Whats wrong with Skylon?

[Sanic]

What's wrong with Skylon?

Everything.

It looks good onpaper, but even after its long development cycle so far it's not progressed much farther than lab tests. It's questionable if it'll be cost effective (again, it looks good on paper, but remember the Space Shuttle), and the SABRE engines may not work as well as they've been designed to.

I'm most interested in its deltaV, does anyone know how much it's supposed to have?

[sevenperforce]

12 minutes ago, fredinno said:

The LACE is an exception, not the rule. All the other engines astronatuix listed have a far lower isp.

http://www.astronautix.com/engines/nk3xlace.htm

http://www.astronautix.com/engines/rb545.htm

The RB545 lists an ISP of 2000s.

But in any case, trying to identify specific performance for specific engines is kind of missing the point. The thrust of an engine depends on one thing and one thing only: instantaneous change in propellant momentum. For a rocket, this is simply your fuel consumption (in mass flow) times your exhaust velocity. But for airbreathers and air-augmented engines, it's more complicated. An airbreather collects the airstream (a net loss of thrust, equal to the mass flow of the airstream times the forward velocity of the engine) and then burns it with a small amount of fuel to accelerate it out the back at a higher speed than it came in; the net thrust is the difference between the momentum going out and the momentum coming in. An air-augmented engine, on the other hand, has a higher fuel mass flow, but doesn't have to slow down the airstream as much, meaning that it can continue to operate with enhanced thrust at far higher forward speeds.

[magnemoe]

On 5.3.2016 at 0:03 AM, AngelLestat said:

2 stages vehicles using a sable engine will have much higher operational cost, refurbish, things to fail, planning and ensemble cost.  Then it will take much more time to launch cargo between launches.
Jet engines are more expensive than rocket engines, sable would be even more expensive than jet engines, and the main point of a sable engine is that it works in the atmosphere upto  match 5 and then as a rocket.  Match 5 is no enough to reduce a lot the cost of a second stage because is still needs most of the deltav to go, but it can give you the benefit of 1 stage to orbit.
You can have few benefits with a first stage sable and a second stage rocket.  But you will not have an economic advantage over falcon9 or heavy, due extra cost of aerodynamics and sable engines.
So if someone should use a sable engine is just for 2 things..  a super fast airplane, or 1 stage to orbit vehicle.

Why? Yes Mach 5 is probably a bit slow, however you already has to load the fragile satellite, second stage will be larger but lighter then empty and designed to fitt

[blowfish]

6 hours ago, sevenperforce said:

Lower cost, no. But a jet turbine engine is taking in poorly-compressed, hot nitrogen and oxygen in a less-than-ideal ratio and burning it with fuel, while a rocket uses pure liquid oxygen. As a result the reaction is fairly inefficient and only a small amount of the chemical potential energy in your fuel is actually converted into thrust. If a highly efficient turborocket gas generator was used to accelerate the same volume of nitrogen and oxygen for the same total net thrust, the fuel and the oxidizer consumed each second would be equal to or lower than the fuel consumed each second for the jet turbine case, for certain high-bypass turbofan engines.

Sorry, but you still have yet to demonstrate that turborockets can even approach the efficiency of turbojets, let alone high bypass turbofans

6 hours ago, sevenperforce said:

If you send the supersonic rocket exhaust plume across the supersonic air flow, I'm pretty sure it should mix.

Not really.  Even at subsonic speeds, you need an exhaust mixer to get the flows to mix efficiently.  At supersonic speeds, you're not going to get much mixing (except maybe a small layer), and ultimately you're just going to cause shockwaves which cut your total pressure.

In fact, many rocket engines rely on flows not mixing at supersonic speeds - many gas generator rockets use turbine exhaust to cool the nozzle extension, creating a thin layer of cooler gas between the nozzle wall and the hot main exhaust.  If these layers mixed immediately, then this method of cooling would be completely ineffective.  Based on heating glow I've seen on some engines, it looks like the streams to start to mix near the end of the nozzle, but the exhaust layer is very thin so that's not surprising.

4 hours ago, sevenperforce said:

Here's a stripped-down diagram of SABRE:

And here's what I mean by an inside-out engine:

The flow following the red airstream need never be choked at sufficiently high speeds in the lower case.

My point is that you would have to close off the paths leading through the precooler.  The walls of the flow path have to be completely straight.

Also, you're going to have trouble with an intake geometry like that - the intake lip shockwave has to be reflected several times to get good pressure recovery, which means that you want a flow path which is very narrow in the radial direction and long along the engine's axis.  You also have no spilling at any mach number, which seems fine at first, but actually results in the engine taking in way more air than it can possibly accept at most flight conditions - there's a reason why supersonic intakes are shaped the way they are (either cone or ramp).

32 minutes ago, sevenperforce said:

The RB545 lists an ISP of 2000s.

But in any case, trying to identify specific performance for specific engines is kind of missing the point. The thrust of an engine depends on one thing and one thing only: instantaneous change in propellant momentum. For a rocket, this is simply your fuel consumption (in mass flow) times your exhaust velocity. But for airbreathers and air-augmented engines, it's more complicated. An airbreather collects the airstream (a net loss of thrust, equal to the mass flow of the airstream times the forward velocity of the engine) and then burns it with a small amount of fuel to accelerate it out the back at a higher speed than it came in; the net thrust is the difference between the momentum going out and the momentum coming in. An air-augmented engine, on the other hand, has a higher fuel mass flow, but doesn't have to slow down the airstream as much, meaning that it can continue to operate with enhanced thrust at far higher forward speeds.

2000s is less than most turbojets on afterburner.  Modern high bypass turbofans are in the several thousand s.

How much you slow down the air doesn't matter - if you're putting it through a compressor, then it's has to be subsonic anyway, and the compressor really only accepts air in the mach 0.3-0.5 range.  And once it's subsonic, it doesn't really matter exactly how fast it's going - flow is mostly isentropic.  This is why most jet engine calculations deal with total (stagnation) pressure and temperature rather than real pressure and temperature - these are quantities that don't care how fast the air is going at any particular point.  If you have no total pressure loss, then you can accelerate it back up to the same speed without adding any additional energy.  Of course, flow through a tube will have some pressure loss, but at supersonic speeds, most of the loss is in the intake.

[mikegarrison]

50 minutes ago, blowfish said:

In fact, many rocket engines rely on flows not mixing at supersonic speeds - many gas generator rockets use turbine exhaust to cool the nozzle extension, creating a thin layer of cooler gas between the nozzle wall and the hot main exhaust.  If these layers mixed immediately, then this method of cooling would be completely ineffective.  Based on heating glow I've seen on some engines, it looks like the streams to start to mix near the end of the nozzle, but the exhaust layer is very thin so that's not surprising.

Film cooling does degrade over distance. It also doesn't block radiative heat transfer.

This takes me back. I used to work on film cooling for gas turbine combustors. Long, long, ago.

[blowfish]

1 minute ago, mikegarrison said:

Film cooling does degrade over distance. It also doesn't block radiative heat transfer.

This takes me back. I used to work on film cooling for gas turbine combustors. Long, long, ago.

Sure (and I think I intended to say as much), but my point is that it doesn't mix with the main flow immediately.

[fredinno]

2 hours ago, sevenperforce said:

The RB545 lists an ISP of 2000s.

But in any case, trying to identify specific performance for specific engines is kind of missing the point. The thrust of an engine depends on one thing and one thing only: instantaneous change in propellant momentum. For a rocket, this is simply your fuel consumption (in mass flow) times your exhaust velocity. But for airbreathers and air-augmented engines, it's more complicated. An airbreather collects the airstream (a net loss of thrust, equal to the mass flow of the airstream times the forward velocity of the engine) and then burns it with a small amount of fuel to accelerate it out the back at a higher speed than it came in; the net thrust is the difference between the momentum going out and the momentum coming in. An air-augmented engine, on the other hand, has a higher fuel mass flow, but doesn't have to slow down the airstream as much, meaning that it can continue to operate with enhanced thrust at far higher forward speeds.

That was air-breathing mode, which just proves my point.

[mikegarrison]

13 minutes ago, blowfish said:

Sure (and I think I intended to say as much), but my point is that it doesn't mix with the main flow immediately.

Anyway, the main point is that it stops being effective at insulating the walls of the flowpath after a non-dimensional distance from the start of the film that depends on the thickness of the film.

Film cooling used to be the way combustors were protected, but I think this has mostly been replaced with transpirational cooling. Film cooling protects the walls from convective heat transfer with the hot gas, but transpirational cooling pulls heat from the walls with convection to the cooling air that is going through walls. (And it also then somewhat replenishes the film.)

Edited March 8, 2016 by mikegarrison

[sevenperforce]

1 hour ago, blowfish said:

Even at subsonic speeds, you need an exhaust mixer to get the flows to mix efficiently.  At supersonic speeds, you're not going to get much mixing (except maybe a small layer), and ultimately you're just going to cause shockwaves which cut your total pressure.

In fact, many rocket engines rely on flows not mixing at supersonic speeds - many gas generator rockets use turbine exhaust to cool the nozzle extension, creating a thin layer of cooler gas between the nozzle wall and the hot main exhaust.  If these layers mixed immediately, then this method of cooling would be completely ineffective.  Based on heating glow I've seen on some engines, it looks like the streams to start to mix near the end of the nozzle, but the exhaust layer is very thin so that's not surprising.

My point is that you would have to close off the paths leading through the precooler.  The walls of the flow path have to be completely straight.

Also, you're going to have trouble with an intake geometry like that - the intake lip shockwave has to be reflected several times to get good pressure recovery, which means that you want a flow path which is very narrow in the radial direction and long along the engine's axis.  You also have no spilling at any mach number, which seems fine at first, but actually results in the engine taking in way more air than it can possibly accept at most flight conditions - there's a reason why supersonic intakes are shaped the way they are (either cone or ramp).

How much you slow down the air doesn't matter - if you're putting it through a compressor, then it's has to be subsonic anyway, and the compressor really only accepts air in the mach 0.3-0.5 range.  And once it's subsonic, it doesn't really matter exactly how fast it's going - flow is mostly isentropic.  This is why most jet engine calculations deal with total (stagnation) pressure and temperature rather than real pressure and temperature - these are quantities that don't care how fast the air is going at any particular point.  If you have no total pressure loss, then you can accelerate it back up to the same speed without adding any additional energy.  Of course, flow through a tube will have some pressure loss, but at supersonic speeds, most of the loss is in the intake.

You seem quite knowledgeable on the subject, so let me ask this: does the deflection angle of an oblique shock change as a function of Mach number, or is it a constant? It's hard to tell from looking at shadowgraphs and diagrams alone.

The idea is to have the precooler and compressors buried behind the drag shadow of the engine cowling and intake ramps. At a standstill, the compressors pull air out radially from the center of the inlet, with an effective intake surface area far greater than the axial inlet cross-section and an induced axial flow component apart from the compressor exhaust itself.  As forward velocity increases, the compressors pull air in as it rushes past. At transonic and low supersonic speed, inlet shocks slow the airstream enough that it can still enter the compressors at optimal Mach numbers. Then, upon transition to hypersonic speed, less and less air enters the compressors because it simply goes straight through the engine, but at that speed it's no longer possible to get the air slowed down enough, so that's fine because the compressors aren't actually in the flow path.

At least that's the theory.  

[blowfish]

6 minutes ago, sevenperforce said:

You seem quite knowledgeable on the subject, so let me ask this: does the deflection angle of an oblique shock change as a function of Mach number, or is it a constant? It's hard to tell from looking at shadowgraphs and diagrams alone.

The idea is to have the precooler and compressors buried behind the drag shadow of the engine cowling and intake ramps. At a standstill, the compressors pull air out radially from the center of the inlet, with an effective intake surface area far greater than the axial inlet cross-section and an induced axial flow component apart from the compressor exhaust itself.  As forward velocity increases, the compressors pull air in as it rushes past. At transonic and low supersonic speed, inlet shocks slow the airstream enough that it can still enter the compressors at optimal Mach numbers. Then, upon transition to hypersonic speed, less and less air enters the compressors because it simply goes straight through the engine, but at that speed it's no longer possible to get the air slowed down enough, so that's fine because the compressors aren't actually in the flow path.

At least that's the theory.  

For oblique shockwaves, if the deflection angle is constant, than the shock angle will decrease with increasing mach number.

The problem with your hybrid scramjet scenario isn't the compressor per se, it's the fact that there's empty space around the ideal flow path (which includes the precooler).  Supersonic flow would expand into this space, then be forced into pressure-reducing shockwaves (oblique vs normal depends on the exact geometry) as it reached the end of that space.

[sevenperforce]

7 minutes ago, blowfish said:

For oblique shockwaves, if the deflection angle is constant, than the shock angle will decrease with increasing mach number.

The problem with your hybrid scramjet scenario isn't the compressor per se, it's the fact that there's empty space around the ideal flow path (which includes the precooler).  Supersonic flow would expand into this space, then be forced into pressure-reducing shockwaves (oblique vs normal depends on the exact geometry) as it reached the end of that space.

If the shock angle decreases as a function of Mach number, then it should be possible to design an asymmetric intake which will focus shock convergence at the appropriate point for each target speed. 

I was wanting to have the precooler double as an inlet ramp to save weight and space; at high speeds, the pressure between the slits will be so high that it is effectively a flat surface. But the precooler and compressors can be entirely cylindrical rather than integrated with the inlet ramp; that's not a problem. The goal is merely to have high intake flow at low speeds without intake obstruction at hypersonic speeds. Then ram compression can do whatever it can to improve ISP via air augmentation and/or fuel-rich afterburning. 

[mikegarrison]

10 minutes ago, sevenperforce said:

If the shock angle decreases as a function of Mach number, then it should be possible to design an asymmetric intake which will focus shock convergence at the appropriate point for each target speed.

Not only is it possible, but it's been done. SR-71, for instance, had a moving inlet spike that adjusted to the Mach so as to position the shocks where they wanted them.

http://www.enginehistory.org/Convention/2013/HowInletsWork8-19-13.pdf

[sevenperforce]

53 minutes ago, mikegarrison said:

Not only is it possible, but it's been done. SR-71, for instance, had a moving inlet spike that adjusted to the Mach so as to position the shocks where they wanted them.

http://www.enginehistory.org/Convention/2013/HowInletsWork8-19-13.pdf

The "moving inlet spike" is the part that I'm trying to avoid. It's (relatively) straightforward to use an inlet spike to move the shock; it's a little trickier to figure out a way to let increasing Mach numbers move the shocks automatically as the shock angle narrows.

[mikegarrison]

2 minutes ago, sevenperforce said:

The "moving inlet spike" is the part that I'm trying to avoid.

Why?

[sevenperforce]

2 minutes ago, mikegarrison said:

Why?

Moving parts are heavy, particularly when you're trying to redirect a flow of superheated air moving at ten times the speed of sound. The thrust-to-weight ratios of scramjet designs are their undoing already.

If the increasing speed of the airflow ends up focusing itself, then you don't have to worry about moving parts and the same engine can function from launch to orbit.

[mikegarrison]

21 minutes ago, sevenperforce said:

Moving parts are heavy, particularly when you're trying to redirect a flow of superheated air moving at ten times the speed of sound. The thrust-to-weight ratios of scramjet designs are their undoing already.

If the increasing speed of the airflow ends up focusing itself, then you don't have to worry about moving parts and the same engine can function from launch to orbit.

You are going in a direction most people are moving away from. Active surfaces can do a lot better than passive ones.

[sevenperforce]

6 minutes ago, mikegarrison said:

You are going in a direction most people are moving away from. Active surfaces can do a lot better than passive ones.

Not if their weight cost makes them prohibitive.

An engine which can using air as its primary working mass from launch to beyond its exhaust velocity makes reusable SSTO not only realizeable, but economical. If it can be lightweight enough.

[blowfish]

6 hours ago, sevenperforce said:

You seem quite knowledgeable on the subject, so let me ask this: does the deflection angle of an oblique shock change as a function of Mach number, or is it a constant? It's hard to tell from looking at shadowgraphs and diagrams alone.

The idea is to have the precooler and compressors buried behind the drag shadow of the engine cowling and intake ramps. At a standstill, the compressors pull air out radially from the center of the inlet, with an effective intake surface area far greater than the axial inlet cross-section and an induced axial flow component apart from the compressor exhaust itself.  As forward velocity increases, the compressors pull air in as it rushes past. At transonic and low supersonic speed, inlet shocks slow the airstream enough that it can still enter the compressors at optimal Mach numbers. Then, upon transition to hypersonic speed, less and less air enters the compressors because it simply goes straight through the engine, but at that speed it's no longer possible to get the air slowed down enough, so that's fine because the compressors aren't actually in the flow path.

At least that's the theory.  

Where the shocks are focused doesn't really matter.  What does matter is that (1) The shock can be reflected off the walls of the intake many times before hitting the intake throat and (2) That at mach numbers lower than the design mach of the engine (5.5 for the SABRE) (and I mean the subsonic part of the engine in your case), at least some of the first shock generated by the intake does not enter the engine.

You've got it backwards.  To bring the air down to subsonic speeds, you need a very long, narrow (at least along one axis) intake geometry that leaves the air only slightly supersonic before the intake throat (this minimizes pressure loss).

Again, for supersonic flow, you want smooth walls with no disruptions of any sort - the precooler doesn't really fit the bill here.  Subsonic flow can mostly just go where you want it, but the same is not true for supersonic flow.  You need to block off everything but the path you want otherwise you will get pressure loss.

[wumpus]

On 3/5/2016 at 6:03 PM, AngelLestat said:

So if someone should use a sable engine is just for 2 things..  a super fast airplane, or 1 stage to orbit vehicle.

This is the real answer for "what is wrong with Skylon".

It doesn't really solve the issue of SSTO, as it still has ~7000m/s to go while carrying heavy air breathing engines (don't be too surprised if this isn't true.  That first few thousand m/s takes nearly all (3/4 to 7/8) of the fuel and the resulting spacecraft might have a chance).

I would expect that as a super fast airplane it might have a chance.  Looking at how the Concorde disappeared without a whimper, I'm guessing the super-rich don't care to travel enough to justify it (more likely use a "traveling office" which makes the time cost moot).  Otherwise a half-hour "UK-NYC" Skylon is a real possibility (don't ask about ticket prices).

And as mentioned a two stage to orbit doesn't make sense at all.  Basically you have something like Space-x (or competitors) only 20 years behind them.  While it does have an advantage in fuel usage, that isn't a concern at all (maybe if they start now, in 20-30 years there will be enough flights to justify high efficiency to orbit ...).

 

[sevenperforce]

8 hours ago, blowfish said:

Again, for supersonic flow, you want smooth walls with no disruptions of any sort - the precooler doesn't really fit the bill here.  Subsonic flow can mostly just go where you want it, but the same is not true for supersonic flow.  You need to block off everything but the path you want otherwise you will get pressure loss.

How smooth does the parallel wall need to be in order to maintain supersonic flow with minimal pressure loss?

E.g., given the following:

As I understand it, a supersonic airstream entering from the left will be deflected parallel to the lower wedge, then flow horizontally (with shock compression) through the portion with parallel walls. From what you're saying, it seems the wedge itself must be perfectly smooth in order to have proper deflection. Would the parallel section (i.e., what is highlighted in green) also need to be perfectly smooth? Or could it have axial slits forming a precooler, flush with the wall?

[blowfish]

4 minutes ago, sevenperforce said:

How smooth does the parallel wall need to be in order to maintain supersonic flow with minimal pressure loss?

E.g., given the following:

As I understand it, a supersonic airstream entering from the left will be deflected parallel to the lower wedge, then flow horizontally (with shock compression) through the portion with parallel walls. From what you're saying, it seems the wedge itself must be perfectly smooth in order to have proper deflection. Would the parallel section (i.e., what is highlighted in green) also need to be perfectly smooth? Or could it have axial slits forming a precooler, flush with the wall?

That green section would need to be covered, yes.  Otherwise you will get an expansion fan at the front and a shockwave at the back.  I don't know exactly how small of holes you would need for it to be effectively flat, but I think it's smaller than the airflow space you'd need on a precooler.

Edited March 8, 2016 by blowfish

[sevenperforce]

1 minute ago, blowfish said:

That green section would need to be covered, yes.  Otherwise you will get an expansion fan at the front and a shockwave at the back.

How smooth of a surface are we talking about needing? Are allowable irregularities on the order of cm? mm? micrometers?

6 minutes ago, blowfish said:

I don't know exactly how small of holes you would need for it to be effectively flat, but I think it's smaller than the airflow space you'd need on a precooler.

So no chance of bleeding exhaust out into the flow path to smooth the surface?

[blowfish]

42 minutes ago, sevenperforce said:

So no chance of bleeding exhaust out into the flow path to smooth the surface?

Maybe?  But you're still talking about a fair bit of moving geometry and probably some pressure loss somewhere in the process.  At some point, a sliding cover is just easier.

[sevenperforce]

12 minutes ago, blowfish said:

Maybe?  But you're still talking about a fair bit of moving geometry and probably some pressure loss somewhere in the process.  At some point, a sliding cover is just easier.

I'm not averse to the thought of a lightweight sliding cover to seal off the precooler inlet, but if bleed air (which already would be used to run the compressor) can be used to form a smooth layer without moving parts, that ought to be simpler.

What do you think, generally, about having a cylindrical turbocompressor mounted into the sidewalls of the intake, to allow mechanical compression of the airstream at subsonic and low-supersonic speeds without impeding ram compression at mid-to-high-supersonic speeds?

SABRE depends on slowing down the airflow to allow turbocompression even at prohibitively high speeds where ram compression (while generally less efficient) would be more realizable and result in less drag. But if you can put the turbocompressor in the sidewalls then you can have both. An additional supersonic-flow scramjet/scramrocket mode would be nice, I suppose, but it's not necessary; the gains are rather low.

[cantab]

On 03/03/2016 at 5:31 PM, wumpus said:

Money.

This. And politics.