The Reaction Engines Skylon Master Thread (1)

[simonbp]

"Man-rating" likely doesn't matter as much as how easy-to-maintain the design is; SABRE is designed for a 2-day turn-around, something SSME has never even tried...

[Hempsell]

one further question:when in rocket mode, why not use the airflow coming into the sabre engine as reaction mass? I.e, air augmented thrust.

At the transition the air temperature is already at the "starting to get uncomfortable" level, if we continued to ingest air above Mach 5.2 we would start to damage the heat exchangers and other internal equipment.  It would also complicate the moving forebody which would how need to cover a wider range.

[sb]

[Deleted, poor cross-post]

[Seer]

one further question:when in rocket mode, why not use the airflow coming into the sabre engine as reaction mass? I.e, air augmented thrust.

At the transition the air temperature is already at the "starting to get uncomfortable" level, if we continued to ingest air above Mach 5.2 we would start to damage the heat exchangers and other internal equipment.  It would also complicate the moving forebody which would how need to cover a wider range.

Sorry, I don't mean take air in and slow it down, but pass it straight through to the combustion chamber or nozzle. Perhaps via a different inlet.

On a different matter, what is the chamber pressure of the sabre engine when in pure rocket mode, and why are there 4 rocket engines/nozzles per sabre?

[adrianwyard]

I certainly don't want to discourage Mark Hempsell from sharing as much info as possible here, but eventually we'll ask questions that touch on proprietary tech and will have to just guess like everyone else.

Seer: The design has shown four nozzles per Sabre for a very long time. Could the reasoning be as simple as redundancy? The hope is to transport people one day, and with engines off-axis on wing-tips, losing one big nozzle would surely be the end. But, perhaps you could recover from losing one nozzle by gimbaling the others to compensate.

[strangequark]

Sorry, I don't mean take air in and slow it down, but pass it straight through to the combustion chamber or nozzle. Perhaps via a different inlet.

On a different matter, what is the chamber pressure of the sabre engine when in pure rocket mode, and why are there 4 rocket engines/nozzles per sabre?

You're going to have to slow it down if you wanted to bring it into the combustion chamber. If you introduce a supersonic flow into the nozzle, you are asking for all kinds of messy shock interactions and flow separation. Ducting supersonic flow is non-trivial too.

I don't know the specifics on why Sabre has 4 thrust chambers and nozzles. However, it is easier to handle acoustic instabilities in a smaller chamber. So, having four chambers fed by one set of turbomachinery makes plenty of sense from that angle. Same thing the Russians do with the RD-170.

[Downix]

"Man-rating" likely doesn't matter as much as how easy-to-maintain the design is; SABRE is designed for a 2-day turn-around, something SSME has never even tried...

Not 100% true.  The Block III SSME was to have dramatically reduced turnaround time. They demonstrated the new the systems for this on three occasions between 2004-2005 before the program was shut down.

They added better monitoring so they could tell what components needed refurbishment and which ones did not, reducing the turnaround time dramatically by elimination of complete teardown ops for diagnostic. (Did you realize that less than 5% of any SSME needs refurbishment between flights, it's the time to diagnose and test each part which eats up the time, not the actual prep work)  It was to be paired with less expensive, more durable replacements for the nozzle and combustion chamber as well, boosting it's performance to 111%.

Never used in a production machine, sadly.  Columbia's loss shut down the Block III program.

[Alpha_Centauri]

I don't know the specifics on why Sabre has 4 thrust chambers and nozzles. However, it is easier to handle acoustic instabilities in a smaller chamber. So, having four chambers fed by one set of turbomachinery makes plenty of sense from that angle. Same thing the Russians do with the RD-170.

If I remember correctly I've seen it mentioned that SABRE has one set of turbomachinery per 2 combustion chambers so there would be 2 whole "engines" per nacelle.



Something I'm curious about is whether the performance of the D1's SABRE 4 engines include the use of altitude compensating nozzles like the one's that have been tested and if not, what kind of performance gain could be expected with it?

[simonbp]

I don't know the specifics on why Sabre has 4 thrust chambers and nozzles. However, it is easier to handle acoustic instabilities in a smaller chamber. So, having four chambers fed by one set of turbomachinery makes plenty of sense from that angle.

Right. Also makes for a shorter engine for a given a chamber pressure and expansion ratio, important for engine like SABRE where aerodynamics are crucial.

Also, they've talked about using expansion/deflection nozzles, which again may be easier at smaller scales...

[maximlevitsky]

Something I always wanted to ask, but only now my account here got approved. I almost though this place is reserved for NASA workers only.

Almost all launch facilities (excluding shuttle) are based on expandable rockets. So naturally tech behind parts of the rocket assumes that they are used on-time only.

Shuttle also is based on that tech, thus even though it returns home, it is essentially damaged/broken by the sole fact that it was flown (because its components are based on the same one-time use tech).
So a lot of work is required to refurbish these components.
This is what makes shuttle so costly in my opinion (its cheaper to make a rocket that lifts 5x more payload then send shuttle 5 times, or even worse)

So the question is, will Skylon be able to be reused without that kind of work?
(I mostly refer to SABREs and auxiliary propulsion systems as these are known to be heavily disassembled during shuttle ground service).

Besides a question to everyone, is it possible to get detailed information on the work that is done on the shuttle between each flight? e.g. How much it costs, what parts of the shuttle are disassembled, etc?

[aero]

The May press release and the referenced report is great news for space advocates. I have a question, though.

The report states that:

Success on future engine test would mean "a major breakthrough in propulsion worldwide"

What does that mean? The engine burns hydrogen fuel and the fuel as the heat sink is required. Does that mean that airplanes will be changing over to hydrogen fuel and the voluminous tankage that entails? Seems doubtful to me but maybe?

[strangequark]

What does that mean? The engine burns hydrogen fuel and the fuel as the heat sink is required. Does that mean that airplanes will be changing over to hydrogen fuel and the voluminous tankage that entails? Seems doubtful to me but maybe?

I think it just means that it will impact space launch around the world, not so much aircraft. Most of the jet guys I know are terrified at the thought of using methane, much less hydrogen. However, for space launch, it could be the equivalent of the Whittle engine. Why does it always seem to be the Brits and the Germans?

[Hermit]

The May press release and the referenced report is great news for space advocates. I have a question, though.

The report states that:

Success on future engine test would mean "a major breakthrough in propulsion worldwide"

What does that mean? The engine burns hydrogen fuel and the fuel as the heat sink is required. Does that mean that airplanes will be changing over to hydrogen fuel and the voluminous tankage that entails? Seems doubtful to me but maybe?

It means that it will be seen as a breakthrough not just in Europe, but worldwide.

Regarding airplanes, there is an ESA study called LAPCAT that looked into hypersonic passenger jets. Reaction Engines submitted a design that applied their precooler technology to the issue, the A2 design.
It should be capable of sustained Mach 5 travel over a design distance from Brussels to Sydney.

[ANTIcarrot]

What does that mean? The engine burns hydrogen fuel and the fuel as the heat sink is required. Does that mean that airplanes will be changing over to hydrogen fuel and the voluminous tankage that entails? Seems doubtful to me but maybe?

Possibly they're referring to non-commercial prospects. Hypersonic cruise missiles and such.

Might also have something to do with fuel tax now accounting for more than 50% of a flight ticket. Which is stupid, because at cruse aircraft are the most efficient form of transport. A long range aircraft that can (at any speed) run on 'green' hydrogen could transform the economics of aviation.

Incidently, most commercial aircraft could run on hydrogen if the entire rear fuselage was one big hydrogen tank; though that would also cut their profit in half. Hmm. Maybe I should dig out my old sketches for a Boeing 7-1/2-7...

[Hempsell]

one further question:when in rocket mode, why not use the airflow coming into the sabre engine as reaction mass? I.e, air augmented thrust.

At the transition the air temperature is already at the "starting to get uncomfortable" level, if we continued to ingest air above Mach 5.2 we would start to damage the heat exchangers and other internal equipment.  It would also complicate the moving forebody which would how need to cover a wider range.

Sorry, I don't mean take air in and slow it down, but pass it straight through to the combustion chamber or nozzle. Perhaps via a different inlet.

On a different matter, what is the chamber pressure of the sabre engine when in pure rocket mode, and why are there 4 rocket engines/nozzles per sabre?

I am sorry Seer I though you were suggesting we reignite the ramjets and run them in parallel to the rocket mode.  There is no practical alternate route to getting air into the combustion chambers, just look at the complex kit we have to get it into the chambers below Mach 5.

Everyone seems pretty much on top of the four chamber configuration in summary
  - Smaller chambers are easier to develop.
  - The arrangement is more compact making the nacelle smaller.
  - In rocket mode there are two independent engines.

The Sabre 3 chamber pressure is around 100 bar in air-breathing mode and around 145 bar in rocket mode.

A thought on possible point to point air travel such as illustrated by the LAPCAT A2 study vehicle; to accommodate the hydrogen mass effectively we feel SKYLON’s truss frame with suspended tanks approach will be needed to keep the structural mass reasonable. Liquid hydrogen does not look good if you house it in a conventional aluminium semi monocoque, you need to move closer to airship style structures then it looks like it has potential whether using conventional jets or pre-cooled cycles.

With regarding to servicing and turnaround we do not yet take the detailed designs and proven demonstrations for the entire vehicle but based on our assessment of the critical areas we have put the following in the SKYLON Requirement Specification.

"2.1.2 The entry into service objectives are: ….

iii – The SKYLON system shall achieve an inherent availability of 2 working weeks or less from a firm request to launch.

2.1.3 The mature operation objectives are: …..

iii – The SKYLON system shall achieve an inherent availability of two working days or less from a firm request to launch after touchdown and 5 hours or less for a vehicle on standby .

6.2.1 The SKYLON system shall have planned servicing after every 40 flights or more.  This servicing shall take no more than 50 hours to accomplish."

Finally thanks to Adrianwyard for the point about propriety information and I am afraid SKYLON D1 and the SABRE 4 is the point where I have to leave you all guessing.

[RobLynn]

Doing a primitive all-rocket SSME comparison with Skylon C1 configuration:
Skylon 1100m³ tanks, 56 tonnes in orbit, of which ~19600kg for 2 Sabre engines.

http://www.spacefuture.com/archive/a_single_stage_to_orbit_thought_experiment.shtml
Using 3x 3200kg SSMEs would reduce vehicle mass by 10 tonnes - so 46tonnes to orbit.
Assuming 9200m/s deltaV and an ascent averaged Isp of 425s gives an initial mass of 418 tonnes needing 1040m³ of 358kg/m³ LOX+LH2.  3xSSME at 109% give lift off thrust to weight about 1.31.  Low drag and High L/D may enable the trajectory to be optimised for lower delta V.

That is 5% smaller fuel volume than Skylon.

There are probably also some additional weight savings to be had (reduced wing and landing gear loads, possibly smaller rudder and canard area requirements with all rockets close to axis in a boattail, and reduced dynamic pressures and pitching moments during ascent).

So if Sabre powered Skylon is feasible then does that also imply that VTHL SSTO is just as feasible?  It would almost certainly have less risk and far lower development costs.

A mountainside launch catapult giving a few hundred m/s might improve it to the point where it was greatly superior to Sabre Skylon.

[MikeAtkinson]

Rob, there are a few problems with your design (as I understand it).

VTHL has problems with abort shortly after take off, loose an engine and the trust to weight is less than 1.0, there isn't enough time or energy to transition to horizontal flight for landing.

I think you need to allow for thrust structure of about 2 tonnes per engine.

3 x SSME are difficult to put on the ends of the wings.

The wings would need to be strengthened to withstand the extra thrust (more mass).

Putting the engines at the back leads to almost all the mass at the back which makes the flying characteristics difficult to design.

A mountainside catapult is a bad idea, for reasons that have already been gone through.

In http://www.spacefuture.com/archive/a_single_stage_to_orbit_thought_experiment.shtml Gary C Hudson showed that a SSTO using SSME was feasible. However, that was not a reusable SSTO, trying to make those designs reusable would probably have pushed the payload negative.

Finally, this is a Skylon thread, if you want to continue discussion of your SSME based design (which inevitably will be nothing like Skylon) then start another thread.

[Seer]

one further question:when in rocket mode, why not use the airflow coming into the sabre engine as reaction mass? I.e, air augmented thrust.

At the transition the air temperature is already at the "starting to get uncomfortable" level, if we continued to ingest air above Mach 5.2 we would start to damage the heat exchangers and other internal equipment.  It would also complicate the moving forebody which would how need to cover a wider range.

Sorry, I don't mean take air in and slow it down, but pass it straight through to the combustion chamber or nozzle. Perhaps via a different inlet.

On a different matter, what is the chamber pressure of the sabre engine when in pure rocket mode, and why are there 4 rocket engines/nozzles per sabre?

I am sorry Seer I though you were suggesting we reignite the ramjets and run them in parallel to the rocket mode.  There is no practical alternate route to getting air into the combustion chambers, just look at the complex kit we have to get it into the chambers below Mach 5.

Everyone seems pretty much on top of the four chamber configuration in summary
  - Smaller chambers are easier to develop.
  - The arrangement is more compact making the nacelle smaller.
  - In rocket mode there are two independent engines.

The Sabre 3 chamber pressure is around 100 bar in air-breathing mode and around 145 bar in rocket mode.

A thought on possible point to point air travel such as illustrated by the LAPCAT A2 study vehicle; to accommodate the hydrogen mass effectively we feel SKYLON’s truss frame with suspended tanks approach will be needed to keep the structural mass reasonable. Liquid hydrogen does not look good if you house it in a conventional aluminium semi monocoque, you need to move closer to airship style structures then it looks like it has potential whether using conventional jets or pre-cooled cycles.

With regarding to servicing and turnaround we do not yet take the detailed designs and proven demonstrations for the entire vehicle but based on our assessment of the critical areas we have put the following in the SKYLON Requirement Specification.

"2.1.2 The entry into service objectives are: ….

iii – The SKYLON system shall achieve an inherent availability of 2 working weeks or less from a firm request to launch.

2.1.3 The mature operation objectives are: …..

iii – The SKYLON system shall achieve an inherent availability of two working days or less from a firm request to launch after touchdown and 5 hours or less for a vehicle on standby .

6.2.1 The SKYLON system shall have planned servicing after every 40 flights or more.  This servicing shall take no more than 50 hours to accomplish."

Finally thanks to Adrianwyard for the point about propriety information and I am afraid SKYLON D1 and the SABRE 4 is the point where I have to leave you all guessing.

Thanks for that very comprehensive reply.

[simonbp]

Given the volume problems of LH2, wouldn't a hydrocarbon fuel make more sense for point-to-point travel?

[simonbp]

Something I always wanted to ask, but only now my account here got approved. I almost though this place is reserved for NASA workers only.

Welcome to posting community, and it sure as heck isn't. An interest in spaceflight is all you need.

WRT turnaround, a lot of the time it took to turnaround Shuttle was due to the extensive use of super-toxic hypergolics for the OMS and RCS, meaning the entire system has to be scrubbed down and flushed after every flight (X-37 is the same). If Skylon can successfully avoid using hypergols, that simplifies turnaround a lot...