The Reaction Engines Skylon Master Thread (2)

[Chris Bergin]

New thread for the epic Skylon and associated Reaction Engines news, updates and discussion.

http://www.reactionengines.co.uk/

http://www.reactionengines.co.uk/space_skylon.html

Keep it on topic.

[65816]

In a hypothetical situation...

Could one launch Skylon into LEO, refuel via fuel depot, than launch for a Apollo 8 style lunar trip? Possibly adding a second Skylon to the mission  that would carry the Moon lander in the hangar? I guess it would need to carry alot of fuel to deaccelerate as it comes back towards Earth at very high delta v....

[93143]

Skylon has about 6.3 km/s of rocket delta-V with a full payload bay, according to the C1 trajectory spreadsheet.  That's probably enough for TLI and propulsive braking into LEO, or at least to the point of being able to make a safe reentry.  The OMS has some delta-V too, but not enough for LOI+TEI.

So no, not Apollo 8, unless there were a depot at the moon too.  Zond, though...?  A small lander could insert into LLO on its own...

Skylon is probably not being designed for the thermal environment, and the four-day on-orbit maximum duration doesn't match the requirements for a lunar free-return loop.  Not sure what the limiting factor is there; presumably some consumable or other...

Can you restart a SABRE in space?

Could you run the engines at a lower mixture ratio to get more Isp?  There's a lot of extra hydrogen tankage that isn't used at 6:1...

...

I don't see anyone ever actually doing this.  A dedicated space tug would probably be better, mostly because it would use about a quarter of the propellant...

[simonbp]

In a hypothetical situation...

More likely you would launch the individual components for a mission on a Skylon and then go from there. A reusable direct lander with Skylon-launched propellant would be quite interesting.

[93143]

SABRE was.

[RanulfC]

REL needs to test the COMPLETE engine as an assembly and not just the part thereof, they need to run it in a full simulated environment tunnel from zero-to-hypersonic and back again and they need to fly some test vehicles of the Skylon itself because it's NOT an "airplane" it is a "spaceplane" and the actual data on such a creature, especially in the regimes that REL is planning is few and far between.
(Hope that's not to far "off-topic" Chris... If it is let me know and I'll start using smaller fonts so you can go blind faster... after all I'm here to help )

Some of the main goals of the next round of funding are the construction of a ground weight complete SABRE design (subscale but complete. I think REL feel the best approach is "build early, test complete device, start modifying.") One of the SSME test rigs had 2000 valves. In the time it took to debug it Rockwell could have built the whole SSME with overweight parts and started the test process.
Design of the full scale SABRE 4 engine.
Design and test of the Nacelle Test Vehicle.

As for "full up testing" that's exactly  what happened in Polaris, Saturn (where key Polaris managers went) and Shuttle. All of which were built without any significant CAD/CAM support.

"Subscale" counts as "full-up" testing because you're testing the system sorry for the confusion
And actually it wasn't until pretty late in the Saturn-V program that they went with "full-up/no-partial" tests and only because they were running out of time. There wasn't such a constraint on the Shuttle, nor really Skylon/SABRE but they HAVE to get that far first

Randy

[RanulfC]

In a hypothetical situation...

More likely you would launch the individual components for a mission on a Skylon and then go from there. A reusable direct lander with Skylon-launched propellant would be quite interesting.

That is the "plan" per what REL has been suggesting. Since they don't actually "plan" on being a "user" of the Skylon the ACTUAL  use will of course be up to the customer but the needed mods for a Lunar velocity reentry are probably a non-starter and it would MUCH easier to simply shuttle payloads up with Skylon and use a dedicated orbit-to-orbit ship for everything else.

Randy

[93143]

the needed mods for a Lunar velocity reentry are probably a non-starter

Like I said, it has (based on the old C1 spreadsheet) about 6.3 km/s of rocket delta-V, not including the OMS and not accounting for the possibility of running at a lower mixture ratio to take advantage of the extra hydrogen tankage.  That should be enough for a Zond-style mission with propulsive braking to LEO entry velocity.  Upgrade that to Apollo-8-style or better with depots at both ends of the trip instead of just in LEO.

Big depots.  Skylon D1 is probably somewhere north of 40 tonnes dry, not counting payload...

And of course there are a lot of niggly technicalities when you try to do this sort of thing with a vehicle that wasn't designed for it...

[Seer]

the needed mods for a Lunar velocity reentry are probably a non-starter

Like I said, it has (based on the old C1 spreadsheet) about 6.3 km/s of rocket delta-V, not including the OMS and not accounting for the possibility of running at a lower mixture ratio to take advantage of the extra hydrogen tankage.  That should be enough for a Zond-style mission with propulsive braking to LEO entry velocity.  Upgrade that to Apollo-8-style or better with depots at both ends of the trip instead of just in LEO.

Big depots.  Skylon D1 is probably somewhere north of 40 tonnes dry, not counting payload...

And of course there are a lot of niggly technicalities when you try to do this sort of thing with a vehicle that wasn't designed for it...

What if you were to put lO2 in one of the hydrogen tanks? modify a hydrogent tank so it had lo2 feedlines as well as hydrogen ones. that should rougly add another 200 tonnes of lo2. the delta vee would be roughly 9 km/s in that case.

[93143]

I suspect it's best to keep hydrogen tanks as hydrogen tanks and oxygen tanks as oxygen tanks.

...

Entertaining your idea for a moment, one of the hydrogen tanks (using C1 numbers here) would hold about 550 tonnes of LOX, not 200.  But 200 tonnes is just about right for burning the other full tank of hydrogen at 6:1, assuming the LOX tanks are empty.  This only gets you about 7.5 km/s, though.

If you're proposing running the engines oxidizer-rich, with 375 tonnes of LOX (both LOX tanks full and 200 tonnes of LOX in one of the LH2 tanks) to just 34 tonnes of LH2, I'm pretty sure that's a bad idea.  Hydrolox engines don't like to run oxidizer-rich; the Isp drops off fast, and I suspect there are chemical susceptibility issues...

If you mean load up the LOX tanks with LH2, that doesn't help much because they can only hold a little over 9 tonnes; you'd have to use extra LOX.  260 tonnes of LOX plus 43 tonnes of LH2 would give you over 8 km/s.

Even filling one LH2 tank and both LOX tanks with LH2 and providing enough LOX to run stoichiometric, you'd still end up a bit shy of 9 km/s, according to my calculations, because you lose something on the order of 20 seconds of Isp by not running fuel-rich.  That's assuming you can even run a SABRE at 8:1...

...

This is getting a little silly.  Skylon is way too heavy to be efficient in this role, even if you wanted to bother converting it to handle the duration, thermal environment, engine restarts if that's an issue, etc.  The only potential advantage is that you can inspect and maintain it on the ground, but I suspect that isn't enough to make it worthwhile over a chemical tug like Fluyt, which could get similar delta-V with the same payload using a quarter of the propellant.

[Kharkov]

I was listening again to Mark Hempsell on a podcast (available at http://www.talkingspaceonline.com/2012-podcasts/episode-434-skylon-with-sabre-single-stage-to-orbit.html) & he was talking about Skylon re-entering the atmosphere. Basically, it doesn't heat up as much as the Space Shuttle did but it does stay in the 'heat zone' for longer, requiring the use of a small amount of liquid hydrogen to cool the airframe & act as a heatsink.

Which raises an interesting question, the Skylon is intended to glide to an unpowered landing but if there's a small quantity of (almost certainly not liquid anymore) hydrogen aboard, could that be harnessed to a greater or lesser degree and give Skylon a powered go-around should there be a last-minute problem with the runway or landing systems?

[simonbp]

You'd probably need quite a bit of fuel to do a go-around; Skylon has some pretty stumpy wings and would need a fair bit of thrust to be able to take off again, which a go-around would involve.

I think the Shuttle experience showed that problems right before landing aren't a really constraining failure mode. The larger constraint is probably the the lifetime of the LH2 in the tanks, limiting how long a Skylon can wait on orbit until the weather calms down. That was definitely an issue for Shuttle (though for a different reason).

[93143]

Skylon's wings are sized for takeoff and climb with full tanks.  On the return leg, the landing speed is quite low and the runway requirements are minimal.

[Carreidas 160]

I've frequently posted random Skylon thoughts, here's another one. Please be thorough, skeptical and unforgiving!

Why does Skylon have two nacelles? Wouldn't it be possible to make the entire vehicle into one big nacelle, where the nose scoops up air and routes it to the back, where the rockets are positioned? That way
- the wings take smaller loads
- the aerodynamic profile is reduced, thus decreasing drag
- potentially less structural mass is required, as you don't need duplicate engines

Less drag & less mass = lower lift requirement = smaller wings & less rocketry (perhaps going from 8 to 7 nozzles) etc. etc.

Unfortunately this setup would
- necessitate an 80m long feedline for the chilled air from front to back, which could be costly
- change load requirements on the hull
- change landing gear position
- need to shift the wings around
- have a problem with nozzle placement, as the back side is tapered upward to allow takeoff

I can't see the COM shifting during ascent, as was the problem with HOTOL.
 
Alternatively, you could ditch the nosecone setup and add some ram scoops behind the wings, but this would turn into even more of a redesign.

EDIT - just realized this was the problem that killed HOTOL as the engine was too far back...

[lkm]

If you want to think about alternate Skylon configurations then this is a good NASA paper to look at.
It's a 1979 study into possible mach 6 hypersonic passenger transports and you can clearly seen Skylon similarities in a number of features of the designs considered.

http://hdl.handle.net/2060/19800006815

[simonbp]

Skylon's wings are sized for takeoff and climb with full tanks.  On the return leg, the landing speed is quite low and the runway requirements are minimal.

They're sized for takeoff under full thrust. Put enough thrust on anything and it will takeoff, you'll just burn a ton of fuel in the process. The amount of LH2 that the Skylon would need to store for a go-around is massively more than the fumes they keep for reentry cooling.

[93143]

The amount of LH2 that the Skylon would need to store for a go-around is massively more than the fumes they keep for reentry cooling.

Very likely.

My point is, Skylon weighs about five times as much taking off as it does landing, even if it's landing with a full payload bay.  Your comment about "stubby wings" is misleading.

[BobCarver]

I was listening again to Mark Hempsell on a podcast (available at http://www.talkingspaceonline.com/2012-podcasts/episode-434-skylon-with-sabre-single-stage-to-orbit.html) & he was talking about Skylon re-entering the atmosphere. Basically, it doesn't heat up as much as the Space Shuttle did but it does stay in the 'heat zone' for longer, requiring the use of a small amount of liquid hydrogen to cool the airframe & act as a heatsink.

Which raises an interesting question, the Skylon is intended to glide to an unpowered landing but if there's a small quantity of (almost certainly not liquid anymore) hydrogen aboard, could that be harnessed to a greater or lesser degree and give Skylon a powered go-around should there be a last-minute problem with the runway or landing systems?

The LH2 for cooling is kept in a different tank, so it doesn't even have a way to be used as fuel for the main engines.

[RanulfC]

I was listening again to Mark Hempsell on a podcast (available at http://www.talkingspaceonline.com/2012-podcasts/episode-434-skylon-with-sabre-single-stage-to-orbit.html) & he was talking about Skylon re-entering the atmosphere. Basically, it doesn't heat up as much as the Space Shuttle did but it does stay in the 'heat zone' for longer, requiring the use of a small amount of liquid hydrogen to cool the airframe & act as a heatsink.

Which raises an interesting question, the Skylon is intended to glide to an unpowered landing but if there's a small quantity of (almost certainly not liquid anymore) hydrogen aboard, could that be harnessed to a greater or lesser degree and give Skylon a powered go-around should there be a last-minute problem with the runway or landing systems?

While there may not be any LH2 in the main tanks there will probably be gasous hydrogen available. The main question would be can the engines USE it?

My guess/gut-feeling is no they won't be able to and the vehicle probably won't have a "go-around" ability at all.

The entire airframe is going to WAY lighter during landing, (not so "much" with a return payload but given the overall mass at take off it's STILL not going to amount to "much" overall with just about any power from the engines (even un-augmented rockets) will probably provide go around power but that as far as I can see is going to take actual LOX and LH2 to feed the engines with.

Randy

[RanulfC]

I've frequently posted random Skylon thoughts, here's another one. Please be thorough, skeptical and unforgiving!

Ok since you asked so nicely

Why does Skylon have two nacelles?

As noted with the problems of the center/aft mounted HOTEL engines the nacelles also allow the engines to avoid having to swallow the fuselage/nose shocks which complicates intake design. Of course not those shocks are going to be impinging on the WINGS instead which is going to be a major stress and heating issue but...

(Note the dame type of configuration was "baselined" for the Lockheed CL-400 SUNTAN aircraft as well. Pretty much for the same reasons. In the end Lockheed went with nacelles buried in the wings and "optimized" for Mach-3 flight, but they weren't going to have to deal with the constant changes from Mach-2 onward up through the Skylon's "depart air-breathing-mode" flight profile)

Wouldn't it be possible to make the entire vehicle into one big nacelle, where the nose scoops up air and routes it to the back, where the rockets are positioned? That way
- the wings take smaller loads
- the aerodynamic profile is reduced, thus decreasing drag
- potentially less structural mass is required, as you don't need duplicate engines

Less drag & less mass = lower lift requirement = smaller wings & less rocketry (perhaps going from 8 to 7 nozzles) etc. etc.

Unfortunately this setup would
- necessitate an 80m long feedline for the chilled air from front to back, which could be costly
- change load requirements on the hull
- change landing gear position
- need to shift the wings around
- have a problem with nozzle placement, as the back side is tapered upward to allow takeoff

Actuall a LOT more "issues" with that in that the intake length would generate far to much drag, (Incoming air is "slowed/compressed" by the intake no matter WHAT shape you make it short of an "air-dumping" style intake such as the "inward-turning" ASTROX type it piles up too fast, slows down and heats up too much with a long intake) worse at LOW speeds you can't get enoug air down a long intake to effectivly run the engine! (Which is why you will see spring-door covered "intake" slots behind a supersonic intake system so that the engine can draw adequate air during taxi and take off)

You WANT multiple engines by the way, as it makes getting adequate thrust from your "engine" easier if you have more than one to spread the load over

You also kind of want to keep your "heavy" bits (such as engines) as close to the CG of the vehicle as possible to help with handling issues. While there would be "horizontal" handling issues from the wing-tip engine mounts should one manage to "un-ingest" the Mach shock, (and yes, boy will they be some FUN issues there you greatly reduce the "longitudal" (is that a word even??) problems for fuel expendature and aero-load shifts during flight.

"Ideally" you want your engines mounted both close to the horizontal AND longitudinal CG of the airframe but you (probably) compromise on some points to get the kind of performance REL is looking at from Skylon.

I can't see the COM shifting during ascent, as was the problem with HOTOL.
 
Alternatively, you could ditch the nosecone setup and add some ram scoops behind the wings, but this would turn into even more of a redesign.

EDIT - just realized this was the problem that killed HOTOL as the engine was too far back...

Yep, like I noted. "Ideally" you want something like the NASP or Model 1074-006 type "body-fed" short intakes as close to the overall vehicle CG as possible.
(See: http://www.secretprojects.co.uk/forum/index.php/topic,11580.msg110297.html#msg110297 for Model 1074-0006)

But that has some "issues" in a reentry vehicle with engine and intake/exhaust system heating during same.

Randy