Whats wrong with Skylon?

[sevenperforce]

1 minute ago, blowfish said:

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.

Well, this would be a high pressure device. Moreover, the compressor serves only to inject the compressed working mass into the exhaust bell; the kinetic energy in the exhaust bell does the work on the mass. It was my understanding that supersonic jets use low-bypass turbofans or turbojets primarily due to heat dissipation problems.

[fredinno]

10 hours ago, Bill Phil said:

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.

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

 

6 hours ago, magnemoe said:

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.

 

4 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.

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

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.

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?

You both have it wrong- Oxy-rich is used for RP-1 Lox primarily since Lox is lighter than Rp-1, so doing so results in higer ISP, despite Lox being corrosive. CH4 and LH2 use fuel-rich since it's lighter than Lox, and results in higher ISP.

Well, our hypothetical Skylon- TSTO would likely have a much larger payload fraction, due to using airbreathing, and can likely go higher than 2 km /s, SABRE can use both air and Lox, you know. It would likely allow for a <30T to LEO TSTO, or maybe a TSTO fully reusable spaceplane? 

18 minutes ago, AngelLestat said:

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.
 

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.

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...  

Yes, Elon actually does have a bit of an edge when it comes to fuel cells. The low density of H2 limits your range capacity, and fuel cells are less efficient than Batteries. Fuel Cells have their own advantages though, such as being able to push much more power more quickly, but that's a major debate in the renewable energy community, along with algae biofuel.

Either way, I'm sure skylon might make some economic sense- but good luck building it. It's like the 2-stage Shuttle back in 1970- when spaceborne liftingbody/ delta-wing/ spaceplane vehicles were not evereven tested before. Nowadays, such a thing (2STO spaceplane) is much easier, but SSTOs are still out of reach.

[sevenperforce]

10 minutes ago, fredinno said:

Oxy-rich is used for RP-1 Lox primarily since Lox is lighter than Rp-1, so doing so results in higer ISP, despite Lox being corrosive. CH4 and LH2 use fuel-rich since it's lighter than Lox, and results in higher ISP.

What would lead you to the conclusion that coking isn't a consideration?

[fredinno]

59 minutes ago, sevenperforce said:

What would lead you to the conclusion that coking isn't a consideration?

Coking IS, just not the primary one.

[wumpus]

1 hour ago, AngelLestat said:

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.

I suspect that the real number is more than a [US] billion.  For one million, NASA could just throw them some spare change (a bunch of [US] companies have received millions to develop potential products for NASA. I suspect that for something as useful as a SABRE, they would easily pay more to a UK company).

I'm also guessing that "making a bunch of assumptions and ignoring the risk that any of them are wrong" is needed to justify "economic sense".  Note that the assumptions include such big things as "all the engineering of an unknown system" works and "there really is a market for putting lots more things [and people.  And the things that are also people*] into space".

In another thread I mentioned that the company behind the SOAR spaceplane wouldn't be profitable if a magic venture cap fairy came down and gave them a first working article of a rocket.  Skylon is different, in that if Reaction Engines had a working SABRE (I still have my doubts about Skylon), they would at least be able to build something profitable (but don't be surprised if it takes *another* billion.  Those things are expensive).  It's that bit about getting the huge pile of money needed to do that, and then finding a way to pay back those who provided that big pile of money.

* A Douglass Adams reference.  Not implying that AI will exist before Skylon flies

[sevenperforce]

In the interest of eliminating heavy moving parts...

There has to be a way of designing inlet geometry such that increase in speed moves the shock surface out of the inlet precisely in accordance with the optimal efficiency requirements. 

[AngelLestat]

2 hours ago, fredinno said:

Yes, Elon actually does have a bit of an edge when it comes to fuel cells. The low density of H2 limits your range capacity, and fuel cells are less efficient than Batteries. Fuel Cells have their own advantages though, such as being able to push much more power more quickly, but that's a major debate in the renewable energy community, along with algae biofuel.

No, he doesn´t.  I know very well of what I am talking about.
But this is not the place..  I made some comments in this topic:

2 hours ago, fredinno said:

Either way, I'm sure skylon might make some economic sense- but good luck building it. It's like the 2-stage Shuttle back in 1970- when spaceborne liftingbody/ delta-wing/ spaceplane vehicles were not evereven tested before. Nowadays, such a thing (2STO spaceplane) is much easier, but SSTOs are still out of reach.

Nobody said that it will be easy, but a 2STO with sable engines does not have much sense, because it will have a huge cost also without the real benefits of 1 stage to orbit in reusability vehicle, mostly for the time it will need to relaunch, which is directly translated to cost.
In that case keep the falcon9 or heavy..  there is no difference.

 

1 hour ago, wumpus said:

I suspect that the real number is more than a [US] billion.  For one million, NASA could just throw them some spare change (a bunch of [US] companies have received millions to develop potential products for NASA. I suspect that for something as useful as a SABRE, they would easily pay more to a UK company).

Ok, my english was no right in that sentence... what I wanna said is that ESA did an study about if skylon has economic sense taking into account all costs and the future demand.. That economic study took 1 year to complete with a cost of 1 millon euros (the economic study).  The cost is just to point it was a serious study.  

[YNM]

I personally think that the problem with Skylon is that no large organization funds them reasonably (ex. CCDev by NASA), while it's still tied to governmental secrecy - the engine design is said to be still in secrecy (while isn't being funded properly). That's why almost nobody gives the attention (and affection) that it needs, because you can't do it easily, and I believe investors have a hard time investing in something secret (unless they're its main contractors, like Lockheed or Boeing).

Regarding trains, anyway :

Spoiler

Trains had it's heyday, certainly. At first, mechanical engines are as large as a room, and only trains could accomodate this. Trains certainly benefitted a lot of nations, including US, Russia, and Europe - coast-to-coast US/Russia can be done under a week, European nations uses them on steep mountains. Even so, when mechanical engines becomes smaller, catchment areas of station becomes larger, and economy fallen, trains becomes less competitive. This is the case in US (where in the US you end up with unbeliaveable huge catchment areas, and expanding train network isn't as easy as expanding road network - also, have you ever consider how slow your freight trains are ?) and GB (say, Beeching Axe, Privatization, even now it's a problem yet again), so trains there slightly fell of the competition. That being said, trains remain to be the main mean of transport in Russia (where you literally have no soil at summer), Japan and Europe (where crowdedness kept catchment areas small).

In comparison to Skylon and conventional rockets, I should say this doesn't quite apply. The only difference is, once again, build cost vs. maintenance cost. Both can easily be the same, so we need certain real-life figures first.

 

[blowfish]

3 hours ago, sevenperforce said:

It was my understanding that supersonic jets use low-bypass turbofans or turbojets primarily due to heat dissipation problems.

That's not really correct.  Subsonic jets would overheat if run at higher speeds, but even if they didn't they loose thrust very quickly (this comes from months of working with AJE's simulations BTW).

[sevenperforce]

23 minutes ago, blowfish said:

That's not really correct.  Subsonic jets would overheat if run at higher speeds, but even if they didn't they loose thrust very quickly (this comes from months of working with AJE's simulations BTW).

Isn't that because the compressor-to-expander cycle really isn't set up to work with really high inlet speed?

I'm convinced (possibly without merit, but who knows) that some kind of high-bypass air turborocket expander is the key to making SSTO instantly realizable. 

[benjee10]

I'm cautiously optimistic about SABRE engines as a concept, potentially as a first stage on a two-stage reusable system. Skyline I would love to see happen but unfortunately I doubt it ever will, at least not until asteroid/lunar mining has been demonstrate to be viable economically, and I don't see that happening on a large enough scale for at least the next fifty years. I hope to see it in my lifetime though. 

The idea of multistage reusable aerodynamic craft has always been appealing to me from a rather non-scientific point of view. You get the excitement of staging events as well as the sci-fi beauty of a ship taking off from a runway, going into space and coming back down again. Always thought this bit of artwork was pretty badass, perhaps something like this could be achieved with the SABRE system:

(artwork by Abiogenesis on DeviantArt)

I can envisage this sort of thing with a payload fairing & upper stage in place of the manned spaceplane in case you wanted a purely cargo launch. 

[YNM]

Subsonic jets have problem going supersonic because their fans are not designed for supersonic flow. While limited supersonic flow can still be reached, for special high-speed application it's a bad idea. Look at how SR-71 have only pure turbojet leading to turbo-ramjet (by means of bypass into the afterburning section) instead of low-bypass turbofan or high-bypass turbofan. The same thing goes to Concorde, where it have pure turbojet. Turbofan running at far higher speeds than Mach 1 can be seen on Tu-144, which suffers from high fuel consumption.

[fredinno]

2 hours ago, AngelLestat said:

No, he doesn´t.  I know very well of what I am talking about.
But this is not the place..  I made some comments in this topic:

Nobody said that it will be easy, but a 2STO with sable engines does not have much sense, because it will have a huge cost also without the real benefits of 1 stage to orbit in reusability vehicle, mostly for the time it will need to relaunch, which is directly translated to cost.
In that case keep the falcon9 or heavy..  there is no difference.

 

Ok, my english was no right in that sentence... what I wanna said is that ESA did an study about if skylon has economic sense taking into account all costs and the future demand.. That economic study took 1 year to complete with a cost of 1 millon euros (the economic study).  The cost is just to point it was a serious study.  

A TSTO SABRE is useful, as it's first stage will be significantly smaller, and can impart a higher velocity to the harder-to-recover 2nd stage.

[magnemoe]

1 hour ago, fredinno said:

A TSTO SABRE is useful, as it's first stage will be significantly smaller, and can impart a higher velocity to the harder-to-recover 2nd stage.

Note that with an faster seperation speed you might even go for an reusable second stage for standard launches. 
Second option return engine and avionic dump the tank. 
For an laugh put an naked satellite on the nose of an lifting body upper stage. It would work and look stupid. 

[sevenperforce]

2 hours ago, YNM said:

Subsonic jets have problem going supersonic because their fans are not designed for supersonic flow. While limited supersonic flow can still be reached, for special high-speed application it's a bad idea. Look at how SR-71 have only pure turbojet leading to turbo-ramjet (by means of bypass into the afterburning section) instead of low-bypass turbofan or high-bypass turbofan. The same thing goes to Concorde, where it have pure turbojet. Turbofan running at far higher speeds than Mach 1 can be seen on Tu-144, which suffers from high fuel consumption.

Indeed. 

To use air as your working mass, you need to add kinetic energy to it. You can do that at low speeds with a simple fan, but supersonic flow doesn't mix with fans, so if you're trying to book it, you need to use an expansion cycle, converting thermal energy to kinetic energy. But that is wildly inefficient unless you have plenty of expansion, which means you need compression at the start. Compressing with a turbine compressor runs into the same problem as a fan unless you slow down the airstream...but doing so heats it up and will melt your compressor. So you need to precool aggressively, or you're stuck with ram compression, which is limited and inefficient. 

Ah, the tyrannies of physics. 

I still say they would do better to skip the air breathing aspect and just use the air as working mass injected downstream of the combustor. 

[blowfish]

Air inside a jet engine is subsonic regardless of airspeed.  The reason why supersonic engines have fancy intakes is to minimize the total pressure loss when decelerating the air to below the speed of sound.  But even the best intakes are not perfect - you loose some pressure, and the fan/compressor has to make up that pressure in order to even produce positive thrust.  That is why low pressure fans don't really work well at supersonic speeds.

@sevenperforce I'm still not sure why you're obsessed with the turborocket idea.  The oxidizer is there, entering the engine, you're only going to increase efficiency by using it.

For those unfamiliar, here is a diagram of the SABRE's engine cycle:

It's a little daunting at first, but the underlying concept is pretty simple: the helium loop acts as a heat engine, extracting energy from the temperature difference between the cryogenic fuel and the superheated air entering the engine, and using that energy to drive the compressor and fuel pump.  This guarantees that (1) very high pressures can be used, which directly increases the efficiency of the engine, and (2) Energy that would otherwise go to waste is extracted to do useful work.

[blowfish]

22 hours ago, KSK said:

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.

Am I missing something?  I see a mention of actively cooling the wings during re-entry, but no indication that this is done with excess fuel.  The user manual states specifically that excess propellants are dumped from the main tanks before circularizing (section 2.3.1).  There might be some hydrogen left in the auxiliary (OMS) tank, but not much.

[KSK]

1 hour ago, blowfish said:

Am I missing something?  I see a mention of actively cooling the wings during re-entry, but no indication that this is done with excess fuel.  The user manual states specifically that excess propellants are dumped from the main tanks before circularizing (section 2.3.1).  There might be some hydrogen left in the auxiliary (OMS) tank, but not much.

Hmm, not sure. The orbital maneuvering engines (SOMA) also use LOX/LH2 (page 5), so presumably there are tanks other than the main tanks making it possible that any active cooling would be done with fuel left in the SOMA tanks. The user manual is pretty quiet about any thermal protection system though - maybe active cooling was discarded somewhere along the way.

Edit. Sorry - managed to miss the last part of your post. Agreed.

Edited March 5, 2016 by KSK

[sevenperforce]

5 hours ago, blowfish said:

 

@sevenperforce I'm still not sure why you're obsessed with the turborocket idea.  The oxidizer is there, entering the engine, you're only going to increase efficiency by using it.

For those unfamiliar, here is a diagram of the SABRE's engine cycle:

It's a little daunting at first, but the underlying concept is pretty simple: the helium loop acts as a heat engine, extracting energy from the temperature difference between the cryogenic fuel and the superheated air entering the engine, and using that energy to drive the compressor and fuel pump.  This guarantees that (1) very high pressures can be used, which directly increases the efficiency of the engine, and (2) Energy that would otherwise go to waste is extracted to do useful work.

I definitely like the helium loop being used to run the compressor and so forth. I hadn't been aware of that before. 

The reason I want to use onboard oxidizer is that it will allow a much higher-volume flow of compressed air. If you're injecting into the combustion chamber, then you can only increase mass flow up to a limit, and then you start to lose efficiency in your burn. Inject downstream, and your air mass flow can be almost arbitrarily high. You can still use a fuel rich combustion if applicable to afterburn. 

[YNM]

10 hours ago, sevenperforce said:

Indeed. 

To use air as your working mass, you need to add kinetic energy to it. You can do that at low speeds with a simple fan, but supersonic flow doesn't mix with fans, so if you're trying to book it, you need to use an expansion cycle, converting thermal energy to kinetic energy. But that is wildly inefficient unless you have plenty of expansion, which means you need compression at the start. Compressing with a turbine compressor runs into the same problem as a fan unless you slow down the airstream...but doing so heats it up and will melt your compressor. So you need to precool aggressively, or you're stuck with ram compression, which is limited and inefficient. 

Ah, the tyrannies of physics. 

I still say they would do better to skip the air breathing aspect and just use the air as working mass injected downstream of the combustor.

Just inserting air into your rocket won't work wonderfully - have you considered that noise - reduced turbojet (which basically adds a little air inlet after the usual engine) have less thrust than unedited ones ?

Edited March 5, 2016 by YNM

[sevenperforce]

7 minutes ago, YNM said:

Just inserting air into your rocket won't work wonderfully - have you considered that noise - reduced turbojet (which basically adds a little air inlet after the usual engine) have less thrust than unedited ones ?

That's what compression is for. 

[Emperor of the Titan Squid]

Skylon is great, but none will prove the tech.

[fredinno]

13 hours ago, magnemoe said:

Note that with an faster seperation speed you might even go for an reusable second stage for standard launches. 
Second option return engine and avionic dump the tank. 
For an laugh put an naked satellite on the nose of an lifting body upper stage. It would work and look stupid. 

A naked satellite would work- if you want its solar panels to be ripped on ascent.

13 hours ago, sevenperforce said:

Indeed. 

To use air as your working mass, you need to add kinetic energy to it. You can do that at low speeds with a simple fan, but supersonic flow doesn't mix with fans, so if you're trying to book it, you need to use an expansion cycle, converting thermal energy to kinetic energy. But that is wildly inefficient unless you have plenty of expansion, which means you need compression at the start. Compressing with a turbine compressor runs into the same problem as a fan unless you slow down the airstream...but doing so heats it up and will melt your compressor. So you need to precool aggressively, or you're stuck with ram compression, which is limited and inefficient. 

Ah, the tyrannies of physics. 

I still say they would do better to skip the air breathing aspect and just use the air as working mass injected downstream of the combustor. 

Air-breathing ISP is leaps above air-augmented.

[blowfish]

@sevenperforce Airflow is determined by the intake and nozzle throat diameter.  At supersonic speeds, conditions inside the engine cannot affect how air flows outside the engine, so there is a maximum airflow determined by the intake.

So if the nozzle doesn't allow as much airflow as the intake, you can simply resize the nozzle.

Now, there is a caveat to this - if you match the intake an nozzle at a particular airspeed, they will not match at other airspeeds.  In particular, if the nozzle is sized for maximum airflow at mach 5, then there will be more air entering the intake than the nozzle can take at lower mach numbers (above mach 1).  The SABRE solves this by allowing the excess air into a series of ramjet bypass ducts around the engine - they aren't shown in the diagram I linked, but plenty of engine cutaways show them.  The bypass ramjets burn the air with hydrogen - since there is no compression beyond whatever ram compression happens in the intake, the bypass jets are not as efficient as the engine core, but it's better than simply bleeding the air overboard.

[magnemoe]

38 minutes ago, fredinno said:

A naked satellite would work- if you want its solar panels to be ripped on ascent.

Satelite is inside cargo hold until released, you have to hold release until out of atmosphere anyway because the more complex separation who need vacuum and free fall.