Whats wrong with Skylon?

[sevenperforce]

4 hours ago, fredinno said:

Air-breathing ISP is leaps above air-augmented.

For a given intake diameter, maybe.

IIRC, high-bypass airline turbofans would actually be more efficient if they were run as turborockets, due to the more efficient burn of liquid oxygen, but the fuel savings wouldn't make up for the extra complexity and reloading cost.

[blowfish]

19 minutes ago, sevenperforce said:

IIRC, high-bypass airline turbofans would actually be more efficient if they were run as turborockets, due to the more efficient burn of liquid oxygen, but the fuel savings wouldn't make up for the extra complexity and reloading cost.

I have studied jet engines rather extensively, and all the research I've done suggests that this is false.  Cite your source.

[sevenperforce]

10 minutes ago, blowfish said:

I have studied jet engines rather extensively, and all the research I've done suggests that this is false.  Cite your source.

Just a remembered anecdote; I'd have to run the numbers myself to know if it was correct.

When I get a chance, I'll look into whether a normal shock and a bow shock could be aligned in a speed-sensitive way so as to maintain ideal inlet flow conditions relative to speed. It annoys me that we can use the atmosphere to decelerate but not to accelerate, you know?

[blowfish]

1 minute ago, sevenperforce said:

When I get a chance, I'll look into whether a normal shock and a bow shock could be aligned in a speed-sensitive way so as to maintain ideal inlet flow conditions relative to speed.

The smallest total pressure loss is achieved by a large number of low-angle oblique shocks before the normal (terminal) shock.  The terminal shock is already far inside the engine (at what's called the intake "throat").  Some intakes are adjustable to adjust the amount of air that they capture - the SR-71 has retractable intake cones, and many ramp intakes can adjust their angles to only capture a certain amount of flow.  But if you're trying to match through a large number of mach numbers, then the adjustment range is going to be quite large.

6 minutes ago, sevenperforce said:

It annoys me that we can use the atmosphere to decelerate but not to accelerate, you know?

That's how non-conservative forces work.

[sevenperforce]

Just now, blowfish said:

The smallest total pressure loss is achieved by a large number of low-angle oblique shocks before the normal (terminal) shock.  The terminal shock is already far inside the engine (at what's called the intake "throat").  Some intakes are adjustable to adjust the amount of air that they capture - the SR-71 has retractable intake cones, and many ramp intakes can adjust their angles to only capture a certain amount of flow.  But if you're trying to match through a large number of mach numbers, then the adjustment range is going to be quite large.

That's how non-conservative forces work.

A theoretical central/open-bypass engine wouldn't have any terminal shock at all, above the design speed. That's part of the range of ideas: turn the engine inside out, and the oblique shocks start to work with you instead of against you. 

Sure, friction (compression, really, but that's beside the point) is nonconservative. But there is a way to use the atmosphere as your reaction mass; it's just wildly inefficient in the ways we have been doing it so far. For reaching orbit, at least. 

And say, we were talking about this earlier, but I forgot...

If the helium loop runs the turbopump off the temperature differential, what runs the turbopump once Skylon shuts off its intakes?

 

[AngelLestat]

20 hours 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.

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.

 

15 hours ago, blowfish said:

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.

Hi blowfish, The one thing that I never understood well is why they use the helium loop instead of just the same hydrogen which would increase the efficiency, because heat exchangers are not 100% efficient.
So you use the same h2 as a heat engine cooling the air that enters and the temperature difference will help to run the h2 and lox pump and compressor. 
Or I am missing something?

Also.. at those temperatures in which the incomming air is cooled, is not possible to split the nitrogen from the oxygen to increase the power of the combustion?

Edited March 5, 2016 by AngelLestat

[sevenperforce]

2 minutes ago, AngelLestat said:

The one thing that I never understood well is why they use the helium loop instead of just the same hydrogen which would increase the efficiency, because heat exchangers are not 100% efficient.
So you use the same h2 as a heat engine cooling the air that enters and the temperature difference will help to run the h2 and lox pump and compressor. 
Or I am missing something?

Helium won't rip their precooler to little bitty pieces. Hydrogen destroys everything. 

[AngelLestat]

2 minutes ago, sevenperforce said:

Helium won't rip their precooler to little bitty pieces. Hydrogen destroys everything. 

I knew that, but how hard it can be?  after all you still use a heat exchanger in which hydrogen go through
Maybe is just an issue that can be solve with material selection...

Edited March 5, 2016 by AngelLestat

[blowfish]

1 hour ago, sevenperforce said:

A theoretical central/open-bypass engine wouldn't have any terminal shock at all, above the design speed. That's part of the range of ideas: turn the engine inside out, and the oblique shocks start to work with you instead of against you.

At supersonic speeds, you need to have a terminal shock - oblique shocks reduce the mach number, but the flow will stay supersonic.  In an ideal inlet, the flow will be only slightly supersonic when it hits the terminal shock, minimizing the pressure loss.

I've seen some theoretical designs for compressors that can operate on supersonic air, but so far no one has actually built an operational engine out of one, and in any case, the efficiency is likely to be less than subsonic compressor stages.

1 hour ago, sevenperforce said:

If the helium loop runs the turbopump off the temperature differential, what runs the turbopump once Skylon shuts off its intakes?

The helium loop bleeds heat from the preburner.  This is the HX3 in the diagram.

1 hour ago, AngelLestat said:

Hi blowfish, The one thing that I never understood well is why they use the helium loop instead of just the same hydrogen which would increase the efficiency, because heat exchangers are not 100% efficient.
So you use the same h2 as a heat engine cooling the air that enters and the temperature difference will help to run the h2 and lox pump and compressor. 
Or I am missing something?

Yeah, I think hydrogen embrittlement is the main issue.  True that you need to run hydrogen through something, but I think the main point is that the main precooler is large and complex, so you want to minimize wear on it / use of exotic embrittlement-resistant materials to maximize reusability and minimize mass.  Plus, you'd need additional turbines in the hydrogen flow, which are also vulnerable parts.

1 hour ago, AngelLestat said:

Also.. at those temperatures in which the incomming air is cooled, is not possible to split the nitrogen from the oxygen to increase the power of the combustion?

I have no idea what that would take or if it's possible.

[sevenperforce]

3 hours ago, AngelLestat said:

Also.. at those temperatures in which the incomming air is cooled, is not possible to split the nitrogen from the oxygen to increase the power of the combustion?

That requires LACE, a Liquid Air Cycle Engine. The air has to be cooled to liquification, then fractionally distilled to separate out the oxygen from the nitrogen. No way to do that fast enough to make it work. Plus, the enthalpy to liquefy air is way too high...you would have a HUGE amount of hydrogen required. Now, if we can make supercooled liquid metallic hydrogen one of these days, all bets are off. 

2 hours ago, blowfish said:

At supersonic speeds, you need to have a terminal shock - oblique shocks reduce the mach number, but the flow will stay supersonic.  In an ideal inlet, the flow will be only slightly supersonic when it hits the terminal shock, minimizing the pressure loss.

I've seen some theoretical designs for compressors that can operate on supersonic air, but so far no one has actually built an operational engine out of one, and in any case, the efficiency is likely to be less than subsonic compressor stages.

A scramjet has no terminal shock at all, right?

While a conventional engine needs a sliding spike to direct the shock to the right places as the speed increases, an inside-out engine could be tuned such that its oblique shocks meet downstream at hypersonic speeds. 

[blowfish]

37 minutes ago, sevenperforce said:

A scramjet has no terminal shock at all, right?

While a conventional engine needs a sliding spike to direct the shock to the right places as the speed increases, an inside-out engine could be tuned such that its oblique shocks meet downstream at hypersonic speeds. 

A scramjet doesn't have any compressor, except for passive ram compression (no moving parts).  My point is that if you want to actually have an actual turbocompressor, you need the flow to be subsonic (again, barring theoretical supersonic compressor designs).

[sevenperforce]

1 hour ago, blowfish said:

A scramjet doesn't have any compressor, except for passive ram compression (no moving parts).  My point is that if you want to actually have an actual turbocompressor, you need the flow to be subsonic (again, barring theoretical supersonic compressor designs).

Exactly. 

So...design an inside-out engine with an open-drum tapering centrifugal turbocompressor, functioning as an ordinary compressor from static to transonic speeds, then using a carefully-shaped inlet to maintain terminal shock right above the compressor surface until the speeds are too great, at which point shocks are oblique through the entire engine and you function as a scramrocket. 

[blowfish]

41 minutes ago, sevenperforce said:

So...design an inside-out engine with an open-drum tapering centrifugal turbocompressor, functioning as an ordinary compressor from static to transonic speeds, then using a carefully-shaped inlet to maintain terminal shock right above the compressor surface until the speeds are too great, at which point shocks are oblique through the entire engine and you function as a scramrocket. 

1. It doesn't matter how close the terminal shock is to the compressor.  If anything, you want some space for the flow to diffuse between the inlet throat and the compressor face because the compressor usually accepts air at mach 0.3-0.5.  This is also where you would put a precooler if you had one.
2. The only way to eliminate the terminal shock is to open up the throat such that the flow doesn't choke.
3. My understanding of scramjets is that efficiency is very sensitive to internal geometry - you can't just redirect flow around the compressor - the path between the inlet and the nozzle has to be dead straight - any deflection will result in additional pressure loss.  So you have have moving parts to close off all other flow paths.
4. I'm still not sure what you're going on about with this "inside out" engine.  The radial arrangement of components inside the engine isn't terribly important.

E: Oh, and the term "scramrocket" is pretty meaningless.  If the flow is already supersonic by the time you mix, then it's really not going to mix much.  You basically just have a scramjet and a rocket.

Edited March 6, 2016 by blowfish

[Nuke]

11 hours ago, blowfish said:

@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.

as i understand it the ram jets are mostly used to reduce drag and dont really produce any meaningful thrust. it also gives a way to dump the surplus hydrogen as well. the precooler needs more for coolant than the engine can burn, so there is a lot of surplus for the ramjets that would otherwise just need to be dumped. i actually think its one of the more clever parts of the design.

Edited March 6, 2016 by Nuke

[Nuke]

8 hours ago, sevenperforce said:

A theoretical central/open-bypass engine wouldn't have any terminal shock at all, above the design speed. That's part of the range of ideas: turn the engine inside out, and the oblique shocks start to work with you instead of against you. 

Sure, friction (compression, really, but that's beside the point) is nonconservative. But there is a way to use the atmosphere as your reaction mass; it's just wildly inefficient in the ways we have been doing it so far. For reaching orbit, at least. 

And say, we were talking about this earlier, but I forgot...

If the helium loop runs the turbopump off the temperature differential, what runs the turbopump once Skylon shuts off its intakes?

 

for a brayton cycle to work you need a cold side and a hot side. the cold side is the lh2 heat exchanger, which is running so long as the engine is in operation. the hot side is primarily the preburner which operates whether its in air breating or lox mode. secondarily the precooler also heats the helium while the intake is open. since the intake air is warmer than the helium, the heat moves from the former into the latter. now when you switch the preburner from air to o2, its going to burn hotter, and so the helium loop will pick up additional heat from the hot side to make up for the heat its not getting from the precooler.

at least thats my educated guess based on the materials that ive read.

Edited March 6, 2016 by Nuke

[mikegarrison]

On 3/5/2016 at 1:50 PM, sevenperforce said:

IIRC, high-bypass airline turbofans would actually be more efficient if they were run as turborockets, due to the more efficient burn of liquid oxygen, but the fuel savings wouldn't make up for the extra complexity and reloading cost.

By what definition of efficiency?

By the practical definition of efficiency, air is free, and it's available enroute so you don't need to carry it around. LOX is not free and you have to carry it with you the whole way up until you burn with it. There is no freaking way it's going to result in a lower cost to move payload from one place to another.

[sevenperforce]

1 hour ago, mikegarrison said:

On 3/5/2016 at 4:50 PM, sevenperforce said:

IIRC, high-bypass airline turbofans would actually be more efficient if they were run as turborockets, due to the more efficient burn of liquid oxygen, but the fuel savings wouldn't make up for the extra complexity and reloading cost.

By what definition of efficiency?

By the practical definition of efficiency, air is free, and it's available enroute so you don't need to carry it around. LOX is not free and you have to carry it with you the whole way up until you burn with it. There is no freaking way it's going to result in a lower cost to move payload from one place to another.

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.

At least, that was the claim I saw; like I said, I'd need to run the numbers myself to be sure.

On 3/6/2016 at 11:57 PM, blowfish said:

1. It doesn't matter how close the terminal shock is to the compressor.  If anything, you want some space for the flow to diffuse between the inlet throat and the compressor face because the compressor usually accepts air at mach 0.3-0.5.  This is also where you would put a precooler if you had one.
2. The only way to eliminate the terminal shock is to open up the throat such that the flow doesn't choke.
3. My understanding of scramjets is that efficiency is very sensitive to internal geometry - you can't just redirect flow around the compressor - the path between the inlet and the nozzle has to be dead straight - any deflection will result in additional pressure loss.  So you have have moving parts to close off all other flow paths.
4. I'm still not sure what you're going on about with this "inside out" engine.  The radial arrangement of components inside the engine isn't terribly important.

E: Oh, and the term "scramrocket" is pretty meaningless.  If the flow is already supersonic by the time you mix, then it's really not going to mix much.  You basically just have a scramjet and a rocket.

Yeah, you'd definitely have space.

The throat would be open and the path would be straight; that's what I mean about an "inside-out" engine. The flow goes through the center of the engine rather than being deflected around a turbine or spike. The convergence point of the oblique shocks changes with respect to speed, allowing you to choke the flow at low supersonic speeds but unchoke it at hypersonic speeds, without moving parts. I'll do a mockup to show you what I'm envisioning.

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

[sevenperforce]

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.

[magnemoe]

On 6.3.2016 at 8:10 AM, Nuke said:

as i understand it the ram jets are mostly used to reduce drag and dont really produce any meaningful thrust. it also gives a way to dump the surplus hydrogen as well. the precooler needs more for coolant than the engine can burn, so there is a lot of surplus for the ramjets that would otherwise just need to be dumped. i actually think its one of the more clever parts of the design.

Ramjet removes under pressure behind plane, reducing drag.

Much like base bled on som artillery shells. Small rocket who don't add much trust but remove the turbulence behind the flat back of shell

[mikegarrison]

3 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.

At least, that was the claim I saw; like I said, I'd need to run the numbers myself to be sure.

What do you think airplanes are for? With very limited exceptions, they carry stuff from here to there. The total cost and time to move payload is the real measure of efficiency.

[sevenperforce]

17 minutes ago, mikegarrison said:

What do you think airplanes are for? With very limited exceptions, they carry stuff from here to there. The total cost and time to move payload is the real measure of efficiency.

...right. 

Which is why adding liquid oxygen tanks to jumbo jets would not be a good idea, even if thrust-specific fuel consumption might be lower. 

[juanml82]

Something I was thinking, and that experts have probably disregarded for some good reason, but couldn't air refueling work for a vehicle like Skylon? 

[fredinno]

4 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.

At least, that was the claim I saw; like I said, I'd need to run the numbers myself to be sure.

Yeah, you'd definitely have space.

The throat would be open and the path would be straight; that's what I mean about an "inside-out" engine. The flow goes through the center of the engine rather than being deflected around a turbine or spike. The convergence point of the oblique shocks changes with respect to speed, allowing you to choke the flow at low supersonic speeds but unchoke it at hypersonic speeds, without moving parts. I'll do a mockup to show you what I'm envisioning.

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

http://www.astronautix.com/engines/lacele5.htm (air augmented lh2 lox) ISP: 600s

http://www.astronautix.com/engines/scramjet.htm (air breathing lh2) ISP: 1500s

 

WHO'S THE MORE EFFICIENT NOW?

[sevenperforce]

3 minutes ago, fredinno said:

http://www.astronautix.com/engines/lacele5.htm (air augmented lh2 lox) ISP: 600s

http://www.astronautix.com/engines/scramjet.htm (air breathing lh2) ISP: 1500s

 

WHO'S THE MORE EFFICIENT NOW?

Look again; the LACE has an ISP of 1200s, not 600s, and can operate in or out of atmo.

And the LACE has a lower ISP because liquifying air is dumb.

[fredinno]

44 minutes ago, sevenperforce said:

Look again; the LACE has an ISP of 1200s, not 600s, and can operate in or out of atmo.

And the LACE has a lower ISP because liquifying air is dumb.

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