Author Topic: The Reaction Engines Skylon Master Thread (5) (Read 736185 times)

adrianwyard · « **Reply #520 on:** 08/05/2015 06:45 pm »

This is not directly Skylon/SABRE related, but since LAPCAT and Ramjets have been discussed recently:

"Concorde Mark 2: Airbus files plans for new supersonic jet - New jet could cut flight time from London to New York to just one hour"

http://www.telegraph.co.uk/finance/newsbysector/industry/engineering/11782446/Concorde-Mark-2-Airbus-files-plans-for-new-supersonic-jet.html

Animations here:

Crispy · « **Reply #521 on:** 08/05/2015 06:58 pm »

Already a thread in this forum

Hankelow8 · « **Reply #522 on:** 08/05/2015 07:57 pm »

I have just finished reading the latest article in Aviation Week focusing on the Sabre demonstrator engine.

It's interesting to note the comments from both AFRL and Sam Hutchison director of corporate development at Reaction Engines. AFRL state that the Sabre's original target, a single-stage-to-orbit space vehicle remains technically "very risky as a first application," but he go's on to say "Sabre may provide some unique advantages in more manageable two-stage-to-orbit Configurations." Sam Hutchison states it might be the best approach to demonstrate the technology by taking it one step at a time.

Could this be a step change in Reaction Engines thinking in going for a full blown Skylon Demonstrator, and they now think that's just "a bridge to far"

Might a demonstrator be built just to prove Sabre up to Mach 5 prior to change over to normal rocket operations be best, the vehicle could be built on a much simpler and smaller airframe and the return re-entry temperatures would be considerably lower. Once confidence had been achieved it might be possible to modify the demonstrator to achieve crossover to rocket power using a small internal Lox tank to test this, it only needs to operate for a short time so re-entry temperatures will again be considerably lower than de-orbit temperatures.

I know Sabre can be fully tested at ground level, but to go straight to a flight model Skylon will be a massive risk, no one should underestimate the complexities involved in this.

In the meantime (to make the financial backers happy) it seems that there are many other potential aerospace applications beyond just the precooler concept that is the heart of Sabre. This could prove to be a massive income earner for Reaction Engines giving the company the income to create a world-beater in Skylon.

Citizen Wolf · « **Reply #523 on:** 08/05/2015 10:35 pm »

@Hankelow8
Yes, this point was mentioned 2 pages back. Why did an REL person say this.

AnalogMan · « **Reply #524 on:** 08/13/2015 12:09 pm »

Came across this recent NASA Ames conference paper on Skylon that I thought might be of interest to followers of this thread.

Skylon Aerodynamics and SABRE Plumes
20th AIAA International Space Planes and Hypersonic Systems and Technologies Conference
6-9 July 2015, Glasgow, Scotland, United Kingdom

Abstract:
An independent partial assessment is provided of the technical viability of the Skylon aerospace plane concept, developed by Reaction Engines Limited (REL). The objectives are to verify REL’s engineering estimates of airframe aerodynamics during powered flight and to assess the impact of Synergetic Air-Breathing Rocket Engine (SABRE) plumes on the aft fuselage. Pressure lift and drag coefficients derived from simulations conducted with Euler equations for unpowered flight compare very well with those REL computed with engineering methods. The REL coefficients for powered flight are increasingly less acceptable as the freestream Mach number is increased beyond 8.5, because the engineering estimates did not account for the increasing favorable (in terms of drag and lift coefficients) effect of underexpanded rocket engine plumes on the aft fuselage. At Mach numbers greater than 8.5, the thermal environment around the aft fuselage is a known unknown−a potential design and/or performance risk issue. The adverse effects of shock waves on the aft fuselage and plume-induced flow separation are other potential risks. The development of an operational reusable launcher from the Skylon concept necessitates the judicious use of a combination of engineering methods, advanced methods based on required physics or analytical fidelity, test data, and independent assessments

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150015818.pdf

[Copy also attached]

john smith 19 · « **Reply #525 on:** 08/13/2015 07:39 pm »

Quote from: AnalogMan on 08/13/2015 12:09 pm

Came across this recent NASA Ames conference paper on Skylon that I thought might be of interest to followers of this thread.

Skylon Aerodynamics and SABRE Plumes
20th AIAA International Space Planes and Hypersonic Systems and Technologies Conference
6-9 July 2015, Glasgow, Scotland, United Kingdom

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150015818.pdf

[Copy also attached]

Interesting report.

Note this suggest REL have been in contact with the NASA team for years before this came out.

Obvious points.

NASA's analysis is that REL figures for lift and drag are conservative and underestimate how good the design is.

While quite a detailed CFD it assumes pefect gas effects, which is very isleading at these temperatures and pressures. Typicially air at this temp dissociates into atoms and there is some ionization.

I'd suggest it sound quite positive within the limits of what's being simulated and how it's being simulated.

It's a good reminder there are still substantial unknowns to the design, some of which will be quite subtle, although the report points out fuselage heating by the engine plumes is a known unknown, and this works starts to quantify it.

t43562 · « **Reply #526 on:** 08/14/2015 02:28 pm »

This looks slightly alarming at Mach 12:

https://lh3.googleusercontent.com/UPElrgfWT-BGhP_sXkswAhG09EMvb829njzzXAPWoik=w1130-h826-no

Can the silicon carbide take it? Could it be actively cooled? Hmm.. I suppose re-entry can't be all that cool but can it compare with this?

Hanelyp · « **Reply #527 on:** 08/14/2015 03:05 pm »

Quote from: t43562 on 08/14/2015 02:28 pm

This looks slightly alarming at Mach 12:

https://lh3.googleusercontent.com/UPElrgfWT-BGhP_sXkswAhG09EMvb829njzzXAPWoik=w1130-h826-no

Can the silicon carbide take it? Could it be actively cooled? Hmm.. I suppose re-entry can't be all that cool but can it compare with this?

Hard to tell how bad that is without a temperature scale or indication of thermal flux. The exhaust flow is well expanded by the time it impinges on the after-body. Heat flow to the skin from this flow should be a small fraction of what the combustion chamber and exhaust nozzle need to deal with.

chipguy · « **Reply #528 on:** 08/14/2015 04:34 pm »

Quote from: john smith 19 on 08/13/2015 07:39 pm

NASA's analysis is that REL figures for lift and drag are conservative and underestimate how good the design is.

I think if you look more closely you'll find that REL figures were conservative at some speeds and optimistic
at others. Another important take away is the thermal flux on the tail is considerable and probably higher
than REL was counting on. Addressing it may cut into margin reserve for mass growth.

However, on the whole the paper represents an important independent assessment of their vehicle concept
and its general agreement with REL's claims should enhance their credibility.

antiquark · « **Reply #529 on:** 08/14/2015 04:46 pm »

Quote from: Hanelyp on 08/14/2015 03:05 pm

Quote from: t43562 on 08/14/2015 02:28 pm
This looks slightly alarming at Mach 12:

https://lh3.googleusercontent.com/UPElrgfWT-BGhP_sXkswAhG09EMvb829njzzXAPWoik=w1130-h826-no

Can the silicon carbide take it? Could it be actively cooled? Hmm.. I suppose re-entry can't be all that cool but can it compare with this?
Hard to tell how bad that is without a temperature scale or indication of thermal flux. The exhaust flow is well expanded by the time it impinges on the after-body. Heat flow to the skin from this flow should be a small fraction of what the combustion chamber and exhaust nozzle need to deal with.

In the article they say "there are regions where static temperatures are roughly 8-16 times greater than the freestream temperature." However I don't know how to fill in the blanks to arrive at an actual temperature value.

Alpha_Centauri · « **Reply #530 on:** 08/14/2015 08:43 pm »

State aid: [European] Commission approves £50 million UK support for the research and development of an innovative space launcher engine.

http://europa.eu/rapid/press-release_IP-15-5495_en.htm

How nice of them.

93143 · « **Reply #531 on:** 08/14/2015 09:00 pm »

At Mach 12, Skylon C1 was at about 62 km, implying an atmospheric pressure of about 15 Pa and a freestream temperature of something like 240 K. At Mach 17, it was at around 71 km, implying 4 Pa and 215 K. Real gas effects tend to result in lower stagnation temperatures (and higher stagnation densities) than one would calculate from the ideal gas law.

adrianwyard · « **Reply #532 on:** 08/14/2015 09:16 pm »

It is curious that the authors didn't take the next step and calculate temps on structures. They're clearly aware that this could be the undoing of Skylon - or the retirement of a critical known risk. In the Takeaway section they state: "If the aft fuselage heating owing to nacelle plumes is an issue that cannot be addressed with appropriate structures and materials, then the overall design of Skylon needs to be modified." By which they appear to mean moving the engines aft (and closer to the HOTOL problems.)

They also make a suggestion I don't really follow: Use Skylon as stage 1 in a TSTO system, launching the second stage from the payload bay at ~7 km/s. But wouldn't plume heating be already high at this speed?

The TSTO comment is curiously consistent with the message from USAF/AFRL, so it makes you wonder if there is some connection.
________
For those new to the forum, this was discussed a little three years ago: http://forum.nasaspaceflight.com/index.php?topic=24621.msg943964#msg943964
http://forum.nasaspaceflight.com/index.php?topic=24621.msg945702#msg945702

lkm · « **Reply #533 on:** 08/14/2015 10:43 pm »

Could the plume impingement be mitigated by switching to having a single large expansion ratio nozzle (400:1) in each SABRE rather than four 120:1 nozzles so that the plumes aren't so under expanded at high mach and using the airbreathing nozzle area for thrust augmenting altitude compensation at low mach?

adrianwyard · « **Reply #534 on:** 08/14/2015 11:30 pm »

The use of altitude compensating Expansion/Deflection nozzles should help also.

Citizen Wolf · « **Reply #535 on:** 08/15/2015 12:36 am »

As Adrianwyard has nicely reminded us, this topic of plume expansion heating the skylon fuselage was queried on the 1st skylon thread back in 2012. The discussion doesn't seem to have gone any further on the thread and I certainly forgot all about it. But one would have thought that REL has considered this issue. At least I would have hoped they did. There has been no hint of any design modifications to the basic skylon shape nor repositioning of the SABRE engines.

And then the apparently odd comments from the AFRL labs and an REL employee (Sam Hutchison) about using SABRE as a TSTO when the whole original concept was for a SSTO.

I guess there's a lot we still don't know yet.

lkm · « **Reply #536 on:** 08/15/2015 09:51 am »

Quote from: adrianwyard on 08/14/2015 11:30 pm

The use of altitude compensating Expansion/Deflection nozzles should help also.

It's only an expansion deflection nozzle in airbreathing mode, in rocket mode it's a 120:1 expansion ratio bell nozzle. Either way an E-D nozzle will still become under expanded once it hits it's critical pressure and the plume will grow.

pippin · « **Reply #537 on:** 08/15/2015 10:42 am »

Quote from: Hanelyp on 08/14/2015 03:05 pm

Hard to tell how bad that is without a temperature scale or indication of thermal flux. The exhaust flow is well expanded by the time it impinges on the after-body. Heat flow to the skin from this flow should be a small fraction of what the combustion chamber and exhaust nozzle need to deal with.

I wouldn't be so sure. All the heat flow is in the boundary shock wave and their indications of more than 10 times the free stream temperature... However the final gas model will be (they do give compensation estimates for real gas), this is massive, we are talking about a kelvin-scale here!

They will probably have to cool the whole body aft of the wing, I doubt you can passively cool this without adding a lot of mass.

This essentially means there's a significant chance the whole airframe concept is not feasible. It's the kind of risks you run into when designing all-new stuff and venturing into unknown unknowns with a lot of elements in your design.

adrianwyard · « **Reply #538 on:** 08/15/2015 03:51 pm »

Assuming plume heating is indeed a problem with D1.5a there's no reason to assume it can't be addressed with relatively simple airframe changes. For example, widening the wing span, moving nozzles further aft, etc.

adrianwyard · « **Reply #539 on:** 08/15/2015 04:01 pm »

Quote from: lkm on 08/15/2015 09:51 am

Quote from: adrianwyard on 08/14/2015 11:30 pm
The use of altitude compensating Expansion/Deflection nozzles should help also.

It's only an expansion deflection nozzle in airbreathing mode, in rocket mode it's a 120:1 expansion ratio bell nozzle. Either way an E-D nozzle will still become under expanded once it hits it's critical pressure and the plume will grow.

Hmmm, I think your saying the E-D benefit occurs it lower altitudes and so is if no help here; that makes sense.

But the nozzle doesn't know if SABRE is in air-breathing or rocket mode, right? Maybe you simply meant earlier in flight when you said 'It's only an expansion deflection nozzle in airbreathing'?

The exhaust from the bypass burners (air-breathing mode) will affect the plume, but that's turned off by the time we're at the problematic speed & altitude/pressure.