The Reaction Engines Skylon Master Thread (1)

[RanulfC]

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.

Since the idea is just for reducing the take-off run, small-high-thrust RATO type rockets COULD work. However the issue isn't with them "falling-off" nor intergration since these are both "in-use" process' that could be adapted to the situation. The biggest issue is that in order to use such a system you would have to HEAVILY reinforce the fuselage near the take-off CG are and that's going to be some mass you can't shed with the boosters. So in the end the take-off assist would probably add more mass than would be acceptable to the overall design itself.

Randy

[RanulfC]

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.

Ugh, talk about picking a wrong example

The Etched-Titanium plates on the SR-71 weren't really a "TPS" as much as a whole lot of individual pieces flying in a very close formation until much later in the flight There was a whole substructure of insulation on top of an inner air-frame to try and keep the majority of the heat from penetrating too far into the main airframe. Lockheed had a heck of a time getting the titanium panels to work at all, and it was only after the SR-71 was excepted for opertion that someone ELSE came up with a set of methods that allowed titanium to be easier to work with and form.

The "original" shuttle metallic-TPS proposal was for a titanium-honeycomb-sandwich similar to the skin of the XB-70, but that went away along with the titanium sub-structure and airframe.

Active cooling and heat-pipe transfer technologies have been available (and used operationally in 'certain' systems, which it's hard to get info on ) since the '60s so we KNOW they work. There is however an "institutional" mis-trust of any "active" systems for reentry cooling as they are not 100% failure proof, but then again neither are "passive" systems.

Randy

[RanulfC]

Thanks to the rocket equation such vehicle are incredibly sensitive, now as Alan point out (but the original observation was I think Bob Parkinson) if the world were 10% smaller it would be very different.

Ahh! So the target market for Skylon is to ship 10% of the Earth into orbit to make things easier all around then?

Randy

[Andrew_W]

Mr. Hempsell,

I think the reasoning goes along these lines:

Skylon has an overall O/F ratio of 2.27. A rocket has an O/F ration of 6. Therefore you can place 75% more propellant into a Skylon vehicle if it is a pure rocket. This brings the end of burn mass down to about 14.5% even before mass savings from lower engine mass, lower landing gear mass (due to vertical takeoff), lower dynamic pressure, etc. Since structural mass scales with propellant volume and not propellant mass that should be a wash.

Thanks, though to copy the Skylon idea more closely I was allowing for greater wing area and heavier landing gear, increasing unladen mass of the C2 excluding engines by 7 tonnes, but then I hadn't figured on such high residuals.

[simonbp]

As adrianwyard points out the black TPS is actually a reinforced high temperature glass (such Pyrosic from Pyromeral Systems)

Skylon: The British Black-Glass Zeppelin That Will Ride Columns of Steam to Outer Space In An Inventive New Fashion!

Sounds rather steampunk/Isambard Kingdom Brunel-ish if you phrase it like that...

Needs more brass fittings!!

Built by a league of extraordinary gentlemen!

I fully expect the passenger module for Skylon to have wood paneling, velvet-upholstered seats, and brass dials and gauges. Also, a rack to put your top hat in microgravity.

[Hempsell]

From Jim Davis
“Since structural mass scales with propellant volume and not propellant mass that should be a wash.”

I am afraid I would take issue with this assumption.  The structure mass is primarily driven by the mass it carries, especially the oxygen mass, all the struts and frames have to be beefed up (bearing in mind our tanks are not load bearing.  If we carry more oxygen, even if the vehicle dimensions do not alter much, the primary structure goes almost linearly up and secondary (but significant) items such as the undercarriage scale strictly linear with the mass they carry.

“Even if the British government had £10 billion burning a hole in its pocket that they wanted to spend on RLV development is it likely that they would hand REL a contract of that size?”

The plan is to finance SKYLON from commercial equity not from public funds so no government is being asked to foot the bill.  It has never been the intention that REL would build SKYLON, but I think whoever does built it is unlikely to make significant changes from D1.

baldusi asks about the propellant ratio.  For SABRE 3 in air breathing mode the equivalence ratio is 2.8 (at Mach 4). In rocket mode the mixture ratio is 6.

simonbp
“I fully expect the passenger module for Skylon to have wood paneling, velvet-upholstered seats, and brass dials and gauges. Also, a rack to put your top hat in microgravity.”

I would rather like that, but as the passenger module is not part of the SKYLON development plan it won’t be up to us.

[fatjohn1408]

Mr Hempsell,

With such a large portion of the volume taken up by the LH2 tanks, i wonder if a kerolox design was ever contemplated.

Keeping the payload target equal, this would mean smaller and lighter tanks and a smaller vehicle in general. With exception of the wings because the mass of the vehicle would probably be larger.
Perhaps the smaller tanks might mean a lower drag and a faster acceleration to Mach 5.5.

I am skeptical wether it would be an improvement, but it would be interesting to know why it was dismissed.

And for the record, thanks for the info...

[Crispy]

There's a good reason why all SSTO designs use LH, not Kero - specific impulse. 450s vs 330s for the very best in class respectively. That's a big enough difference to outweigh any advantage made in tank weight.

[Hempsell]

Mr Hempsell,

With such a large portion of the volume taken up by the LH2 tanks, i wonder if a kerolox design was ever contemplated.

Keeping the payload target equal, this would mean smaller and lighter tanks and a smaller vehicle in general. With exception of the wings because the mass of the vehicle would probably be larger.
Perhaps the smaller tanks might mean a lower drag and a faster acceleration to Mach 5.5.

I am skeptical wether it would be an improvement, but it would be interesting to know why it was dismissed.

And for the record, thanks for the info...

No kerolox was never considered for either HOTOL or SKYLON because both Alan Bond’s pre-cooled engines (the RB545 and the SABRE) need to use liquid hydrogen for the heat absorbing properties and the engine performance (you take a big specific impulse hit with kerolox) and need to get a mass ratio of around 7%.

The use of the truss framework means the mass hit for the large tank volume is not as bad as you might imagine and a smaller structure would have some downsides.  First the ballistic coefficient goes up so you need heavier more fragile TPS, and second, assuming you keep the minimum supersonic drag form of SKYLON the cross section of the payload bay reduces below the 4.6 m which we consider the minimum for a 10 tonne plus payload.  If instead you keep the payload cross section you have a stubbier shape that actually make the supersonic drag worse or you have a lot of empty space which pushes the structure mass up.

[Seer]

Have you considered advanced carbon carbon for the aeroshell? It flew on the x37 and is meant to be only a quarter the density of RCC. It is also stronger and more oxidant resistance than RCC.

[simonbp]

I would rather like that, but as the passenger module is not part of the SKYLON development plan it won’t be up to us.

Interesting. So REL wouldn't design a baseline passenger module, but would leave that up to the customers (unless, presumably, they hire you do it)? Sounds like a reasonable conservative approach.

How involved will REL be in the design of the ground facilities?

[aceshigh]

Have you considered advanced carbon carbon for the aeroshell? It flew on the x37 and is meant to be only a quarter the density of RCC. It is also stronger and more oxidant resistance than RCC.

how about STARLITE plastic, from the other thread? (plastic rocket engine)

[tnphysics]

Could a passenger module be built for the standard SKYLON that would provide the needed functions?

[adrianwyard]

Re passenger module: Annex D of the Skylon User's Manual describes a conceptual Personnel/Logistics Module that would accommodate 24 people.

http://www.reactionengines.co.uk/downloads/SKYLON_User_Manual_rev1-1.pdf

It sounds as though the current 'D' Skylon designs will have more internal space than when this was written, so perhaps more capacity for passengers.

[kkattula]

There's a good reason why all SSTO designs use LH, not Kero - specific impulse. 450s vs 330s for the very best in class respectively. That's a big enough difference to outweigh any advantage made in tank weight.

Actually, no.  There have been SSTO designs that used Kero (Roton comes to mind) and plenty of existing kero first stages that are barely SSTO with no payload.  In vertical launch rockets, tank mass is proportional to propellant volume, not mass, and kero engines have better T/W.

Take any notional LH2 SSTO design, fill the tanks with kerolox, and add enough say RD-180s for the extra weight.  See what mass ratio and hence delta-v you get.  Not that I'm saying SSTO is easy, those designs tend to make very optimistic assumptions. Also, kero SSTO would be much more sensitive to mass budget overuns.


However, as explained above, in the case of Skylon, there are other considerations dictating LH2 as the appropriate fuel. This probably applies to any HTHL SSTO*, but more so Skylon because of the cooling required.


* You can stick a big hammerhead fairing on top of a VL, and get above the atmosphere before drag eats your lunch. Not so HTHL, especially airbreathing HTHL.

[tnphysics]

Makes sense! One other idea-what about another heat exchanger, from the hot He from the precooler directly to the GH2 from the first heat exchanger it passes through? Might cut the amount of LH2 needed significantly, at the expense of very high temperature hydrogen heat exchangers being needed (but doesn't the preburner already need that)?

I am not quite sure I understand this one.  The precooler (which is staged so is HX1 and HX2) is followed by HX3 in the preburner to further heat the Helium up so it has the power (100’s megawatts) to drive the turbines and pumps, we do not want to do any cooling of the Helium until it has done its work.  It is a classic thermodynamic cycle using the temperature difference between the heating end (HXs 1 to 3) and the cooling end HX 4 and we want to maximise the temperature difference.

This would be a separate heat exchanger that would operate in parallel (not series) to HX1 and HX2 and their associated turbomachinery. It would do no work, but it would cool air, allowing lower H2 flows-and thus greater airbreathing Isp.

My understanding of it is that there is an abundance of power at these high airspeeds.

OK I see what you mean now. The problem is that it would reduce the energy from the airflow into the Helium which is where we need it and nowhere to we have too much power HX3 is always doing some top up. Also the "excess" hydrogen is not wasted as it produces propulsion in the ramjet and even when just venting, being hot hydrogen, it can be made to produce significant thrust so there is little drive to minimise it from its current level.

What is the Isp of the ramjet?

[baldusi]

baldusi asks about the propellant ratio.  For SABRE 3 in air breathing mode the equivalence ratio is 2.8 (at Mach 4). In rocket mode the mixture ratio is 6.

Am I mistaken in thinking that the stoichiometric ration is 8:1? I've seen how the RL10 has improved efficiency going from 5, to 5.5 in the A4 and 5.88 in the B2. The RS-25D, RD-0120 and RD-0146 have a ratio of 6 to 1, even the proposed Raptor had a ratio of 6:1. When I use the mole solution I get a ratio of 8 to 1. Yet, all current and new future high efficiency LH/LOX seem to converge to 6:1. What am I missing? In particular, wouldn't it improve the mass fraction in the skylon?

[kraisee]

Its not just engine efficiency that counts.   The tanking mass for the vehicle increases a lot when using larger ratio's of LH2 to LOX, due to LH2's very low density.

So it becomes a trade at the full system level, between the best Isp from the engine vs. greater dry mass in the stage.

Experience shows that the best overall system performance tends to be in the 5:1 to 6:1 range.

Ross.

[baldusi]

Its not just engine efficiency that counts.   The tanking mass for the vehicle increases a lot when using larger ratio's of LH2 to LOX, due to LH2's very low density.

I might have understood it backwards, but the mixture is usually oxidized to fuel. So a mixture of 8:1 would be better than a 6:1. LH2 is about 16 lighter than LOX. So 8:1 would mean 15% less tank volume.

[93143]

If the nozzle had an infinite expansion ratio, stoichiometric would be the way to go.  But it doesn't, and under the circumstances, a ratio in the vicinity of 4:1 to 5:1 or so usually gives you the best Isp.  The trouble with that is that the tank and fluid handling equipment get large, so the best compromise ends up in the 5:1 to 6:1 range.