liquid air fed scramjet?!

[Arugela]

If you put a scramjet in a fitting where you can add intake to an intake. IE a prechamber before the normal intake. Can you fill it with air converted from liquid air to feed a scramjet with an artificial atmosphere?

https://www.grc.nasa.gov/www/k-12/airplane/ramth.html

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The flow exiting a scramjet inlet is supersonic and has fewer shock losses than a ramjet inlet at the same vehicle velocity. In the burner, a small amount of fuel is combined with the air and ignited. In a typical engine, 100 pounds of air/sec. is combined with only 2 pounds of fuel/sec.

If a scramjet takes 100lbs of air per second per 2 lbs of fuel(not sure which type), and air is 710 less dense than liquid air why do we not feed it into a scramjet with reserves of liquid air turned back into air to make perfect potentially vacuum based scramjets?

https://www.physicsforums.com/threads/liquid-air-and-compressed-air-density.975471/

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Densities of Normal Air is 1.225 kg/m3 at STP, whereas the density of Liquid Air is 870 kg/m3. That means liquid air is 710x denser that normal air.

Is there some technological hurdle that stops this? From a fuel storage concept this is easy. The weight of storage to feed this is 50/710=0.070423= 7.0423% of the mass. This seems as ideal as possible for a rocket.

Does this ratio change when using something like a sabre engine with liquid hydrogen fuel?

If you take this concept and add the ability to generate hydrogen and or liquid air in flight you can just keep extending the rockets abilities potentially. At which point is it feasible to generate clouds sufficiently from a crafts body to collect the water to make some of this?

If you do a tank of 183 tons of liquid hydrogen and have around 12 tons of liquid air to feed it. That is near 200 tons of fuel. Wouldn't that allow a vehicle of 600 tons to orbit and back potentially. 800 tons on takeoff and 600 for body/cargo?

Is the sabre 16 or 24 tons per engine. It is 14:1 thrust to weight. But is that at 440k lbs of thrust at sea level or 660k thrust in vaccum. If it's at sea level you can do 36 engines at 16 tons each for 581 tons and around 19-24 tons of cargo depending on hull weight. Not sure how this works enough yet.

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9.81*3600*ln(800/600) = 10159.78007070709523344346

9.81*3600*ln(800/605) = 9866.69955050332329811207

 

Edited March 16, 2023 by Arugela

[Piscator]

Wait, wait, wait, isn't the advantage of jet engines that you don't have to bring the air along and thus get a lot of your reaction mass for free? Making your intake air from a liquid air storage seems to defeat the purpose and would very likely result in a system massively less efficient than a dedicated rocket engine.

Also "generating hydrogen in flight" is a very strange idea. The reason why you use fuel, is so that you have a nice energy storage medium without having to carry the infrastructure around that generated the energy in the first place. Or looked at it the other way round, if you had a sufficiently strong and portable energy source, you could  do more efficient things with it than somehow collect water, split it and then recombine it in an engine.

[sevenperforce]

9 hours ago, Arugela said:

If a scramjet takes 100lbs of air per second per 2 lbs of fuel(not sure which type), and air is 710 less dense than liquid air why do we not feed it into a scramjet with reserves of liquid air turned back into air to make perfect potentially vacuum based scramjets?

It's not entirely clear what you're proposing here.

Are you saying "let's bring along liquid air so that we can feed it to our scramjet once it runs out of atmospheric air"?

If so, congratulations, you've invented rocketry.

[Arugela]

It's so you can run a scramjet like the sabre at 3600 isp for higher in the atmosphere and up to a higher mach speed..

[sevenperforce]

1 hour ago, Arugela said:

It's so you can run a scramjet like the sabre at 3600 isp for higher in the atmosphere and up to a higher mach speed..

A scramjet only gets 3600 isp because you aren't counting the mass of the oxidizer, since it's collected as you go.

If you're bringing your oxidizer with you, then you have a rocket, not an airbreather. Airbreathing engines don't get high efficiency by magic; they actually get lower efficiency than a rocket BUT they benefit because they don't have to carry their oxidizer with them, so you don't have to count it as part of the propellant mass flow.

Specific impulse is thrust divided by mass flow rate. In a rocket engine, the mass flow is all the propellant: both fuel and oxidizer. In an airbreathing engine, the mass flow is the fuel alone. But once you're bringing along liquid air to operate your engine, you now have to count the liquid air in your mass flow, so you're no longer getting 3600 seconds of specific impulse.

Edited March 16, 2023 by sevenperforce

[Piscator]

I'm pretty sure scramjets only reach this kind of Isp because they get a large part of their reaction mass for free. After all, when calculating Isp you only have to take the propellant into account that you actually have on board. If you were carrying your own air supply, the number would drop considerably.

[darthgently]

2 hours ago, sevenperforce said:

congratulations, you've invented rocketry

Says all I was going to say

[StrandedonEarth]

Thrust is generated by throwing reaction mass (which could be anything) out the back. Propellers are immersed in an ocean of reaction mass (water or air). Jet engines suck in a huge amount of air for reaction mass, then burn a little fuel in it to heat it up, which makes it expand and go shooting out the back. Rockets need to carry their own reaction mass, and then throw it out the back. The faster it’s thrown, the more efficient it is. It can be thrown by thermal expansion either by running it through something really hot (NTR) or by using a fuel and an oxidizer as reaction mass and burning them together. Some compounds decompose exothermally, sometimes needing a catalyst. Or it can be thrown electrically in an ion engine or rail gun. Or just use pressurized gas, just like an untied balloon. 

Bit if you’re carrying liquid air around, it’s not getting the benefits of being an airbreather anymore  

Fun fact: Car engines run on air. The more air you can pack in, the more fuel you can add to generate power, hence the addition of turbo/superchargers. It’s the expansion of the air/exhaust products that pushes the piston down  

 

[sevenperforce]

Modern turbofan engines produce an exhaust speed of approximately 500 mph (223 m/s) at the exhaust nozzle.  A large engine like the GE90 which produces up to 432.8 kN of thrust with a bypass ratio of 9:1. If you divide 432.8 kN by 223 m/s, you get a total reaction mass flow of 1941 kg/s.

However, the thrust-specific fuel consumption of the GE90 is only 7.9 grams per second for each kN of thrust, meaning that its actual fuel flow is only 3.42 kilograms per second. So the bulk of its thrust comes from the ~1,938 kilograms (1.9 tonnes) of air it's pushing through the engine each second.

That bypass ratio of 9:1 means that only 194 kilograms of air is actually flowing through the engine each second; the other 1,744 kilograms of air goes through the fan bypass. Without that fan bypass, the thrust would be much lower, but the energy going through the engine would be the same, and so the exhaust speed would be correspondingly higher. Do the math and you get an exhaust velocity of about 350 m/s with a total mass flow of 197 kg/s: 69 kN.

So what's the specific impulse of a pure turbojet without the bypass fan? Well, it would be 35.7 seconds. That's...terrible. Absolutely terrible. If your specific impulse is that low, you will not go to space today. However, a turbojet gets by because the only propellant flow that actually matters is coming from the fuel it carries, and the fuel it carries is just 1.52% of the total propellant flow. Subtract out the mass of the airflow and do the math again, and boom: your pure turbojet now has a specific impulse of 7,930 seconds. That's fantastic!

The problem, of course, is that your pure turbojet will only be able to maintain thrust at relatively low speeds. The faster it goes, the faster the airflow into it, and the less work it is able to do. Net thrust is the momentum of the exhaust coming out of the back of the engine MINUS the intake drag: the momentum of the air coming into the front of the engine. At 200 m/s airspeed, its net thrust is the 69 kN of thrust out the back minus 38.8 kN of intake drag or a total of 30.2 kN, and so your effective specific impulse drops to 900 seconds. At 250 m/s airspeed, intake drag reaches 48.5 kN and so net thrust drops to 20.5 kN, a specific impulse of 611 seconds. At the speed of sound -- 343 m/s -- intake drag is 66.5 kN and so net thrust is just 2.5 kN, a specific impulse of 75 seconds. At 355 m/s, net thrust is zero: all of the thrust out the back of the engine is being used to counteract the intake drag, and your specific impulse is zero.

Unfortunately, 355 m/s is only 4.5% of the velocity you need to reach orbit.

You can, of course, carry your own oxidizer (in the form of liquid air or simply liquid oxygen) in tanks. As your speed increases, you can slowly close the intakes to reduce intake drag, and you can inject liquid oxidizer into the engine to make up for the lost oxidizer. But now you're going to have to include the mass flow of your liquid oxidizer in the equation. And so by the time you've completely closed your intakes and you're now relying entirely on liquid oxidizer, you're still only getting that same specific impulse of 35.7 seconds I calculated above. You'll need a much more efficient rocket engine to get to space.

[magnemoe]

2 hours ago, sevenperforce said:

A scramjet only gets 3600 isp because you aren't counting the mass of the oxidizer, since it's collected as you go.

If you're bringing your oxidizer with you, then you have a rocket, not an airbreather. Airbreathing engines don't get high efficiency by magic; they actually get lower efficiency than a rocket BUT they benefit because they don't have to carry their oxidizer with them, so you don't have to count it as part of the propellant mass flow.

Specific impulse is thrust divided by mass flow rate. In a rocket engine, the mass flow is all the propellant: both fuel and oxidizer. In an airbreathing engine, the mass flow is the fuel alone. But once you're bringing along liquid air to operate your engine, you now have to count the liquid air in your mass flow, so you're no longer getting 3600 seconds of specific impulse.

This, now it has been suborbital space plane concepts using an rocket engine going from mach 2-3 up to 6-8 then you deploy upper stage in space and you don't need fairings. 
This is very similar an real rocket engine would have better ISP but if you just need say 100 m/s to raise AP you could probably heat up lox and inject oxygen into the scram jet. 
If you want to make an SSTO, stop it  Make second stage reusable instead. 
Well if you have high TWR fusion reactors you can but it changes everything.  Or you can build some support mega structure who only makes sense if you launch thousands of ton of payloads daily. 

[CBase]

Thank you @Arugela for provoking @sevenperforce great explanation into air fed engines

[sevenperforce]

1 hour ago, CBase said:

Thank you @Arugela for provoking @sevenperforce great explanation into air fed engines

One thing I didn’t discuss above is the advantage created from ram air effect. A turbojet engine operating on the run away is using a significant portion of its reaction energy to compress atmospheric air and push it into the combustor. This creates a limit on the static thrust that the engine can produce. However, once the engine really starts to get moving, the air begins to be compressed simply by the forward motion of the engine forcing the air into the intake. This means the compressor doesn’t have to do nearly as much work, which means the turbine isn’t extracting quite as much energy out of the exhaust stream, which means higher exhaust velocity and better specific impulse than you would otherwise expect.

Unfortunately, the ram air effect really doesn’t provide much help until you are starting to edge toward transonic velocities. Jet engines can be designed to fly at well above the speed of sound by using a much more fuel-rich combustion and thus pumping more energy into the exhaust. Even so, the region in which the ram air effect starts to provide a significant advantage tends to be rather narrow, because even the most energetic hydrocarbon fuels soon reach a point where they can no longer overcome intake drag. There are scramjet designs using liquid hydrogen to dump as much energy into the exhaust as possible and thus push the airbreathing mode to the max possible velocity, but hydrogen is fluffy and not great at providing thrust at the initial low speeds.