, especially given that carbon fibre was still a lab curiosity when the report was written.

Other potential design features would be making the body as a Sears Haack shape for minimal drag (as Skylon does) and possibly use winglets to lower drag further (as they won't have to withstand full reentry).