The Reaction Engines Skylon Master Thread (1)

[Hempsell]

RobLynn – on comparison with pure rocket SSTO

I think the best way to look at this is consider the required mass ratio.  For an SSTO the orbit insertion mass will be around 12% the take off mass and whether this is achievable is a debate point. For SKYLON the number is around 21%.  OK we will have to account for heavier engines and it is lower that typical aircraft but still much more comfortable than any SSTO concept.


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

For very high speed flight I think you need the energy and cooling capabilities of hydrogen – anyway with the Truss framework / suspended tank does not introduce a mass penalty.


Simonbh – on the use of hypergolics

SKYLON only has hydrogen and oxygen on board – no hypergolics!

[RobLynn]

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.

I suggested 3 SSME strictly to be as close as possible to Skylon size for the purposes of comparison to see if Sabre has a performance advantage over rocket (beyond operational issues).  4-6  engines obviously make more sense for redundancy.

Your estimate for thrust structure weight appears far too high.
http://www.columbiassacrifice.com/techdocs/techreprts/NASA_CR-2002-211249.pdf
LOX/LH2 VTHL SSTO, 3500kg thrust structure for 7 x SSME class engines, ~500kg per SSME.

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.

The extra weight for wing tip engine mounting is probably minor ~0.15% of engine thrust if using carbon fiber composite structure (eg 2MN thrust via 25m of purely compressive and tensile loaded columns in an equilateral triangle planform loaded at ±1GPa = 0.05m³ volume, in Carbon Fiber that is 80kg, Titanium about 230kg).  So 200 tonnes extra thrust might cost another 300kg.

If you are VTHL on rockets without needing aero lift then engines in tail are no problem, it shortens load paths, eliminates nacelle drag, reduces base drag and without lifting flight the longitudinal aero bending loads are very small.  But perhaps there is some advantage in wing mounted engines due to reduced loads (and those loads being tensile) in rear of fuselage.  Would be interesting to see a full trade-study

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

Easy to make assertions like that. I think you are wrong. A weight and pulley powered catapult on a track up a mountainside can provide a safe abort mode for a VTHL vehicle as well as a very cheap increase in payload.

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.

I'm not designing another vehicle, I'm asking if Sabre and all of the challenges, cost and risk it represents are sensible or necessary.

[adrianwyard]

The ESA positive technical (and economic) reviews really are pretty exciting. As someone who remembers HOTOL it's quite weird to see this edging toward reality. I just found a couple of interesting things in the following article: http://www.bbc.co.uk/news/science-environment-13520948

+ A sub-scale version of SABRE 4 engine testbed could be running in 2014.
+ A one-fifth scale test rocket-aircraft is also envisioned - the Nacelle Test Vehicle (NTV) - which will reach Mach 5 and test the transition from air-breathing. At 1/5 scale it will be over 50 feet long, so not a toy by any means!

It's not clear if the NTV will share the outer mold-line of the whole Skylon, or just the wings and nacelle. I wonder if it will also test the TPS and fuel-tanks/aero-shell construction techniques and components intended for the full Skylon.

Once there's 'bent-metal' actually flying, and real-world data on elements like TPS, engine performance, tankage, then it'll just be a case of finding people to invest a few measly billions!

[majormajor42]

Just discovered this thread recently, and the Skylon concept. Pretty cool. Looked at their website, watched the videos. Read some history. Good for Britian.

Can't seem to find anything on runway lengths for landing/take-off? What about for the A2?

Do the X-43/51 project results translate to useful info for SABRE?

I would think that there would be tremendous military interest (again) if the scaled versions are successful. Money too. Looking at the x-37, USAF likes wings.

And why that bent shape of the nacelles? Is that for what I read on the wiki page, for slowing the air? Wasn't clear to me.

[Space OurSoul]

And why that bent shape of the marcella nacelles? Is that for what I read on the wiki page, for slowing the air? Wasn't clear to me.

They are angled because the thrust vector has to be directed through the center of mass, but the inlet has to be directed into the air flow. Skylon accelerates in atmo at a slight angle of attack, hence the bend.

I learned this from a recorded presentation by Alan Bond, the link for which I can't find right now. I believe it was at the U of Strathclyde, but I could be wrong (prolly am, or I woulda found it). A very illuminating talk, if you can find it.

(And I'm right there with you on the cool factor. SR71 crossed with a Naboo cruiser. Now THAT's a rocket ship!)

[adrianwyard]

There are very knowledgable people around here, but here's a couple of thoughts:

Re engine shape: I had heard the same reason for the downward bend at the rear as explained by the previous poster. Your wikipedia reference is probably to the spike/nacelle which manages the the air on its way to the precoolers. The spike moves laterally to position the shockwave internally as the speed changes - just like on an SR-71.

I have wondered if the downward bend of the rear engines might help landing characteristics, behaving a bit like flaps...

I doubt if the X-43 or X-51 research was that useful because SABRE only airbreathes up to Mach 5, and the X-craft only start airbreathing at about that speed. (As I'm sure you know, SABRE is not a SCRAMJET.) So high-supersonic aerodynamics would have been applicable, perhaps.

It sounds as though unfortunately there can be little direct collaboration  with the US because if any US high-technology (military-related research) ends up on Skylon, REL will be unable to sell it as freely as they'd like. Or something like that. It sounds silly to me.

And it sounds like they envision new runways being built for it, so presumably that means looong. Also strong as the under-carriage is small to keep weight down.

The hope is it will be able to self-ferry home from abort/alternate sites. My guess is that these will not have to be custom extra-long Skylon abort runways and it can in fact take-off from normally long runways with a partial fuel load. It's just a guess though.

[Crispy]

And why that bent shape of the marcella nacelles? Is that for what I read on the wiki page, for slowing the air? Wasn't clear to me.

They are angled because the thrust vector has to be directed through the center of mass, but the inlet has to be directed into the air flow. Skylon accelerates in atmo at a slight angle of attack, hence the bend.

I learned this from a recorded presentation by Alan Bond, the link for which I can't find right now. I believe it was at the U of Strathclyde, but I could be wrong (prolly am, or I woulda found it). A very illuminating talk, if you can find it.

(And I'm right there with you on the cool factor. SR71 crossed with a Naboo cruiser. Now THAT's a rocket ship!)

Here's that presentation:

Alan Bond gave the Inaugural James Weir lecture a year ago and covered a lot of the ground of this thread.

[grdja]

The ESA positive technical (and economic) reviews really are pretty exciting. As someone who remembers HOTOL it's quite weird to see this edging toward reality. I just found a couple of interesting things in the following article: http://www.bbc.co.uk/news/science-environment-13520948

+ A sub-scale version of SABRE 4 engine testbed could be running in 2014.
+ A one-fifth scale test rocket-aircraft is also envisioned - the Nacelle Test Vehicle (NTV) - which will reach Mach 5 and test the transition from air-breathing. At 1/5 scale it will be over 50 feet long, so not a toy by any means!

It's not clear if the NTV will share the outer mold-line of the whole Skylon, or just the wings and nacelle. I wonder if it will also test the TPS and fuel-tanks/aero-shell construction techniques and components intended for the full Skylon.

Once there's 'bent-metal' actually flying, and real-world data on elements like TPS, engine performance, tankage, then it'll just be a case of finding people to invest a few measly billions!

Those look like excellent news. Best of fortunes to them.

[A_M_Swallow]

{snip}

And it sounds like they envision new runways being built for it, so presumably that means looong. Also strong as the under-carriage is small to keep weight down.

The hope is it will be able to self-ferry home from abort/alternate sites. My guess is that these will not have to be custom extra-long Skylon abort runways and it can in fact take-off from normally long runways with a partial fuel load. It's just a guess though.

The runways may need to be built out of special materials to survive having liquid hydrogen spilt on them.  Particularly around the refuelling points.

[majormajor42]

Here's that presentation:

Alan Bond gave the Inaugural James Weir lecture a year ago and covered a lot of the ground of this thread.

[/quote]

Thanks for the video link. Watched it. Also went back and read this whole thread.

Read about the debate on ITAR
Read about the debate on sled launch.
then finally the return of Hempsell last month.
seaplane launch, and so on...

okay, all caught up now.

Realized I asked some questions a few posts ago that were already covered. I guess any interest from the USAF will only be as a buyer later on but not as an investor or development partner. Hard to imagine the military not wanting these eventually but as Hempsell has implied, the USAF might be buying these the same way Qantas does.

I'm glad they seem to be on their way to more funding. Surprised investors don't use Concorde as a excuse to dismiss Skylon. I suppose one reason that exclusive airports might be built, besides those already mentioned, is runway debris. Is it also a potential problem for Skylon? Perhaps the new runways that might be built could include advanced runway debris sensors/detectors?

After watching that video, gonna read up on who Kenneth Clarke is. Probably not a good idea for Bond to so strongly state his political feelings (unless that bridge has already been burned).

I like Bond's mention of Earth's mass being a bit too much, by 10%. Never though about it that way. These Super Earths that Kepler is starting to find might therefor be poor candidates for spacefaring alien civilization homeworlds.

With all the talk about different take-off options, one simple idea might be small solid rocket boosters like they have on the C-130s, RATO or RATOG. Might not technically make it a SSTO anymore. Besides, I think the alternate ideas have been discussed enough here.

Is the 10km higher reentry (than shuttle) due to higher surface area?
So Bond says in the video that 200kg of liquid hydrogen needs to be retained for renetry as part of the TPS. What if something were to happen where that H2 wasn't there. Would the Skylon burn up? Does this consideration limit the amount of time that a Skylon can stay in orbit? (not that there is currently any reason for a Skylon to stay in orbit for an extended period of time)

Good Luck REL

 

[maximlevitsky]

I'm glad they seem to be on their way to more funding. Surprised investors don't use Concorde as a excuse to dismiss Skylon. I suppose one reason that exclusive airports might be built, besides those already mentioned, is runway debris. Is it also a potential problem for Skylon? Perhaps the new runways that might be built could include advanced runway debris sensors/detectors?

Bah, Concorde really wasn't dismissed on technical grounds. Accidents happen.
If you look at history of any aircraft currently in service you'll find a lot of crashes.
It just wasn't that profitable. And its really sad it was dismissed that way.
But anyway back on topic,

With all the talk about different take-off options, one simple idea might be small solid rocket boosters like they have on the C-130s, RATO or RATOG. Might not technically make it a SSTO anymore. Besides, I think the alternate ideas have been discussed enough here.

In my humble opinion anything falling off the spacecraft will kill the turnaround rate and thats one of the strongest points of the skylon.
Cause for each launch you need another set of the boosters, you need to attach it, you can't stop them as soon as they ignite, and since skylon is quite heavy to add any meaningful delta-v you will need anything but small boosters.

Is the 10km higher reentry (than shuttle) due to higher surface area?

I guess so, that what FAQ on REL site says.

So Bond says in the video that 200kg of liquid hydrogen needs to be retained for renetry as part of the TPS. What if something were to happen where that H2 wasn't there. Would the Skylon burn up? Does this consideration limit the amount of time that a Skylon can stay in orbit? (not that there is currently any reason for a Skylon to stay in orbit for an extended period of time)

Well, Hempsell, said here earlier that all reaction control systems (aka these small thrusters that orient the spacecraft and the ones that provide small delta-v changes to reach space station or to execute a deorbit burn are powered by oxygen and hydrogen, so I guess that skylon will have to have some fuel onboard, otherwise it won't be able to do a deorbit burn, so having a bit more hydrogen won't make a difference).

However, I am aware of the fact that Apollo spacecraft whose S-IVB stage was restarted had to dump some of the hydrogen that was boiling off to prevent overpressure, like what is done to any cryogenic launch vehicle while it is on the lauchpad.

So, don't know if problem of storing deep cryogenic fluids for prolonged time in space is solved.
Thats why hypergolic fuels are used in space usually, but apart from long term storage, they are really nasty stuff.

Good Luck REL

I wish you the best of luck. Really make that happen!

In my opinion 21 century hasn't yet started.

In 2000s we really didn't have anything new, except maybe small form factor computers (smartphones, mp3 players, etc), but these are really toys.
Computers while did increase their speed manyfold, they are still well same computers.
And sadly lion share of their performance increase is just consumed by inefficient programs which do not much more that their older counterparts did.

So make it happen, start the real space age!

And of course big thanks for all responses. I am busy lately so, thats just my 2nd post.

[ciscosdad]

RE the 200kg Hyrdogen for reentry cooling.


This is really making a virtue of a necessity. Very few (if any) tank / propellant systems allow 100% usage. The 200kg would likely be there wheter intentional or not. Makes good use of an otherwise wasted resource.

[Hempsell]

Perhaps a word on propellant handling on SKYLON.

The main tanks are loaded with sub-cooled propellants that do not start to boil for over 2 hours by which time we have either launched or abandoned the launch attempt and defuelled - so no venting after fuelling or during the flight. After MECO the main tanks are vented and end up filled with ambient temperature gas at around 1 bar.

The in orbit hydrogen and oxygen are housed in separate auxiliary tanks which are more heavily insulated. The fuel cells ensure that consumption always exceeds boil off.  It is the capacity of these tanks that in part sets the 2 day nominal life with 2 day contingency, we could go longer by designing bigger tanks but no one has yet asked us to.  The 200 kg used to absorb the re-entry heating soak comes from those tanks and is a straight mass penalty, i.e. it does not use up ullage or otherwise unusable propellant.

And on runways

The length for C1 is 5.5 km including about 1.5 km runoff for aborted take-offs, the remaining 4 km includes an allowance for an “engine out” take off.  It does have to be of higher strength that normal runways but not with special material just more concrete and reinforcing than normal.  Without the oxygen on board SKYLON can use normal runways for both takeoff and landing.

LAPCAT A2 is designed to use existing airport runways inline with the ESA Study brief.

[simonbp]

How do you handle the forces from venting the tanks? Just do them in two opposite directions?

(BTW, thanks for being so accessible on this site; it's great!)

[Crispy]

Will you be able to release information on the progress of the precooler tests? Or will it have to wait for the full report?

[majormajor42]

Would a dry lake bed runway like those at Edwards not work for the C1. I know we are not talking about USAF involvement, just asking about the geology. If dry lakes work, I wonder if there are any good sites closer to the equator. Facing east toward an ocean too while at it.

edit: another question
The SABRE is still a rocket engine with many similarities to engines like the SSME. What makes REL think that they can turn these things around in two days when NASA takes a bit longer than that.  Even the new unused SSME design that had more sensors to indicate parts needing replacement might have take more than two days to evaluate. What is it about the SABRE that allows the quick turn around? Are there examples of other reusable rocket engines that can turn around quickly too?

[adrianwyard]

Re runways: if it will require hardened concrete runways, my guess is a dry lake bed wouldn't work. 5.5km sounds like a lot, but it's only a bit more than the Shuttle Landing Facility (18000 vs 15000ft).

If ESA were to continue their involvement then I imagine they'd build the runway at the ESA space center in Kourou French Guyana. The terrain there looks pretty flat on the coast... If they were to do a sub-orbital satellite launch with an upper stage, or an abort downrange they're heading for Africa, then India.

[adrianwyard]

I'd like to pick up on a theme that's been in several recent posts: the ambitious, optimistic nature of the Skylon project. There's no doubt in my mind that philosophically, this project is a descendent of the kind of 'right stuff' thinking that led to the likes of Concorde and the Space Shuttle. Those will go down in history as crowning technical achievements, yet these days many people will happily call them failures and mistakes as well. This is most clearly seen in the return to the capsule on 'big dumb booster' design, and an increased emphasis on abort systems.

When the Shuttle designers chose a vehicle with no early launch abort system, this was because all things considered, they didn't think one would be needed. Contrast this with SpaceShipTwo which automatically orientates itself to re-entry attitude. My guess is you could be tumbling with a full electrical failure on SS2 and still make it back OK. With the Space Shuttle, if the TPS is undamaged, then you're in good shape unless the redundant hypergolic RCS were to fail.

With Skylon, we're back to the Shuttle era philosophy with 'right stuff' expectations that all will go well. The question is, who's more correct: the optimistic Skylon approach, or the cautious (expect everything will break) approach. Personally, I'm undecided.

As a previous poster noted, the tanks need to be pressurized to maintain structural integrity for re-entry and landing: a leak on orbit would be bad. Some other optimistic aspects of the design include:

+ The foreplanes need to be actively cooled. A failure of this system would presumably be the end of Skylon.
+ Parts of the wing also need to be actively cooled.
+ Skylon places cryogenic temp tanks ahead of RCC wing leading edges, so icing will need to be controlled.

Some responses to these concerns are as follows:

1] Redunancy. For example, duplicate the cooling loops and pump systems. The OMS fuel can pressurize the main tanks if needed.
2] Fail-safe. Perhaps the foreplanes could sustain major damage when not cooled, but stay intact.
3] Flight history. There's nothing builds confidence like actual flight history and real-world testing. It's through this route that Skylon might be able to compete with capsules on safety stats. We've become comfortable boarding passenger aircraft with no parachutes, and with sufficient history, would accept the active cooling etc. on Skylon. The nature of Skylon would allow for a rapid testing program that can't be matched by boosters.

[Gregori]

What amazing TPS does this thing have that's cheaper, easier and better than the Shuttle?

[Downix]

What amazing TPS does this thing have that's cheaper, easier and better than the Shuttle?

A metalic one similar to what the Shuttle was supposed to use (until politics blocked NASA's access to the Titanium needed) paired with an active cooling system using residual LH2 piped to key areas.

It's nothing amazing, the SR-71 used a similar TPS.