Whats wrong with Skylon?

[fredinno]

12 hours ago, p1t1o said:

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).

No, it's basically that staging is a lot more effective than 0 staging, which is what Skylon uses, to becoming economical.

10 hours ago, Streetwind said:

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.

Spoiler

Neither SpaceX Mars nor Skylon will happen by 2030.

 

7 hours ago, Bill Phil said:

There were trains. Demand already filled to a degree.

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

No, trains are limited to their rails. Cars were much more versatile on where they could go.

1 hour ago, sevenperforce said:

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.

Yeah, and the insulation mass for slush-liquid methane is going to be very high. Ask Elon with the F9FT.

[sevenperforce]

1 hour ago, fredinno said:

 

  Reveal hidden contents

Neither SpaceX Mars nor Skylon will happen by 2030.

Yeah, and the insulation mass for slush-liquid methane is going to be very high. Ask Elon with the F9FT.

Better than using liquid hydrogen. 

[Nibb31]

7 hours 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.

Most of its technologies have never been tested even at a subscale level: the tanks, airframe, the TPS, the engine of course, and probably plenty of other stuff. We have no idea how mature those technologies are for an operational vehicle, how fragile or durable they are, their cost, or their maintenance requirements. That is unproven.

There's a huge difference between a paper study and actual engineering. Look at the engineering that goes into producing an airliner or a conventional rocket, which is based on decades of experience, existing facilities, certified suppliers, etc... For Skylon, you need to throw that experience away and reinvent everything, from the supply chain to the manufacturing facility, including tooling, test fixtures, training personnel... You basically need to build a whole industry from scratch.

5 hours ago, sevenperforce said:

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. 

The LH2 is also used for cooling the engine and the ceramic composite skin on reentry. Methane won't work.

Edited March 4, 2016 by Nibb31

[blowfish]

3 hours ago, sevenperforce said:

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.

I don't think this is true.  Air is about 23.2% oxygen by mass, but you only need 1 kg of hydrogen for every 8 kg of oxygen, so you're really getting most of the mass you're combusting from the air.  In other words, you'd need to have several times the fuel flow to get the same thrust if you carry the oxidizer.

Density does affect drag, but ultimately on a single stage vehicle you need all the efficiency you can get, and methane doesn't really cut it.  And a higher takeoff mass means larger wings, which means more drag anyway, especially at higher mach numbers.

Everything you're mixing in the combustion chamber should be at the same pressure, otherwise you're loosing efficiency.  And the precooling is not really about temperatures after combustion so much as temperatures in the compressor - the SABRE has no moving parts in the flow after the compressor, the compressor itself being driven by the helium loop.  This, combined with the precooling, means that the air can be compressed much more than in traditional jet engines, increasing the specific impulse significantly.

[blowfish]

@Nibb31 Source on hydrogen cooling the skin on re-entry?  I recall reading in the user manual that all excess propellant in the main tanks is dumped before even circularizing the orbit.

[Temeter]

Many people here alrady described it quite well: Reaction Limited is an engine producer, and the whole Skylon craft is mostly just a concept. While the engines certainly are the most remarkable part of the craft, the remainder of it mostly exists on paper. It's very far away from actually being produced, and not sure if it's ever gonna happen. Hopefully, but don't get your hopes up just yet.

Oh, and good old Elon was critical about it once, so some people just took that stance and became quite dismissive about it.

[KSK]

38 minutes ago, blowfish said:

@Nibb31 Source on hydrogen cooling the skin on re-entry?  I recall reading in the user manual that all excess propellant in the main tanks is dumped before even circularizing the orbit.

It's an old source but judging by the authors, is pretty authoritative. See second page, right hand column, where active cooling is briefly mentioned. It's mentioned in a little more detail here as well.

 

[Streetwind]

I just came across this, which seems relevant: http://www.space.com/32115-skylon-space-plane-engines-air-force-vehicle.html

Salient points:

"Since January 2014, the Air Force Research Laboratory (AFRL) has been developing hypersonic vehicle concepts that use the Synergetic Air-Breathing Rocket Engine (SABRE) (...) In April 2015 (...) an AFRL study had concluded that SABRE is feasible. (...) The lab will reveal two-stage-to-orbit SABRE-based concepts either this September (...), or in March 2017 (...)."

"AFRL (...) views a single-stage-to-orbit Skylon space plane as "technically very risky as a first application [of SABRE]," and this is why the lab is developing two-stage-to-orbit concepts."

"Reaction Engines is planning to have a ground-test SABRE engine ready by the end of the decade. However, Jeremy Nickless (...) distanced the company from the milestones described (...) previously."

[Nibb31]

Yes, the USAF is interested in the SABRE engine, but they have no interest in Skylon and have doubts that it's a viable vehicle.

Edited March 4, 2016 by Nibb31

[Bill Phil]

5 hours ago, fredinno said:

No, it's basically that staging is a lot more effective than 0 staging, which is what Skylon uses, to becoming economical.

  Reveal hidden contents

Neither SpaceX Mars nor Skylon will happen by 2030.

 

No, trains are limited to their rails. Cars were much more versatile on where they could go.

Yeah, and the insulation mass for slush-liquid methane is going to be very high. Ask Elon with the F9FT.

That's why I said filled to a "degree." And that limit was always rising, as more tracks were being built.

Cars are only really more versatile in cities, and even then you could get around by biking, walking, or taking a subway.

Trains allow for fast and efficient travel over long distances. Then you could get a car, or, a hundred something years ago, get a horse, and still get to your destination within a day or two, and it still takes a while for cars to go places. Just going to a nearby state in the U.S. is usually hours, except on the eastern coast.

7 hours ago, sevenperforce said:

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.

Last I recalled oxygen rich is more efficient, the Russian RD-180s do this, with ISPs up to 311.

[KSK]

1 hour ago, Bill Phil said:

Cars are only really more versatile in cities, and even then you could get around by biking, walking, or taking a subway.

Trains allow for fast and efficient travel over long distances. Then you could get a car, or, a hundred something years ago, get a horse, and still get to your destination within a day or two, and it still takes a while for cars to go places. Just going to a nearby state in the U.S. is usually hours, except on the eastern coast.

It's getting a bit off-topic but I'd really have to dispute that, at least in the UK. Driving in cities isn't a great deal of fun and there tend to be sufficient mass-transit options to provide feasible and convenient alternatives to travelling by car. Outside of cities, travel by car is far more versatile, especially if you're going somewhere that requires multiple journey stages by mass-transit, or are going somewhere small and poorly served by bus or train routes

[Nibb31]

There has always been demand for fast transportation. Ever since humanity exists, there has been a need to go from point A to point B because point B was a destination where people needed to meet other people, trade stuff, visit things, etc...  Horses, ships, cars, planes were invented to meet a strong demand that already existed. Roads, railroads, shipping lines, and airport infrastructure were built to connect the As and the Bs of the world.

Space is not an appealling destination for mass transport. There are no friends or family to visit, there is no business or trade to do, and there isn't even anything much to visit. Space will remain a niche until someone comes up with a strong business model that requires frequent launches to orbit. And that might never happen, just like we have never found a massive interest in building an international airports on the Southern Sandwich Islands or a high speed train bridge to the middle of the Atlantic Ocean. 

Most people on these forums only look at the technical side of things, but the barriers are rarely technological. When the need for mass transportation arises, the technical solutions will come naturally. The problem is finding the "killer app" for space travel, a compelling reason for people to want to launch stuff into space. Until that happens, reusability and fast turnaround are pointless. Something like Skylon is a bridge to nowhere.

Edited March 4, 2016 by Nibb31

[Kertech]

Even if skylon goes belly up the SABRE engines have had a lot of development (in fact other than some cool graphics the rest of the craft is a "we'll come up with it later" affair). Let's hope no one gets spiteful with the patents if it does go, those enginges do look like an SSTO spaceplane maker

[magnemoe]

11 hours ago, Nothalogh said:

I thought you were gonna say military applications

Military makes more sense, its an interest in an hypersonic cruise missile, rapier might work out better than scramjet. 
An hypersonic passenger plane would be very expensive to develop and operate. Saftey and turnover requirements are far higher.

[magnemoe]

20 hours ago, wumpus said:

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).

An suborbital only Skylon makes some sense, limit it to around 2 km/s as I understand this is the limit of the air breathing engines
Far larger cargo hold, who holds payload and upper stage, takeoff normally speed up and do the climb, then in space (air resistance at 2 km/s can be ignored) you release upper stage and payload who ignite and do the rest of the trip, Skylon reenter the atmosphere but don't require serious heat shielding as speed is manageable. 
it return to base for next mission. 
Benefit is that upper stage and payload is released higher and faster than normal two stage rockets, smaller stage, lower TWR requirements and no need for fairings. 
As its an plane its easier to use multiple bases for launch and it can cruise from there its serviced to there its launched. 

Yes it will still be expensive to develop but far more realistic than an SSTO.

[John FX]

Is the reason large upfront costs for debatable savings?

[Nibb31]

15 minutes ago, John FX said:

Is the reason large upfront costs for debatable savings?

What?

[John FX]

13 minutes ago, Nibb31 said:

What?

What do you mean by what?

EDIT : Ah, does my post make more sense if you replace the word `reason` with `answer`?

Your post needs many more words to make sense though...

Edited March 4, 2016 by John FX

[sevenperforce]

8 hours ago, blowfish said:

I don't think this is true.  Air is about 23.2% oxygen by mass, but you only need 1 kg of hydrogen for every 8 kg of oxygen, so you're really getting most of the mass you're combusting from the air.  In other words, you'd need to have several times the fuel flow to get the same thrust if you carry the oxidizer.

Density does affect drag, but ultimately on a single stage vehicle you need all the efficiency you can get, and methane doesn't really cut it.  And a higher takeoff mass means larger wings, which means more drag anyway, especially at higher mach numbers.

Everything you're mixing in the combustion chamber should be at the same pressure, otherwise you're loosing efficiency.  And the precooling is not really about temperatures after combustion so much as temperatures in the compressor - the SABRE has no moving parts in the flow after the compressor, the compressor itself being driven by the helium loop.  This, combined with the precooling, means that the air can be compressed much more than in traditional jet engines, increasing the specific impulse significantly.

The goal would be to have a lower fuel+oxidizer flow, actually, but with a mass flow several times greater. You already have to have a preburner or gas generator to run your turbopump, so beef it up slightly and run it to your compressor. Cool the incoming airstream only as much as is needed to keep your compressor from melting, and inject the pressurized air into the exhaust stream. So, basically a precooled air turborocket.

Come to think of it, you don't actually have to have a preburner or gas generator. You can bleed high-pressure gas straight from the combustion chamber and use it to run your compressor and turbopump, then exhaust it into the airflow that is compressed by the compressor.

Up to around 3 km/s, the exhaust flow would be more air than propellant.

5 hours ago, Bill Phil said:

Last I recalled oxygen rich is more efficient, the Russian RD-180s do this, with ISPs up to 311.

The Russians used oxy-rich preburning for the turbopumps because preburning fuel-rich RP-1 will coke and choke. Don't think it's an efficiency thing. Oxy-rich preburning is horribly corrosive and generally a nasty nasty affair but it's better than filling your engine with coal tar.

1 hour ago, magnemoe said:

An suborbital only Skylon makes some sense, limit it to around 2 km/s as I understand this is the limit of the air breathing engines
Far larger cargo hold, who holds payload and upper stage, takeoff normally speed up and do the climb, then in space (air resistance at 2 km/s can be ignored) you release upper stage and payload who ignite and do the rest of the trip, Skylon reenter the atmosphere but don't require serious heat shielding as speed is manageable. 
it return to base for next mission. 
Benefit is that upper stage and payload is released higher and faster than normal two stage rockets, smaller stage, lower TWR requirements and no need for fairings. 
As its an plane its easier to use multiple bases for launch and it can cruise from there its serviced to there its launched. 

Yes it will still be expensive to develop but far more realistic than an SSTO.

I like this idea. However, it's not too far from what SpaceX is already doing. Staging speed for the reusable first stage of the Falcon 9 is around 2 km/s and it takes place out of atmo. You'd have to show how doing this with a partial airbreather would be better than doing the same identical thing with the Falcon 9 first stage. Also, what would the second stage engine need to look like?

[wumpus]

19 hours ago, Matuchkin said:

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

Looks like the winds have died down and countdown is scheduled for tonight.  Space-x vs. my misstatement: go!

Reaction Engines' problems (which can really boiled down to too many money issues) break down into two sets:

Skylon and SSTO issues: SSTO is a basically a converse error falacy*: since an efficient rocket will have 0 stages, SSTO believers seem to think that 0 stages mean an efficient rocket.  It doesn't work that way.  In any event, Skylon will have to pull ~7000m/s delta-v with the dead weight of the SABRE engine.  While this is likely an improvement over previous SSTO ideas (which needed a couple thousand more delta-v), it still is fighting the rocket equation (and that dead weight in the engine isn't helping).

SABRE issues: I'll assume that these are largely financial, but the hydrogen is likely a problem (anybody know if could work on jet fuel or even methane?).  Rebuilding SSMEs was a nasty business and of all the criticisms of SLS, building non-resuable RS-25s is low on the list.  

I use this argument typically when people insist on bringing up hydrogen cars, but even in hydrogen's best enviornment (space) it has use: consider the biggest jets, for England-Australia a jet carries roughly half its weight in kerosene.  Aircraft manufactures and their customers wildly value low weight in aircraft, yet won't even consider a hydrogen powered plane.  Assuming you could supply sufficiently long flights with a takeoff weight of 2/3 or so of a kerosene plane, why aren't they jumping for it?  Presumably the answer is that hydrogen is just that bad, and that you really want to avoid it for anything but the upper stages of rockets.

Note: Spaceshipone has a delta-v of 1,700 m/s, less than what the SABRE can presumably provide with air-breathing mode.  A London-New York flight would presumably need ~4,000 m/s.  Making this a "stepping stone" to space makes a whole lot more sense than trying to get to orbit in one go, but I suspect that they can't even raise the money for that.

[Frozen_Heart]

The vehicle itself looks fine. What makes me think that it will never happen is the company. They are completely unequipped ad unable to make a spacecraft or launch vehicle and no matter how much money is poured in they will just produce fancy powerpoint presentations.

[blowfish]

2 hours ago, sevenperforce said:

The goal would be to have a lower fuel+oxidizer flow, actually, but with a mass flow several times greater. You already have to have a preburner or gas generator to run your turbopump, so beef it up slightly and run it to your compressor. Cool the incoming airstream only as much as is needed to keep your compressor from melting, and inject the pressurized air into the exhaust stream. So, basically a precooled air turborocket.

Come to think of it, you don't actually have to have a preburner or gas generator. You can bleed high-pressure gas straight from the combustion chamber and use it to run your compressor and turbopump, then exhaust it into the airflow that is compressed by the compressor.

You're not going to get anywhere close to the I_sp of the SABRE with that.  Think about it - a jet engine or turborocket takes energy from combustion to impart energy to the air flowing through it.  So in order to impart the same amount of energy to the air, you really need about the same amount of combustion mass.  And most of that combustion mass is oxidizer

BTW, the SABRE doesn't really function the same way as other rockets/jets as far as the pumping/compressing goes.  Both the compressor and fuel pumps are driven by the helium loop, which gets its energy from extracting a temperature difference between the fuel and the incoming air.  At lower speeds, the incoming air isn't very hot so it also needs a heat exchanger after the preburner, but there are no turbines in the actual propellant/air flow path.  Once you get up to higher mach numbers, it's not actually tanking any energy from the combustion process.

[sevenperforce]

22 minutes ago, blowfish said:

You're not going to get anywhere close to the I_sp of the SABRE with that.  Think about it - a jet engine or turborocket takes energy from combustion to impart energy to the air flowing through it.  So in order to impart the same amount of energy to the air, you really need about the same amount of combustion mass.  And most of that combustion mass is oxidizer

BTW, the SABRE doesn't really function the same way as other rockets/jets as far as the pumping/compressing goes.  Both the compressor and fuel pumps are driven by the helium loop, which gets its energy from extracting a temperature difference between the fuel and the incoming air.  At lower speeds, the incoming air isn't very hot so it also needs a heat exchanger after the preburner, but there are no turbines in the actual propellant/air flow path.  Once you get up to higher mach numbers, it's not actually tanking any energy from the combustion process.

Well, regarding energy, I'm not interested in adding an equivalent amount of energy to the airflow. I'm interested in adding substantially lower energy, actually, but adding it to a much larger mass flow, so that the momentum impulse is higher. Lower fuel consumption but greater thrust. 

Curious, about the helium loop. How does the engine get started, with no initial airflow?

Edited March 4, 2016 by sevenperforce

[AngelLestat]

I was many times in this discussion like many others here so I will be brief..
The demand and economic argument: some believe that there is no enough. I believe that space demand will rise by a lot, and it is already rising even at current costs, ESA make an economic study (1 millon euros cost 1 year to complete) about skylon and they found that it has economic sense.
Of course that the development cost is very high, the same as any airplane.. due mostly to aerodynamics.  But no so high as many might think, the main goal at first is just transport cargo, a normal airplane needs to make an airplane that will work the 100% of the time 24hs 365 days all years with 400 innocent passenger who paid its ticket.
Yeah, skylon has many other difficulties to overcome, but all are completely in the range of our current technology.
 

11 hours ago, Temeter said:

Oh, and good old Elon was critical about it once, so some people just took that stance and became quite dismissive about it.

Of course he criticize skylon... if investor think that in less than 10 years spacex may have problems competing with skylon, they may choose not to invest today in spacex.  (I am not saying that it would)
He also said that fuel cell vehicles does not have sense..  but they have even more sense (in the transport business) than normal battery vehicles.
He needs to show that their products are the best with no question to make the best sales and attract investors.

10 hours ago, Nibb31 said:

Yes, the USAF is interested in the SABRE engine, but they have no interest in Skylon and have doubts that it's a viable vehicle.

USAF review only the engine...   so said that their are no interested in skylon is just your assumption. They said that it will be nice to start with a 2 stages vehicles at first..  but I dont remember nothing about a skylon comment...  

[blowfish]

53 minutes ago, sevenperforce said:

Well, regarding energy, I'm not interested in adding an equivalent amount of energy to the airflow. I'm interested in adding substantially lower energy, actually, but adding it to a much larger mass flow, so that the momentum impulse is higher. Lower fuel consumption but greater thrust. 

Curious, about the helium loop. How does the engine get started, with no initial airflow?

Low pressure, high flow devices loose thrust and efficiency very quickly above mach 1.  It's why supersonic jets all use low-bypass turbofans or turbojets.

I don't know exactly how they plan to start it.  Since it really only needs to start once per flight, they might just vent a bunch of pressurized helium through it to start.