Whats wrong with Skylon?

[SinBad]

Hello,

this is my first thread in the ksp forums, but ive been lurking a while. So ive been reading and reading, and whenever the subject of ssto, cheap reuseable lifters or promising new space craft is discussed, skylon hardly gets mentioned. If it does it gets dismissed.

why? The last i heard it was showing a lot of promise as a cheap, short turn around, fully reuseable moderate load lifter. Did i miss a press release or something?

 

 

 

[p1t1o]

As far as I can tell, Skylon, as a craft, looks totally viable, and awesome cool.

As far as problems go, I think that it is about whether or not air-breathing SSTOs will ever be the most economically viable way of getting to space. Whilst Skylon and SABRE are going through all sorts of tests and research and scale-ups, Elon Musk is bouncing rockets all over the show, doing tricks, showboating. etc.

For example, Elon's rockets can land and take off from pretty much any flat, tidy place already. Skylon will most likely require specially reinforced runways to be built.

But at this state of the game, i think there is everything still to play for. One advantage of Skylon is that the air-breathing engine technology can be adapted for use in a number of projects (eg: an airliner).

[Darnok]

I wonder why nobody is trying to make Skylon with 4 engines (2 jets and 2 rocket)?

[p1t1o]

10 minutes ago, Darnok said:

I wonder why nobody is trying to make Skylon with 4 engines (2 jets and 2 rocket)?

The engines can essentially interconvert between airbreathing and conventional rocket modes and will do so at the point in the flight regime where altitude makes airbreathing becomes untenable. Quite efficient LH2/LOx ones too. It will also have a fairly decent set of orbital maneuvering engines, similar to the space shuttle.

[ment18]

The problem with Skylon is that it is new, untested technology and no where near done.  Many programs have gotten much further than it and been cancelled.  

I hope it succeeds as well.  The vac ISP is 460(theory) for the SABERs, which is quite good.  The OMS also runs on Lh2/LOX so its has similar ISP.

Edited March 3, 2016 by ment18

[DerekL1963]

1 hour ago, SinBad said:

why? The last i heard it was showing a lot of promise as a cheap, short turn around, fully reuseable moderate load lifter. Did i miss a press release or something?

Because pretty much nobody other than Reaction Engines Ltd. and a few remaining heavilty blinkered fanboys believe it shows any promise at all.   It's an extraordinarily complex and expensive system with many, many potential bugs.

That and, as p1t1o points out, the bulk of the fanboys tend to chase the latest shiny.

[wumpus]

Money.  A coworker (with both pilots license and A&R qualifications) once mentioned that in school for one of those an instructor asked what kept a plane up in the air.  The "correct" answer was money.

To really get the idea of Skylon's limitations, you need to use the "realism overhaul" (this should work better with 1.1, or maybe with Linux).  They might work great (in KSP) to get a delta-v of ~3000m/s, but Earth requires a delta-v of over 9000 m/s.  Since Skylon requires its own fuel after ~2000m/s of flight, the entire engine is essentially "dead weight" after that.

Compare this to space-x.  Hopefully, space-x will finally launch their rocket sometime next week.  There aren't as much hopes for sticking the landing.  Nevertheless, the rocket will launch.  Here is the rub: the market for launches is so thin that the cost of throwing away a full blown rocket is less than the cost of the non-recoverable-engineering to build a fully reusable (and air-breathing) rocket.  While I'd love to see a three-stage system where the first two stages are recoverable (and the first stage breathes air), I just don't expect the space market to pay for it.

Personally, I felt that escape dynamics had a much better idea (they didn't require carrying an oxidizer, but could keep functioning all the way into space), but of course the initial costs were far too high.  It all comes down to money, and how much it costs to launch each craft into space.  Its hard enough to get the money to launch the first craft into space with rockets, and everything else is just too expensive for that first one (and for Skylon and Escape Dynamics it is more like that first hundred.  I'm guessing there are 100 launches on order for all commercial launchers, but it isn't much more than that, at least not enough to expect a profit).

[sevenperforce]

I think Skylon hasn't gotten a lot of press because it has made only very incremental developments. While important developments, they are still "small stuff" by press standards. 

It doesn't get a lot of discussion on the forums because it's a fairly noncontroversial and limited thing. If it works, great; we'll have a cheaper way of getting to orbit. If not, oh well. But it doesn't really expand our access to space. It is limited to LEO; it can't send something on GTO like SpaceX's Falcon 9. And that's if it works at all, which remains to be seen. 

Using airbreathers to orbit is a really tricky proposition. It's not just about altitude, either. The faster you're moving, the greater the kinetic energy of the incoming air. You've got to get that air moving out the back of your engine faster than it came into your engine. So even if you can design a high-thrust engine and somehow minimize drag, there is a limit to how fast you can be going and still gain anything. Eventually, the airstream will be moving in faster than you can accelerate it back out. 

[Nibb31]

1 hour ago, SinBad said:

Hello,

this is my first thread in the ksp forums, but ive been lurking a while.

So maybe you should do a search on the forums. Skylon has been discussed over and over for years.

1 hour ago, SinBad said:

So ive been reading and reading, and whenever the subject of ssto, cheap reuseable lifters or promising new space craft is discussed, skylon hardly gets mentioned. If it does it gets dismissed.

why? The last i heard it was showing a lot of promise as a cheap, short turn around, fully reuseable moderate load lifter. Did i miss a press release or something?

There is not enough demand for a quick turn around to be of any use, and there is no indication that it will be cheap. It's the bigger than an A380, and much more complex, it uses multiple unproven technologies and techniques, and there is no demand for building large numbers of it. Why would it be cheaper ?

The numbers, when compared to the actual demand for orbital launches, simply don't add up. Reducing costs is a chicken and egg thing. Low cost requires high demand. High demand relies on low cost, but also on an actual business model, that nobody has invented yet. Ultimately, investing billions in a system that only makes sense with launch rates that are 100 times higher than they are today simply doesn't work. 

And the whole concept revolves around all of those unproven technologies meeting all the planned cost and performance requirements as they do on paper. When was the last time any large engineering project achieved that? The reentry system that vaporizes hydrogen on the leading edges has never been experimented. The fuselage built around the self carrying composite tankage needs a whole need industrial infrastructure, and again, we have no experience with any of those materials on an actual aircraft, let alone in space and reentry conditions. The engines work on paper, but we don't know anything about their maintenance, reliability, or durability.

If a single one of those technologies underperforms in any way, then Skylon is not going to orbit and you have wasted billions.

 

[Streetwind]

The main thing that makes me dismiss Skylon is its roadmap.

From what the latest plans look like, the SABRE engine is scheduled to have a working prototype no sooner than 2020. And there'll be another 2 years or so from a working prototype to a production ready engine, and another year after that to get a set of four to six of them tested and flight certified.

And all that is before they even start on Skylon.

You need to understand that Reaction Engines is, as their name says, first and foremost an engine manufacturer. Skylon is a concept study that they are considering to produce, eventually, but their main focus is likely going to market the SABRE engine once they've finished it. There are other entities interested in using it for their own stuff, primarily various militaries who wish for faster airplanes.

All in all, I consider Skylon to start to become interesting to talk about around the time SpaceX establishes their Mars colony. It may happen by 2025... or it may happen sometime after 2030. Maybe it won't happen at all, who knows. The point is, it's not something happening in the near future. There's little point speculating about things a decade out.

[A35K]

47 minutes ago, Nibb31 said:

It's bigger than the A380

Only in terms of length... Its weight and wingspan are much smaller. And plus, don't you think sending an A380 sized plane in space is awesome?

2 hours ago, p1t1o said:

Skylon will most likely require specially reinforced runways to be built.

I don't see why this should be the case. Unless it lands exceptionally fast, there is no reason it shouldn't be able to land at most major airports. I guess it depends on how good it's braking systems are. However, looking at its small wings, it seems a high landing speed is inevitable.

[Admac]

The other thing is the fuel it uses is liquid hydrogen which is super terrible to work with. It embrittles things, and it has to be handled differently than other cryofuels. The result of this is aerospace grade liquid hydrogen costs a bit more at about 2-10 dollars a kilo, compared to about 40 cents per kilo for methane. The high fuel costs mean, best case scenario, based on the skylon working perfectly, have a full launch manifest, and lasting for decades of daily service, they are pretty close to 1000 USD per kilo to LEO. Still way better than $15000 per kilo with the shuttle program, but higher than the $500 dollar a kilo system than SpaceX is targeting at with their 10m Methalox launcher, that should be ready to go much sooner(By decades?), with 1/100th the potential hurdles.

To be fair though, it's all still rocket science! But really all it comes down to is price per kilo to orbit, and reusable methalox rockets are the cheapest thing on the horizon, which is why we see SpaceX and Blue Origin both basing their next generation of systems on it. Oh, and Skylon wouldn't work on Mars...

[Nuke]

i have a feeling we will see the engines at least. i view the plane itself more as a loose concept rather than a final product, to give potential buyers of the engine an idea of what it can do. id love to give a couple of those engines to scaled composites and see what they could come up with.

[Bill Phil]

1 hour ago, Nibb31 said:

So maybe you should do a search on the forums. Skylon has been discussed over and over for years.

There is not enough demand for a quick turn around to be of any use, and there is no indication that it will be cheap. It's the bigger than an A380, and much more complex, it uses multiple unproven technologies and techniques, and there is no demand for building large numbers of it. Why would it be cheaper ?

The numbers, when compared to the actual demand for orbital launches, simply don't add up. Reducing costs is a chicken and egg thing. Low cost requires high demand. High demand relies on low cost, but also on an actual business model, that nobody has invented yet. Ultimately, investing billions in a system that only makes sense with launch rates that are 100 times higher than they are today simply doesn't work. 

And the whole concept revolves around all of those unproven technologies meeting all the planned cost and performance requirements as they do on paper. When was the last time any large engineering project achieved that? The reentry system that vaporizes hydrogen on the leading edges has never been experimented. The fuselage built around the self carrying composite tankage needs a whole need industrial infrastructure, and again, we have no experience with any of those materials on an actual aircraft, let alone in space and reentry conditions. The engines work on paper, but we don't know anything about their maintenance, reliability, or durability.

If a single one of those technologies underperforms in any way, then Skylon is not going to orbit and you have wasted billions.

 

What about the demand for autos? As far as I can tell there was none a century and a half ago.

[SuperFastJellyfish]

33 minutes ago, Bill Phil said:

What about the demand for autos? As far as I can tell there was none a century and a half ago.

There was a demand for people to travel from A to B faster than by horse.  There is little demand to go to space besides comsats, GPS, and science packages.

Edited March 3, 2016 by SuperFastJellyfish

[Bill Phil]

52 minutes ago, SuperFastJellyfish said:

There was a demand for people to travel from A to B faster than by horse.  There is little demand to go to space besides comsats, GPS, and science packages.

There were trains. Demand already filled to a degree.

There is demand, it's just drowned out by the enormous costs.

[Matuchkin]

3 hours ago, wumpus said:

Hopefully, space-x will finally launch their rocket sometime next week.  There aren't as much hopes for sticking the landing.

Umm. I think, uhh... *ahem* I think you missed out on something...

Edited March 3, 2016 by Matuchkin

[SinBad]

thanks for all the replies.

so what it comes down to is that its not at a point in its development where the media can take pretty pictures of it.

its unproven technology or technology that hasn't been used this way before.

there is doubt about the financial viability of its business model.

it assumes the use of hydrogen as fuel will become more common and the cost of cryogenic lh2 handling will come down in the future.

is that about right?

[Matuchkin]

I do not think it is unproven. In fact, there were many SSTO designs, one of which being tested in real life. The largest problem here is the financial viability, after all.

[sevenperforce]

1 minute ago, Matuchkin said:

I do not think it is unproven. In fact, there were many SSTO designs, one of which being tested in real life. The largest problem here is the financial viability, after all.

Getting that kind of mass fraction to orbit is definitely unproven. 

[Matuchkin]

2 minutes ago, sevenperforce said:

Getting that kind of mass fraction to orbit is definitely unproven. 

Okay, sure it is unproven, but there is a lot of experience in the area.

[sevenperforce]

2 minutes ago, Matuchkin said:

Okay, sure it is unproven, but there is a lot of experience in the area.

No experience getting that kind of mass fraction to orbit. 

[Bill Phil]

The problem is that whenever you want to multiply your deltav by a certain factor, without changing your ISP, you have to raise your mass ratio to that factor. It's the real issue with any SSTO. 

Skylon tries (or will try) to beat this by using atmospheric oxygen for some part of the journey, but you still need pretty darn good ISP.

[Nothalogh]

9 hours ago, p1t1o said:

(eg: an airliner)

I thought you were gonna say military applications

[sevenperforce]

1 hour ago, Bill Phil said:

The problem is that whenever you want to multiply your deltav by a certain factor, without changing your ISP, you have to raise your mass ratio to that factor. It's the real issue with any SSTO. 

Skylon tries (or will try) to beat this by using atmospheric oxygen for some part of the journey, but you still need pretty darn good ISP.

And I'd wager that their primary boost to effective ISP comes from passing a large bulk of nitrogen through their engine.

In fact, I bet you could get significantly better effective ISP and a lower dry mass by relaxing the combustion requirements and opting for a less demanding, denser cryogen like slush-liquid methane. Plan on carrying all your oxidizer, use fuel-rich staged combustion for your turbine, and set up the turbine to also run an air compressor. The compressed air gets pumped straight into the neck of your exhaust bell, with an intended air mass flow greatly exceeding your fuel mass flow. As you speed up, use the slush methane to partially precool the incoming air...but not as much as SABRE, since you aren't going to need to combust it. Should be able to operate further than SABRE can and with a far greater effective ISP.