Reaction Engines SABRE engine (Skylon Spaceplane)

[steelpillow]

@ Zootycoon
The air entering the compressor is pre-cooled by a helium cycle, I assume you are referring to the H2 which cools the used He via the secondary heat exchanger? That would make sense.

I appreciate your comments about the Mach number, however I suspect you err too far the other way. The quote says Mach 5, so it is taking the transit time from the Mach 5 entry point, through the shock compression steps and then on through the heat exchanger. Even if you call that say 5 metres in length, the number is still out by a factor of 10. The speed of sound, the local Mach number, depends heavily on temperature. As you say, the deceleration is not linear but mostly takes place in a series of steps before reaching the HTX. But these heat the gas, increasing the absolute value of Mach 1, so a local near-Mach 1 flow of superheated air is travelling faster than you might think, perhaps 50% faster, when it hits the HTX. Then again, a local near-Mach 1 flow of supercooled exit air is correspondingly slower than normal, approximately halved. So the deceleration is not as non-linear as your arguments might suggest and there is still a factor of 3 or so to account for. Close enough to be lost in the small print, perhaps.

 

[Zoo Tycoon]

Yes, LH2 is used to cool Sabres air charge via the He loop. One reason for the He loop is to ensure the LH2 is fully isolated from the air because the diffusion rate of H2 through the very thin HTX nickel tubes is such that, if it diffused into air, could lead to down stream flammability issues. (This was seen as a Showstopper for Rolls Royce with the RB545 .......until RE figured out a way to fix it)

As for cooling times Reaction Engines website advises;-

“ These are capable of cooling airstreams from over 1,000°C to -150°C in less than 1/20th of a second with world leading compactness and low weight.”

This is completely accurate and is subtly different to the BBC claim.

Another consideration in slowing the air velocity is the 90 degree turn required to enter the HTX. It’s a general rule of thumb that a media will lose velocity, maybe up to 50% depending on the abruptness of the turn. Another little trick in making this architecture work.

 

[steelpillow]

Another consideration in slowing the air velocity is the 90 degree turn required to enter the HTX. It’s a general rule of thumb that a media will lose velocity, maybe up to 50% depending on the abruptness of the turn. Another little trick in making this architecture work.

Usually, a 90 deg turn that sharp loses velocity by creating turbulence and that is just wasted energy. It would indeed be a neat trick if the HTX micro-tubes straightened it out again and recovered that energy, but they would have to perform that little bit higher to do so. Really, it's just the Supermarine Spitfire radiator inlet on a larger scale.

 

[Flyaway]

From https://www.theengineer.co.uk/leading-rolls-royce-polishing-jewel/

”Back with propulsion technologies, Rolls-Royce recently announced that it was among a group of investors taking stakes in Reaction Engines, the Oxfordshire-based company developing an air-breathing hybrid engine that can act like a jet in atmosphere and a rocket in space. This is an example of technology going full circle, Stein commented, as technology originated with thermodynamicist Alan Bond as part of a project called Hotol some 30 years ago. The company is working with Reaction Engines on developing “quite a large number of technologies,” Stein said."

 

[Rhinocrates]

Regarding energy lost from incoming air with that 90 degree bend, as far as I understand SABRE is best understood as a temporarily air-breathing rocket, not a jet. It doesn't need air rushing straight through it, it needs LOX coming through pipes into a rocket combustion chamber. As a rocket, it's horribly heavy and complicated and as a jet it's horribly inefficient, but for a spacecraft it's meant to be lighter and simpler than having to carry both. Even as a 'mere' hypersonic vehicle or 1st stage of a 2STO, it's able to exit the atmosphere, unlike a scramjet-powered vehicle.

What interested me about the BAE Systems concept I've linked to upthread is that it has the intake and LOX-extraction plant in the nose and the rocket bits in the tail with the fuel and payload in the middle. Rather than a duct running nose to tail, it would have plumbing.

 

[Flyaway]

Farnborough International Air Show 2018

Reaction Engines will be in Hall 4 and the Space Zone in Hall 1!

Farnborough International Airshow is a week-long event that combines a major trade exhibition with a public airshow.

https://www.reactionengines.co.uk/farnborough-international-air-show-2018/

 

[steelpillow]

Regarding energy lost from incoming air with that 90 degree bend, as far as I understand SABRE is best understood as a temporarily air-breathing rocket, not a jet. It doesn't need air rushing straight through it, it needs LOX coming through pipes into a rocket combustion chamber. As a rocket, it's horribly heavy and complicated and as a jet it's horribly inefficient, but for a spacecraft it's meant to be lighter and simpler than having to carry both.

At risk of monotony, a sharp 90 deg bend is bad for any fluid flow under extreme and marginal conditions, liquid or gas. It can be avoided with an appropriate alignment of the HTX tubes, so it is a no-brainer. As I understand it, in air-breathing mode the oxidant is still gaseous, being cooled to just above its liquefaction point - otherwise it would not need a compressor stage! The main rocket chamber receives gaseous hydrogen from a pre-burner, but appears to be able to cope with both gaseous air and liquid oxygen, unless a vaporiser is not shown on the simplified schematic. Definitely not your normal liquid-fuel rocket chamber.

What interested me about the BAE Systems concept I've linked to upthread is that it has the intake and LOX-extraction plant in the nose and the rocket bits in the tail with the fuel and payload in the middle. Rather than a duct running nose to tail, it would have plumbing.

Where does a narrow duct end and large plumbing begin? Because the gaseous air is so chilled, it is well shrunk and the duct needed for it is so narrow it looks like a fat pipe or two. History teaches us that long ducts are bad, so I think a nose-to-tail engine is a bit fanciful. What with all the heat exchangers and turbopumps winding around everywhere, realistically the space between HTX and rocket chamber will be too busy for bulky stuff.

 

[Rhinocrates]

At risk of monotony...

Nope. I'm no expert, so the explanation is appreciated.

I think a nose-to-tail engine is a bit fanciful.

Admittedly it's just a concept. It would be interesting to know how serious it is and whether they've progressed to deal with the problems you point out, or whether they've made the tradeoffs that make it worthwhile in total.

 

[Flyaway]

A forum member on NASAspaceflight has noticed something interesting about this Tweet from REL, amongst the expected names in the partners list there is one that is unexpected namely Ariane Group.

https://twitter.com/ReactionEngines/status/996361621130956801?s=20

 

[FighterJock]

A forum member on NASAspaceflight has noticed something interesting about this Tweet from REL, amongst the expected names in the partners list there is one that is unexpected namely Ariane Group.

https://twitter.com/ReactionEngines/status/996361621130956801?s=20

Was not expecting that Ariane Group to be one of the backers of Skylon, especially when they are mostly involved in rocket production such as Ariane 5.

 

[TomcatViP]

Ariane lost on the ground of the Skylon[otaur]… Ouch ouch.

 

[Flyaway]

A forum member on NASAspaceflight has noticed something interesting about this Tweet from REL, amongst the expected names in the partners list there is one that is unexpected namely Ariane Group.

https://twitter.com/ReactionEngines/status/996361621130956801?s=20

Was not expecting that Ariane Group to be one of the backers of Skylon, especially when they are mostly involved in rocket production such as Ariane 5.

Especially when they are the midst of developing Ariane 6.

 

[martinbayer]

A forum member on NASAspaceflight has noticed something interesting about this Tweet from REL, amongst the expected names in the partners list there is one that is unexpected namely Ariane Group.

https://twitter.com/ReactionEngines/status/996361621130956801?s=20

Was not expecting that Ariane Group to be one of the backers of Skylon, especially when they are mostly involved in rocket production such as Ariane 5.

Especially when they are the midst of developing Ariane 6.

Never hurts to hedge your bets.

Martin

 

[FighterJock]

A forum member on NASAspaceflight has noticed something interesting about this Tweet from REL, amongst the expected names in the partners list there is one that is unexpected namely Ariane Group.

https://twitter.com/ReactionEngines/status/996361621130956801?s=20

Was not expecting that Ariane Group to be one of the backers of Skylon, especially when they are mostly involved in rocket production such as Ariane 5.

Especially when they are the midst of developing Ariane 6.

Never hurts to hedge your bets.

Martin

Of course we won't know anything about the possible success or failure of Skylon/Sabre until it gets to the point where we get actual hardware developed and tested in flight. :-\

 

[martinbayer]

A forum member on NASAspaceflight has noticed something interesting about this Tweet from REL, amongst the expected names in the partners list there is one that is unexpected namely Ariane Group.

https://twitter.com/ReactionEngines/status/996361621130956801?s=20

Was not expecting that Ariane Group to be one of the backers of Skylon, especially when they are mostly involved in rocket production such as Ariane 5.

Especially when they are the midst of developing Ariane 6.

Never hurts to hedge your bets.

Martin

Of course we won't know anything about the possible success or failure of Skylon/Sabre until it gets to the point where we get actual hardware developed and tested in flight. :-\

That's why it is a bet ;D.

 

[Flyaway]

REL have given their site a revamp.

https://www.reactionengines.co.uk/sabre/core-engine-demonstrator

https://www.reactionengines.co.uk/sabre/htx

https://www.youtube.com/watch?v=bIO_B40g03U

 

[SteveO]

Hmmm... can't see Skylon on the new website! Maybe it's time to rename this thread or start a new one and call it "Reaction Engines Ltd - Synergetic Air Breathing Rocket Engine (SABRE).

I started this thread nearly 11 years ago as a die hard Skylon fanboy who hadn't got around to reading all his books on space exploration and rocket development. Now I'm a little bit wiser on the technical difficulties and economics of single stage to orbit and full reusability it makes sense that Reaction Engines is concentrating on SABRE for multiple applications and seems to be making good progress.

It's great to see SpaceX leading the way with making space access more affordable. However, Skylon still looks awesome

 

[FighterJock]

Hmmm... can't see Skylon on the new website! Maybe it's time to rename this thread or start a new one and call it "Reaction Engines Ltd - Synergetic Air Breathing Rocket Engine (SABRE).

I started this thread nearly 11 years ago as a die hard Skylon fanboy who hadn't got around to reading all his books on space exploration and rocket development. Now I'm a little bit wiser on the technical difficulties and economics of single stage to orbit and full reusability it makes sense that Reaction Engines is concentrating on SABRE for multiple applications and seems to be making good progress.

It's great to see SpaceX leading the way with making space access more affordable. However, Skylon still looks awesome

Too true about Skylon, when I first saw it I could not stop looking at it.

 

[steelpillow]

The revamp finds space for LAPCAT:

"During a 2008 EU-funded study called LAPCAT I, Reaction Engines found that a SABRE variant named Scimitar would be capable of powering a vehicle at cruise speeds of up to Mach 5. This was the first major government research contract and was worth €2 million. The LAPCAT I contract for the pre cooler represented the first opportunity to demonstrate its world-beating heat exchanger design and manufacturing capabilities."
https://www.reactionengines.co.uk/about/story-so-far

It does look as if Skylon has been airbrushed out. That NASA analysis can't have pulled its punches!

This looks interesting:
https://www.reactionengines.co.uk/application/files/4515/3080/7680/2018-07-05_17-21_364.jpg

 

[Archibald]

Hmmm... can't see Skylon on the new website! Maybe it's time to rename this thread or start a new one and call it "Reaction Engines Ltd - Synergetic Air Breathing Rocket Engine (SABRE).

I started this thread nearly 11 years ago as a die hard Skylon fanboy who hadn't got around to reading all his books on space exploration and rocket development. Now I'm a little bit wiser on the technical difficulties and economics of single stage to orbit and full reusability it makes sense that Reaction Engines is concentrating on SABRE for multiple applications and seems to be making good progress.

It's great to see SpaceX leading the way with making space access more affordable. However, Skylon still looks awesome

don't be too harsch with yourself. Unlike so many paper projects, Skylon got a positive assessment by ESA some years ago. So it was - and still is - a little more than hype.

 

[Flyaway]

https://twitter.com/ReactionEngines/status/1059424468039335936

http://www.imeche.org/news/news-article/the-great-british-space-age-tackling-the-satellite-backlog

One concept, the Skylon spaceplane, has been “a little bit of a distraction” in the public eye from the company’s engine development, says a spokesman. The firm is considering “a number” of concepts, and will speak to vehicle developers after testing of Sabre.

 

[Archibald]

Dang. So REL is now a little wary about all the hype about their beloved Skylon ? I know many people at NASAspaceflight.com that gonna be quite heartbroken. ;D

 

[Flyaway]

ESA completes further design validation of Reaction Engine's revolutionary air breathing SABRE rocket engine

The development programme of the world’s first air-breathing rocket engine has taken an additional significant step forward, which will lead to major testing milestones being undertaken within the next 18 months.

Reaction Engines has received further endorsement of its revolutionary SABRE[emoji769] engine design via its collaboration with ESA and the UK Space Agency (UKSA). The two agencies recently reviewed the preliminary design of the demonstrator engine core of the Synergetic Air-Breathing Rocket Engine (SABRE), which Reaction Engines will use to undertake ground-based testing at its under-construction test facility at Westcott, Buckinghamshire.

 

[Nigelhg]

https://www.bbc.co.uk/news/science-environment-47832920
New update on testing in the States.

 

[steelpillow]

Nice graphic clearly showing the outer bypass duct. Very reminiscent of the J58 installation in the Lockheed Blackbirds. Presumably current thinking is to have a multi-cycle engine: air-breathing rocket for low speed/altitude, hybrid ramjet for ca. Mach 2-4 and high altitude, rocket for Mach 5+ and space.

If I was them, I might install a drop tank with a little LOX to shorten the takeoff run.

 

[Flyaway]

Mon, 2019-04-08 08:06
A key element of the revolutionary SABRE™ air-breathing rocket engine successfully passes the first phase of high-temperature testing. Precooler technology will enable a wide variety of high-speed flight and advanced propulsion systems.

Reaction Engines’ precooler heat exchanger successfully achieved all test objectives in the first phase of high-temperature testing designed to directly replicate supersonic flight conditions and future tests are planned at even higher temperatures. The precooler is a key element of Reaction Engines’ revolutionary SABRE engine and is a potential enabling technology for advanced propulsion systems and other commercial applications.

The ground-based tests saw Reaction Engines’ unique precooler successfully quench the 420°C (~788°F) intake airflow in less than 1/20th of a second. The intake temperature replicates thermal conditions corresponding to Mach 3.3 flight, or over three times the speed of sound. Mach 3.3 matches the speed record of the SR-71 Blackbird aircraft, the world’s fastest jet-engine powered aircraft produced to date and is over 50% faster than the cruising speed of Concorde.

In the recent tests, the compact precooler achieved all test objectives and achieved 1.5 MW of heat transfer, the equivalent to the energy demand of 1,000 homes; successfully cooling incoming air from a temperature at which hot steel starts to glow. The tests are the first phase in an extensive test programme which will see the precooler test article (HTX) exposed to high-temperature airflow conditions in excess of the 1,000°C (~1800°F) expected during Mach 5 hypersonic flight.

The significant testing milestone occurred at Reaction Engines’ recently commissioned TF2 test facility located at the Colorado Air and Space Port, US. The TF2 test facility has been constructed by Reaction Engines to undertake ground based ‘hot’ testing of its precooler technology. The technology has already passed an extensive range of tests in the UK where its performance was fully validated at ambient air temperatures.

Commenting, Mark Thomas, Chief Executive, Reaction Engines, said:

“This is a hugely significant milestone which has seen Reaction Engines’ proprietary precooler technology achieve unparalleled heat transfer performance. The HTX test article met all test objectives and the successful initial tests highlight how our precooler delivers world-leading heat transfer capabilities at low weight and compact size. This provides an important validation of our heat exchanger and thermal management technology portfolio which has application across emerging areas such as very high-speed flight, hybrid electric aviation and integrated vehicle thermal management.”

To replicate the conditions the precooler will experience at hypersonic speeds, the TF2 test facility uses a General Electric J79 turbojet engine formerly used in a McDonnell Douglas F-4 Phantom aircraft to provide high-temperature airflow. Engineers at Reaction Engines’ Culham headquarters constructed the HTX precooler test article and after initial testing it was shipped to Colorado at the end of 2018, and ‘hot’ tests commenced in early March 2019.

In addition to the hot precooler tests being conducted in the US, Reaction Engines is in the final stage of constructing its TF1 test facility at Westcott, Buckinghamshire, UK, where it will undertake ground-based testing of a SABRE engine core. Over the last four years Reaction Engines has raised over £100m from public and private sources and has secured investment from BAE Systems, Rolls-Royce and Boeing HorizonX.

For more information please contact:
Richard Webster-Smith
Finsbury (communications agency) ReactionEngines@Finsbury.com +44 (0)20 7251 3801

Lorraine Fry
Head of Marketing and Communications, Reaction Engines UK
Lorraine.Fry@reactionengines.co.uk +44 (0) 7827 989 536

Adrian Tansing
Communications Manager, Reaction Engines Inc.
Adrian.Tansing@reactionengines.com +1 970-214-6406

Or visit the Reaction Engines website: www.reactionengines.co.uk
Images of the TF2 test site and HTX test article are available for download here.[/quote]

https://www.reactionengines.co.uk/news/reaction-engines-test-programme-successfully-proves-precooler-capability-supersonic-heat-conditions

 

[Flyaway]

EUCASS 2019 | 1 – 4 July 2019 | Madrid, SPAIN – 8th European Conference for Aeronautics and Space Sciences

www.eucass2019.eu

REL's papers:

Our first paper will be on Monday 1 July 2019 at 17:10.

Titled: SABRE Technology Development: Status and Update. Paper number: 307.

Our second paper is jointly presented with CNES and Onera and will be on Wednesday 3 July 2019 at 09:20.

Titled: Conceptual Design of a two-stage-to-orbit vehicle using SABRE and a low-cost LOX/CH4 Upper Stage Engine. Paper number: 454

 

[steelpillow]

Very interesting to see the move from SSTO Skylon to two-stage as the options crystallise. However in my naive way I am surprised that SABRE should power the first stage rather than the second. If the Shuttle could lug a useless third engine all the way into orbit, why can't a SABRE in space mode lug its useless air-breathing precooler system? When BAe moved away from SSTO they stuck HOTOL on top of the Antonv An--5 Mriya. What has changed? Want to read that paper!

 

[martinbayer]

Hello Steelpillow,

airbreathers are always more useful at the beginning of the ascent trajectory, since in a first order approximation, the specific impulse of any airbreathing propulsion scheme tends to taper off pretty much linearily as a function of flight velocity for any airbreathing engine concept, whereas it stays pretty much constant for rockets, so there is an optimized switch-off point from air breather to rocket for every realistic ascent trajectory optimization scheme.

Best wishes,

Martin

 

[Flyaway]

REL Press Release:

Reaction Engines participating in UK MOD - funded research programme to develop hypersonic propulsion systems :: Reaction Engines

The Ministry of Defence intends to place a £10M contract to develop hypersonic propulsion systems for increased aircraft performance and capability.

www.reactionengines.co.uk

 

[FighterJock]

REL Press Release:

Reaction Engines participating in UK MOD - funded research programme to develop hypersonic propulsion systems :: Reaction Engines

The Ministry of Defence intends to place a £10M contract to develop hypersonic propulsion systems for increased aircraft performance and capability.

www.reactionengines.co.uk

That is excellent news for Reaction Engines. Hope that accelerates the development and testing of the SABRE engine.

 

[RanulfC]

REL Press Release:

Reaction Engines participating in UK MOD - funded research programme to develop hypersonic propulsion systems :: Reaction Engines

The Ministry of Defence intends to place a £10M contract to develop hypersonic propulsion systems for increased aircraft performance and capability.

www.reactionengines.co.uk

That is excellent news for Reaction Engines. Hope that accelerates the development and testing of the SABRE engine.

It will help though it would be more directly towards LAPCAT than SABRE I'd think given the program goals. Still anything that advances the technology and moves REL forward.

Randy

 

[steelpillow]

airbreathers are always more useful at the beginning of the ascent trajectory, since in a first order approximation, the specific impulse of any airbreathing propulsion scheme tends to taper off pretty much linearily as a function of flight velocity for any airbreathing engine concept, whereas it stays pretty much constant for rockets, so there is an optimized switch-off point from air breather to rocket for every realistic ascent trajectory optimization scheme.

Yes indeed. That is exactly the condition which SABRE's switch to pure rocket mode is intended to meet. All the stranger why they should now abandon one of SABRE's key technical advantages.

 

[RanulfC]

airbreathers are always more useful at the beginning of the ascent trajectory, since in a first order approximation, the specific impulse of any airbreathing propulsion scheme tends to taper off pretty much linearily as a function of flight velocity for any airbreathing engine concept, whereas it stays pretty much constant for rockets, so there is an optimized switch-off point from air breather to rocket for every realistic ascent trajectory optimization scheme.

Yes indeed. That is exactly the condition which SABRE's switch to pure rocket mode is intended to meet. All the stranger why they should now abandon one of SABRE's key technical advantages.

One thing to note about what is considered the 'standard' Two-State-To-Orbit concept vehicle is that there has always been a 'point' which martinbayer eluded to where the "air-breathing" portion begins to lose effect but before the second stage 'rocket' portion becomes effective. In most concepts this means the first stage has to have a set of rockets of its own and must use them to bridge that operational gap. SABRE as you note would not have that issue and could therefor power through that segment at a higher effective overall rate.

The reason for not hauling extra mass to orbit is quite simple since that mass can now be used for payload and/or vehicle structure in the orbital stage. In the Shuttle example the 'useless' mass it dragged to orbit was FAR more than just a 'third engine' but in context it had to drag a lot of that mass to orbit and back simple due to the design. In other words the Shuttle would have greatly benifited from being a true TSTO rather than a hybrid 1.5STO vehicle and this was well known and understood but budgetary and operational restraints meant it was instead designed and built the way it was.

Similary Skylon and SABRE have been subject to real-world restraints that have moved it away from the SSTO model. Given that SABRE can operate at altitudes higher than 'standard' air-breathing engines can in an single engine the odds are quite good that it can be argued that it has enough utility to be progressed as a first stage multi-mode engine which is currently where interest is.

Randy

 

[Flyaway]

I am cross posting this article that has more info into this thread as well:

British military scrambles to speed up work on hypersonic engines, weapons

The future Tempest fighter aircraft could be in line for a high-Mach propulsion system.

www.defensenews.com

 

[steelpillow]

Thanks. RanulfC, you are correct as far as you go. However, SSTO projects used to be all the rage and they carry far more dead weight into orbit. Any kind of two-stage system is a great advance on them in the terms you describe. The original B.Ae concept when migrating HOTOL to a more realistic two-stage scenario was to use a subsonic mothership for the low-altitude, low-speed bit, where the demands on engine intake geometry vary dramatically from those at high Mach numbers. That really did relieve the Reaction Engines team of some notoriously intractable intake optimisation problems. That they now think it is worth grappling with those problems for the sake of lower orbiter mass is the bit I find interesting.

 

[RanulfC]

Thanks. RanulfC, you are correct as far as you go. However, SSTO projects used to be all the rage and they carry far more dead weight into orbit. Any kind of two-stage system is a great advance on them in the terms you describe. The original B.Ae concept when migrating HOTOL to a more realistic two-stage scenario was to use a subsonic mothership for the low-altitude, low-speed bit, where the demands on engine intake geometry vary dramatically from those at high Mach numbers. That really did relieve the Reaction Engines team of some notoriously intractable intake optimisation problems. That they now think it is worth grappling with those problems for the sake of lower orbiter mass is the bit I find interesting.

I recall how much of a 'rage' SSTO's were... Still are for some

IIRC the original HOTOL issue was not the intakes but the engine weight and balance which was all over the place due to the aft rockets massing more than the air-breathing segments. Interim-HOTOL simply dumped the air-breathing aspect but kept the rocket engines which is why Bond was so peeved since he, (rightly) pointed out that they could simply move the engines from the aft towards the more nominal CG/CP location. (On the other hand the aerodynamic and thermal loads were even more a bear which is why engine designers normally hsould not 'do' airframe design and why REL moved the engines to the wing-tips )

HOTOL used body compression and B-70-ish ramps to manage the supersonic/hypersonic airflow which was arguably more heavy and complex than a translating spike would be but that also comes back to the overall design too. Part of the simplicity of the Skylon aero-form is the engines and airframe don't interact but while that allows a rather simpler engine and intake design it's not as efficent or capable as an intergrated body-intake system. Really there's been a huge amount of work on intergrated body-intake systems for high speed flight so the actual work won't be nearly as tough I suppose.

And it's probably not 'just' Orbiter mass that will come about but the ability to deliver to higher and more diverse orbits as well which has always been the bugaboo for SSTO.

Randy

 

[steelpillow]

Not that moving the engines to the wing tips helped, as rocket exhaust flares sideways far more than jet exhaust and cooked Skylon's tail section. Then there are little things like the one-engine-out condition when too high up for the rudder to work. If the Space Shuttle can get the CG right with rear-mounted rockets, I don't see why B.Ae can't.

 

[Archibald]

Thanks. RanulfC, you are correct as far as you go. However, SSTO projects used to be all the rage and they carry far more dead weight into orbit. Any kind of two-stage system is a great advance on them in the terms you describe. The original B.Ae concept when migrating HOTOL to a more realistic two-stage scenario was to use a subsonic mothership for the low-altitude, low-speed bit, where the demands on engine intake geometry vary dramatically from those at high Mach numbers. That really did relieve the Reaction Engines team of some notoriously intractable intake optimisation problems. That they now think it is worth grappling with those problems for the sake of lower orbiter mass is the bit I find interesting.

Interim HOTOL (the one using the An-225) was not Alan Bond team but BAe, after the two went their own, separate ways (REL was funded in 1989).

 

[RanulfC]

Not that moving the engines to the wing tips helped,

It helped a great deal on the issue they were focusing on which was reducing the intake complexity

... as rocket exhaust flares sideways far more than jet exhaust and cooked Skylon's tail section.

While it needed to be better addressed than REL did the answer was beefing up the insulation and adding a heat resistant structure. One of the reasons for having a TSTO instead of an SSTO is that you can adjust the structures of both stages to optimized levels.

Then there are little things like the one-engine-out condition when too high up for the rudder to work.

That's a misson abort, throttle down to minimum of the working engine and heavy use of the RCS till you get low enough for the rudder to be effective and limp to the nearest airfield that can handle the vehicle. It was a risk but keep in mind the vehicle was designed to show off the engines and was a very conservative design given the parameters so it could afford some added structure and weight gain. Most other SSTO's can't.

If the Space Shuttle can get the CG right with rear-mounted rockets, I don't see why B.Ae can't.

HOTOL had issues because it had two engine systems in a way since the intake and heat-exchanger mass was forward, the Cargo Bay over the 'nominal' CG but coming down sans propellant and cargo the CG drifted to far to the rear which made the design far to touchy during landing. That was on the rockets which were put on the back as per designs like the Shuttle to avoid any issues with the vehicle structure and the exhaust. The 'easiest' fix for this was to remove the air-breathing system and run it as a straight rocket which is what B.Ae did much to Bond's disgust. Alternatly the idea of moving the intake and pre-cooler systems aft was looked at, (found this on wikipedia and it's an actually good configuration that I'd not seen before, https://en.wikipedia.org/wiki/HOTOL#/media/File:HOTOL.JPG) but the CG issue remained problematic. (In this case IIRC without cargo in the bay the balance was off whereas with it was just ok and since it would be without on most flights...)

But Bond was wrong too and it was always going to be unlikely that the CG/CP issue could be fixed using the HOTOL design as it was. Skylon 'fixes' the engine CG issues but as we've seen has issues of it's own but it's closer to a solution. Some of the suggested "alternate" Skylon designs shorten the fuselage aft of the engines and buff it up, some suggest moving the engines to different positions on the fuselage and REALLY beefing up the aft structure which would be used as an exhaust expansion ramp system. (Not unusual in the air-breathing community but less studied when using rockets. Tricky but doable, and likely easier on a first stage vehicle)

Personally I'm in favor of using the Beta I, Beta II, or Boeing TSTO designs as a starting point:

https://www.secretprojects.co.uk/threads/beta-beta-ii-spaceplane.5217/

TSTO spaceplanes | Boeing two stage to orbit - SSME orbiter | robotpig.net

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Randy