Will Skylon Actually Fly?

[Der Anfang]

Sorry to be off-topic, but every time I see this thread, I can't help but think of how great the name "Will Skylon" would be for a sci-fi character

You have inspired me. Consider yourself thanked. I love that! XD

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I was under the impression that liquid helium was used as the precooler coolant, and that the hydrogen was used for regenerative engine cooling only, something methane could easily do

Liquid Helium is indeed used in the cooling cycle of the Rapiers. Not sure why others are talking about Hydrogen, unless I missed something.

[Caroliano]

I was under the impression that liquid helium was used as the precooler coolant, and that the hydrogen was used for regenerative engine cooling only, something methane could easily do

Liquid Helium is indeed used in the cooling cycle of the Rapiers. Not sure why others are talking about Hydrogen, unless I missed something.

You are missing magnemoe and SargeRho answers: the hydrogen is the one that cools that helium (for patent avoidance as well for a technical reason I forgot)

Not sure about how much one kg of methane cools compared to one kg of hydrogen.

You need about 6 times more heat to rise 1°C on 1kg of hydrogen than methane. So, much more.

Methane is the most advantageous early in the flight for a regular rocket, because you can get better TWR and lower volume helps with the aerodinamic losses. But skylon don't need those high TWR on take-off, as it does so horizontally, and the lower volume it occupies is actually undesirable in skylon: it would require a complete redesign of the hypersonic/re-entry aerodinamics of the spacecraft, and I'm not sure if it is even possible.

For the late part of the flight, where you don't need to cool the incoming air anymore, the higher ISP of hydrogen wins hands down on performance over methane.

Methane isn't viable for skylon for various reasons.

EDIT: A simplified view of the cooling system:

Edited October 16, 2015 by Caroliano

[Der Anfang]

You are missing magnemoe and SargeRho answers: the hydrogen is the one that cools that helium (for patent avoidance as well for a technical reason I forgot)

You need about 6 times more heat to rise 1°C on 1kg of hydrogen than methane. So, much more.

Methane is the most advantageous early in the flight for a regular rocket, because you can get better TWR and lower volume helps with the aerodinamic losses. But skylon don't need those high TWR on take-off, as it does so horizontally, and the lower volume it occupies is actually undesirable in skylon: it would require a complete redesign of the hypersonic/re-entry aerodinamics of the spacecraft, and I'm not sure if it is even possible.

For the late part of the flight, where you don't need to cool the incoming air anymore, the higher ISP of hydrogen wins hands down on performance over methane.

Methane isn't viable for skylon for various reasons.

EDIT: A simplified view of the cooling system:

http://upload.wikimedia.org/wikipedia/en/d/dc/Sabre_cycle_m.jpg

The reason why hydrogen is used is because it doubles as the closed cycle fuel as well as a coolant for the helium, which in turn the chilled helium cools the intakes when they breathe the air.

From wikipedia:

"In either case the end result is an engine that has a poor thrust to weight ratio at high speeds, resulting in an engine that is too heavy to assist much in reaching orbit.[41]

The SABRE engine design aims to avoid this by using some of the liquid hydrogen fuel to cool helium in a closed-cycle precooler, which quickly reduces the temperature of the air at the inlet.[41] The air is then used for combustion much like in a conventional jet,[41] and once the helium has left the pre-cooler it is further heated by the products of the pre-burner, giving it enough energy to drive the turbine and the liquid hydrogen pump."

[Sanic]

Spoiler Alert: Probably not.

[Der Anfang]

Spoiler Alert: Probably not.

Then the Skylon's gotta go fast.

[Caroliano]

The reason why hydrogen is used is because it doubles as the closed cycle fuel as well as a coolant for the helium, which in turn the chilled helium cools the intakes when they breathe the air.

I'm not sure what you are answering to. What I didn't remember is why the helium is used at all, besides HOTOL patent avoidance. In there hydrogen was used to cool the air directly.

[Wjolcz]

I hope so. A flying SSTO would probably greatly boost space exploration. Once it works, everyone will want to have one.

But as others also are, I am skeptical. As much as I would love it to work and fly, and all that, I wonder why other companies aren't developing their own SSTO. Probably because it's either super hard to make one or it's not worth it.

[Der Anfang]

I'm not sure what you are answering to. What I didn't remember is why the helium is used at all, besides HOTOL patent avoidance. In there hydrogen was used to cool the air directly.

Oh derp. I thought I was answering to a question you asked, which I misread. Whoops! So, what's the issue with "patent avoidance?" I'm not entirely sure I understand.

[magnemoe]

Oh derp. I thought I was answering to a question you asked, which I misread. Whoops! So, what's the issue with "patent avoidance?" I'm not entirely sure I understand.

Someone probably patented cooling using hydrogen directly as the cooling medium.

Make an question why they don't work with hydrogen directly always posible to sign an deal skylon is unlikely to fly commercial before the patent expires anyway.

You are free to make prototypes of patented stuff, you are not allowed to sell or make money of them without a deal.

[Caroliano]

That someone was their team while working on HOTOL. Then when the British goverment pulled the funding it also seems to have used the Official Secrets Act to turn those patents secret (I'm not sure how it works) and now they can't use it.

[softweir]

There are special clauses under the British Patents Act which allow the British Government to "hide" any patents that are deemed to be a risk to British Security. Indeed, there is a Civil Service agency that scans patents as they are filed, and jumps on them. I assume this law would have been used in the case of HOTOL.

It is self-defeating. Companies that have patents blocked have no protection for their ideas, and therefore nothing prevents inventors in other nations where this law doesn't apply from filing similar patents. It is suspected that several agencies throughout the world watch for British patents that are filed and then disappear, and actively bring their contents to the attention of tech firms as being "interesting". Because of this several British firms now refuse to file patents in the UK, and there are cases where they have moved to other countries.

[Nibb31]

But as others also are, I am skeptical. As much as I would love it to work and fly, and all that, I wonder why other companies aren't developing their own SSTO. Probably because it's either super hard to make one or it's not worth it.

Both. It's also extremely inefficient. If you can design an SSTO that can launch a 1-ton payload to orbit, you could also make a multi-stage version that could launch a 10-ton payload.

[shynung]

Both. It's also extremely inefficient. If you can design an SSTO that can launch a 1-ton payload to orbit, you could also make a multi-stage version that could launch a 10-ton payload.

I don't think there's someone crazy enough to put something like REL's SABRE (or basically any rocket-jet hybrid motor) on a disposable stage. Stuff like precoolers aren't exactly cheap.

[Nibb31]

I don't think there's someone crazy enough to put something like REL's SABRE (or basically any rocket-jet hybrid motor) on a disposable stage. Stuff like precoolers aren't exactly cheap.

Who said it had to be disposable? The only purpose of having an SSTO is to make reusability easier, SSTO has no other merit. However, there is nothing that says that you can't have reusable multi-stage launcher.

The margins are tight on Skylon, and although it does seem to work on paper. In real-life, pretty much every aerospace engineering project ends up slightly overweight or slightly underperforming, or slightly more expensive than predicted... Compromises have to be made, and because of the paper-thin margins on Skylon, any compromise in weight, performance, or cost, means that it's not going to orbit.

Aiming for SSTO just for the sake of SSTO is stupid. If they designed it as a reusable suborbital hypersonic first stage, it might make more sense.

[magnemoe]

Who said it had to be disposable? The only purpose of having an SSTO is to make reusability easier, SSTO has no other merit. However, there is nothing that says that you can't have reusable multi-stage launcher.

The margins are tight on Skylon, and although it does seem to work on paper. In real-life, pretty much every aerospace engineering project ends up slightly overweight or slightly underperforming, or slightly more expensive than predicted... Compromises have to be made, and because of the paper-thin margins on Skylon, any compromise in weight, performance, or cost, means that it's not going to orbit.

Aiming for SSTO just for the sake of SSTO is stupid. If they designed it as a reusable suborbital hypersonic first stage, it might make more sense.

Yes, more so as anything outside of GEO will need an second stage to reach it anyway, making the second stage a bit larger will not increase cost much.

I guess even an SSTO Skylon would go suborbital for GTO payloads.

Again to go real fancy you could use the drop fuel tank and recover engines and system on this upper stage if fancy.

And it would be interesting to try the sabre engine on an supersonic cruise missile instead of scramjet.

No the supersonic cruice missile is probably not an very practical weapon, however the US air force is interested and will pay for development. It has the benefit that you can build an small engine who don't have to last long

[shynung]

Who said it had to be disposable? The only purpose of having an SSTO is to make reusability easier, SSTO has no other merit. However, there is nothing that says that you can't have reusable multi-stage launcher.

Aiming for SSTO just for the sake of SSTO is stupid. If they designed it as a reusable suborbital hypersonic first stage, it might make more sense.

So, something like Stratolaunch, except the carrier aircraft had SABREs on them? Seems like a neat idea.

Also, suborbital-capable hybrid motor might be interesting for military projects. SABRE attack drones, anyone?

[magnemoe]

So, something like Stratolaunch, except the carrier aircraft had SABREs on them? Seems like a neat idea.

Also, suborbital-capable hybrid motor might be interesting for military projects. SABRE attack drones, anyone?

No, this will work pretty different, the plane will go into space and have an cargo hold like the space shuttle.

It will however not reach orbital speed, just suborbital think 1700 m/s in KSP. it will release an satelite or probe with an smal second stage, think 3rd stage on ariane rockets who will accelerate up to orbital speed and the wanted orbit. The difference between orbital and suborbital speed is far higher in real world than in KSP.

Just going suborbital give you the benefit that you don't need to carry so much fuel and oxidizer as you would need to go orbital, reentry heat will be far lower on reentry as you move slower.

Since you release the payload in space it don't need faring and the second stage don't need high trust.

Downside is that you will need an second stage but you would need an second stage going anywhere except GEO anyway.

[KerikBalm]

Seems to me like what you described is a high altitude hypersonic stratolaunch... I don't see how what you described is fundamentally different from what he said...

And yea... I think a very high altitude, hypersonic stratolaunch rather than SSTO seems like an awesome idea.

I'd think the carrier craft (whether internal bay or cargo externally slung) would probably benefit from switching to closed cycle and boosting the payload a bit more, since you already have all the engines capable of that right there)... I'd think getting the payload to 3,000 m/s velocity at 60 km would not be unreasonable... at which point the second stage should easily be able to achieve orbit (the space shuttle+ ET had about... what, 8,000 m/s of delta V? the remaining 1,000-1,500 m/s was supplied by the SRBs?)

I see no reason that the second stage couldn't be a smaller reusable craft too.

[Nibb31]

Seems to me like what you described is a high altitude hypersonic stratolaunch... I don't see how what you described is fundamentally different from what he said...

Stratolaunch is a subsonic air launch. It's very different from a hypersonic high-altitude first stage in pretty much every way. One is a carrier plane, the other is a spacecraft.

[prophet_01]

From my point of view there isn't anything major that would prevent them from using the currently planed skylon for this.

Thrust and max weight on lift off seem like the main obstacles here. However, launching skylon with only partly filled tanks and a disposable stage + payload inside the cargo bay could make this a viable choice. It could drastically increase the max payload for a mission if required.

Especially if the final version of skylon can only reach orbit with a very tiny payload, it could increase the potential share of the market by a lot.

Edit: I don't think it's a good idea to design such a second stage reusable. Picking it up with skylon isn't rly an option and designing it with reentry protection would be a rly deep cut into your payload fraction.

Edited October 19, 2015 by prophet_01

[SargeRho]

An empty upper stage wouldn't weigh all that much I think, it'd probably be within Skylon's retrieval mass range. It's basically just one engine, avionics and empty fuel tanks. Park it in LEO, pick it up with the next orbital Skylon flight.

[magnemoe]

From my point of view there isn't anything major that would prevent them from using the currently planed skylon for this.

Thrust and max weight on lift off seem like the main obstacles here. However, launching skylon with only partly filled tanks and a disposable stage + payload inside the cargo bay could make this a viable choice. It could drastically increase the max payload for a mission if required.

Especially if the final version of skylon can only reach orbit with a very tiny payload, it could increase the potential share of the market by a lot.

Edit: I don't think it's a good idea to design such a second stage reusable. Picking it up with skylon isn't rly an option and designing it with reentry protection would be a rly deep cut into your payload fraction.

No and it would be practical for launching things outside of LEO where you will need an upper stage anyway.

Only issue is cargo bay size, it would be nice to use hydrogen for upper stage because of ISP and Skylon is designed to handle it anyway but it require large tanks.

[KerikBalm]

Stratolaunch is a subsonic air launch. It's very different from a hypersonic high-altitude first stage in pretty much every way. One is a carrier plane, the other is a spacecraft.

Conceptually, its the same, at least these are the similarities:

An reusable air breathing carrier that releases a payload at higher altitude and lower atmospheric pressure, with some horizontal velocity already.

A SABRE craft would just release it at even higher altitudes with even more horizontal velocity.

Of course there are going to be many design differences to deal with hypersonic flight regimes.

The SABRE craft doesn't need to be a spacecraft... if it only goes to 60 or even 80m... that would still be a massive benefit, but it would not pass the Karman line, and wouldn't be a "spacecraft"

Another difference is the sabre craft may cease operating as an airbreather at some point.

There's just no need to take the whole craft to orbit, you don't even need to take the whole craft to space. The aerodynamics of the release shouldn't be much of a problem if the release is at 60km.

Its the stratolaunch concept, taken an order of magnitude further in terms of velocity and altitude of the release... to the point where it may actually justify doing it.

[magnemoe]

An empty upper stage wouldn't weigh all that much I think, it'd probably be within Skylon's retrieval mass range. It's basically just one engine, avionics and empty fuel tanks. Park it in LEO, pick it up with the next orbital Skylon flight.

Problem is that you then you would need an arm like the shuttle have to put it into the cargo bay, upper stage would also need twice the amount of fuel to return to LEO, be able to operate until next launch and then close in so it can be grabbed.

An reusable tug operating out of an space station makes more sense, yes its require more infrastructure but don't cut down in launch weight so much.

This would only be relevant for standard equator orbits anyway

[78stonewobble]

Hmm, I'll say that... IF it works as advertised, it would be a rather brilliant thing to have (like so many many other things). I just don't see how they could get to proving that it works considering current economical and political climate.

As others point out, there is no demand for fast launch rates and without fast launch rates there won't be demand. A gordian knot of a problem...

Unless someone subsidises "fast" launch rates/access to space to promote an actual space industry and population.