Half a croissant, on a plate, with a sign in front of it saying '50c'
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Jet engine + lox = rocket

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So. Model jet engines have now reached a very high level of maturity. For a few thousand pounds, you can buy a plug-and-play jet engine with a thrust of up to 150kgf, and a weight of <<30kg. These things typically drink a very few litres per minute.

Clearly, you can't run a jet engine in a vacuum. However, why can you not simply supply oxygen (either as gas from LOX, or more likely as an aerosol of LOX), mixed if necessary with nitrogen (since I'm sure the engines are optimised for regular air)? In the simplest system, the oxygen would simply be sprayed directly at the intake of the jet engine. This may not be the most efficient, but on the other hand it must surely work, since this is effectively what happens in atmospheric flight.

If fuel consumption is litres per minute, then oxygen consumption (as LOX) would be a few fold higher - still not outrageous. Even if nitrogen is needed as a 'filler', consumption is still OK.

I know there is nothing radically new here, and I also know that people have tried (with some success) to develop big advanced hybrid rocket/jet engines.

All I'm saying here is that, given that small efficient jets are baked, why can't they be turned into rockets by just spraying LOX at their intakes?

(And yes, I know the expansion ratio at the back end would be non-ideal, but that's a detail. Also, dynamic jet nozzles are well-baked and robust.)

MaxwellBuchanan, Jan 22 2013

Lox on a bagel http://www.bphope.c...ghters_bagel-an.jpg
.... if lox could kill :-) ha [xenzag, Jan 22 2013]

lox rocket http://science.hows...rks.com/rocket5.htm
AWW SHUCKS someone else thought of it first. [Brian the Painter, Jan 22 2013]

Nitrous vs oxygen facts http://www.noswizar...chnical-information
sorry I couldn't find a better link, I'm tires [Brian the Painter, Jan 22 2013]

XCOR "Tea Cart" Engine http://www.xcor.com..._rocket_engine.html
15lbf N2O/Ethane [Klaatu, Jan 23 2013]

[link]






       Even if you could somehow accomodate the internal shapes for a much lower speed, I imagine you'd end up either having to use the fuel as coolant/motive charge or burn out the engine pretty fast with nothing to soak up the heat except the engine. And given that the LOX is compressed there's no need for a compressor/turbine arrangement.   

       If you subbed in Ar for the missing N2 you could get some interesting results, given the noble gases' ridiculous thermal properties. Might even lower fuel consumption.
FlyingToaster, Jan 22 2013
  

       Yes and no.   

       If you feed the LOX in as gas (or, probably, aerosol; and possibly mixed with nitrogen), then the engine sees exactly the same as it sees when flying in atmosphere.   

       As for not needing a compressor, that applies only if you can feed the oxygen in above the pressure inside the engine. Conventional rockets either have turbopumps, or rely on highly pressurized fuel/oxidiser tanks which are weighty.   

       However, I see one possible flaw. Does a regular jet engine suck in a lot more air than is needed for combustion? If so, then this is doomed.
MaxwellBuchanan, Jan 22 2013
  

       I imagine it does, but even if it didn't, in the LOX version you've only got 1.3x the amount of gas you started off with (3O2 + hydrocarbon -> 2H2O + 2CO2), as opposed to 5.3x, to distribute the heat of combustion.   

       So, in order to keep the engine from melting you need to add more O2 or more fuel to keep the exhaust gas temperature down.   

       At which point you might as well use compressed air instead of O2... or maybe supplemental water-injection to save space.
FlyingToaster, Jan 22 2013
  

       Is this for space?
Kansan101, Jan 22 2013
  

       //you need to add more O2 or more fuel to keep the exhaust gas temperature down.// Yes, probably true. I was thinking LOX plus liquid nitrogen (which, I guess, is liquid air).   

       //Is this for space?// Yes. Air-breathing up to some maximum altitude, then progressively supplemented with LOX or liquid air.   

       Main question remains - does a jet engine suck in a lot more air than it needs for combustion?
MaxwellBuchanan, Jan 22 2013
  

       Isn't this a Soyuz rocket, with the added complexity and inefficiency of unnecessary turbines? Actually scratch that, a Soyuz probably still has turbines (turbopumps) that do about the same job of fuel/lox delivery, so this is really just a unique rocket engine design.   

       //Main question remains - does a jet engine suck in a lot more air than it needs for combustion?// Yes, lots more. Most of it is bypass, which you can do away with, but some extra air is still needed for cooling.
DIYMatt, Jan 22 2013
  

       I thought this would be similar to Reentry Fried Chicken (but with cream cheese and lox instead). I'm disappointed.
phundug, Jan 22 2013
  

       If you try to spray LOX at the front of a jet engine in space very little of it would make it inside, I am afraid, as there is no sucking in a vacuum.   

       If it was injected inside where combustion takes place, probably better, but some of the combusted gasses would exit the front of the engine. You would need closeable shutters on the front to prevent this.   

       If this were for near-space, very high altitude, I think it might work as is.
Kansan101, Jan 22 2013
  

       Would be even better with some capers.
normzone, Jan 22 2013
  

       //Yes, lots more. Most of it is bypass// I was afraid of that. If it's just a cooling question, that's bad enough. But if the jet uses the extra intaken air (heated by the combustion) as a major contributor to thrust, then that's a big problem.   

       //very little of it would make it inside, I am afraid, as there is no sucking in a vacuum. // Also good point. I was assuming that you could have a chamber in front of the engine (an extended intake, really) into which you'd pump oxygen fast enough to maintain the pressure there at something like atmospheric; then the jet "thinks" it's operating in normal air.   

       // is that not the Skylon British spaceplane wossername...// Yes, and I think the engine is being designed by Reaction Engines? That's the big expensive version. I was only wondering why you can't feed air (or oxygen) to a regular model jet engine and have it work the same way. Again, though, the problem comes down to how much "excess" air a jet eats, beyond that needed for combustion.
MaxwellBuchanan, Jan 22 2013
  

       OK, so it turns out that turbojets (as used on Concorde, and on some high-speed fighters) have a bypass ratio of 0:1, meaning that all of the air coming into the engine goes through the combustion chamber. More fuel efficient turbofan engines send only some of the air through the combustion chamber to power the turbine, which then drives the inlet fans, which send some air to the combustion chamber but most of the air around it - in effect, a turbofan is a turbojet with extra big fans at the front which provide the bulk of the thrust.   

       Now, my question is: does a 0:1 bypass ratio mean that all the oxygen in the incoming air is consumed in combustion? I.e., does it mean that these engines eat air stoichiometrically with respect to fuel? Or does it mean that there's a lot of surplus air (a very lean mixture), all of which passes through the combustion chamber?
MaxwellBuchanan, Jan 22 2013
  

       OK again. So it turns out that only a proportion of the air entering a turbojet (non-bypass jet engine) is used in combustion, the rest being 'bulk' which provides cooling and, by expanding as it absorbs heat, contributing to some of the thrust. The highest proportion of air "used" in combustion seems to be about 25%.   

       Thus, if we have 1kg of fuel, we will need to feed it about 4 times the stoichiometric amount of oxygen. This means roughly (very roughly) 4kg of oxygen. But we are feeding it air (because the figures for turbojets are based on air, not oxygen), so that's about 20kg of air.   

       So now suppose that we have a model turbojet engine (I don't know if the currently available model jets are turbojet or turbofan), weighing 30kg and providing a thrust of 150kg. It's going to use something on the order of 2kg of fuel and 40kg of liquid air per minute. Call it 50kg in total per minute.   

       This is not great, but remember that the engine can breathe atmospheric air up to a considerable altitude. Thus, if we had a gross lift-off weight of say 100kg (30kg engine, 50kg fuel+liquid air, 20kg structure and payload), we'd probably start feeding it liquid air at (I'm guessing) 40,000ft, after which we'd have one minute of operating time before the liquid air ran out. (The fuel is only a minor part of the weight, so we don't need to run out of that.)   

       At the start of this minute, we have 150kg force (1500N) and a mass of around 100kg, giving an acceleration of A=F/M = 15m/s/s. Towards the end of the minute, we have a mass reduced to around 50kg, giving twice that acceleration. But then there's air resistance (constantly reducing, and already lowish at 40,000 feet). So let's assume an average acceleration of 10m/s/s for one minute... Awwww bugger. That only gives us 600m/s.   

       Oh well.
MaxwellBuchanan, Jan 22 2013
  

       I bet there IS a way this is workable. As I said, a kerosene and lox powered rocket like the soyuz has a lot in common with a turbojet. You would just be relocating the axial compressor/turbopump to the front of the combustion chamber and adding a turbine in the exhaust stream to drive it. Yes, it would be less efficient than either a rocket or a jet. But it might be more efficient than having seperate jets and rockets for high alitude flight.
DIYMatt, Jan 22 2013
  

       And at which point do the bagels feature in the idea?
xenzag, Jan 22 2013
  

       //a kerosene and lox powered rocket like the soyuz has a lot in common with a turbojet//   

       Yes, you're right! One question is *why* it would be less efficient to replace conventional rocket fuel pumps with the compressor stage of a turbojet - perhaps it needn't be that much less efficient and, as you noted, the ability to air- breathe for much of the flight is an advantage.   

       Still, I guess we are just converging toward the Reaction Engines/Skylon concept, i.e. it's not just a question of squirting liquid air into the intake of an otherwise off-the-shelf miniature jet engine.
MaxwellBuchanan, Jan 22 2013
  

       the "why" of it being inefficient is that the engine now has a hole at the front where is doesn't need one. The hole and the motion of the engine through the air delivers air to the turbine under substantial pressure, operating the turbine stationary, or without this source of air pressure substantially derates the output, and in effect makes the design operate rather poorly.
WcW, Jan 22 2013
  

       // why can't they be turned into rockets by just spraying LOX at their intakes? //   

       Apart from the above-mentioned problems, we think it is also useful to point out that the LOX will scavenge heat from its surroundings in a quite vicious way. Pretty soon, everything forward of the combustion chamber will be at -180 C.   

       Odd things happen to metals at such low temperatures.   

       Further, you may have noted that every device which employs compressed or liquid oxygen is lavishly provided with cheerful labels that read "USE NO OIL". There is an excellent reason for this. The consequences of ignoring the advice of the aforementioned labels are dramatic, fascinating, and most enjoyable, providing the observer is positioned a sufficiently long way from the object under provocation, and has something substantial behind which to take cover.   

       Turbojet engines require their bearings to be lubricated. Traditionally, this is by means of oil. Minute traces of oil inevitably escape from the front bearings of the N1 spool into the airflow, where they are entrained and conveyed into the combustion chamber, which is all very fine and splendid and makes no difference to the operation of the engine.   

       However, an intake compressor which contains a mixture of oil and oxygen , even cold, is liable to something called "compressor blowback" (See above regarding safety distances and hiding behind something solid).
8th of 7, Jan 22 2013
  

       //the engine now has a hole at the front where is doesn't need one.// Not so. In a true turbojet, the front of the engine is a compressor - essentially an axial turbopump delivering oxidiser (air) to the combustion chamber. This compressor is being driven by the blades at the exhaust end of the engine.   

       In a conventional liquid rocket, there are turbopumps delivering oxidiser and fuel into the combustion chamber. These turbopumps are normally driven by a turbine which is powered by bleeding off some of the fuel (or the hot exhaust).   

       Thus, as [DIYMatt] pointed out, the only real difference is in the relative arrangements of the components.
MaxwellBuchanan, Jan 22 2013
  

       Just add something with a high hydrocarbon content, like cream cheese, into the mix. That's should make for a tasty fuel mix.   

       I must try rocket, next time. Quite fond of rocket.
UnaBubba, Jan 22 2013
  

       we could name this new invention the "liquid fuel rocket"
Brian the Painter, Jan 22 2013
  

       Or you could use yellow rope as a fuel. Don't eat it though
Brian the Painter, Jan 22 2013
  

       Or polybutadiene and nitrous oxide... etc.
UnaBubba, Jan 22 2013
  

       good call, nitrous has 3x's the oxygen by volume.
Brian the Painter, Jan 22 2013
  

       I think your wrong, I shall post a link henceforth.   

       "Furthermore, oxygen can only be ‘readily’ stored in a compressed ‘gaseous’ form, without being stored in a special cryogenic thermos cylinder (a cylinder within a cylinder with a vacuum between the two walls) and as a gas it loses the cooling effect that nitrous offers by being available as a liquid. Adding the oxidiser as gaseous oxygen would displace more air than adding nitrous in liquid form, resulting in a lower total power capability. In other words; by using nitrous oxide we can squeeze in more oxygen atoms in a more beneficial form, containing substantial amounts of detonation suppressing nitrogen, than would be the case with gaseous oxygen"
Brian the Painter, Jan 22 2013
  

       The nitrogen also reacts with one of the combustion byproducts, from memory.   

       Liquid oxygen is volatile and highly reactive, unlike nitrous oxide (which is used as a mild anaesthetic).
UnaBubba, Jan 23 2013
  

       I would like to retract my 3x's more oxygen statement. It does in some circumstances, help put 3x's more in the car; but that wont work in outer space.
Brian the Painter, Jan 23 2013
  

       Easier and cheaper just to use a liquid fuel rocket engine. A turbojet has a lot of heavy parts that a rocket doesn't need.On a small-scale rocket, you could do away with the need for fuel/oxydizer pumps simply by pressurizing the tanks. If you wanted a high-altitude, near-space engine, say for altitudes around 80-100,000 ft, borrow an idea from the SR-71 and bypass the compressor turbines for ramjet mode.
whlanteigne, Jan 23 2013
  

       N2O's not bad: 2:1 ratio of filler:oxidant compared to air's 4:1, and a BP of -80C.   

       NO2 looks interesting for a rocket oxidiser: BP of 21C.
FlyingToaster, Jan 23 2013
  

       //do away with the need for fuel/oxydizer pumps simply by pressurizing the tanks.// That's used for small liquid rockets; the problem, though, is that the entire fuel/oxidiser storage and piping system has to be able to withstand whatever pressure you have in the combustion chamber.
MaxwellBuchanan, Jan 23 2013
  

       Re: cooling using bypass air. Liquid oxygen is quite good at cooling things.
Ling, Jan 23 2013
  

       I understand that lots of rockets use liquid oxygen and/or hydrogen, and before combustion it's piped around the parts of the engine to keep them within their thermal tolerance. This solves both problems at once.
It does however make for quite a complicated system - they're not just spraying liquid oxygen in and hoping for the best.
Loris, Jan 23 2013
  

       Amazing number of rocket scientists we have on here.
RayfordSteele, Jan 23 2013
  

       Since when do we need those pesky "qualifications" for the bakery?
DIYMatt, Jan 23 2013
  

       //not just spraying liquid oxygen in and hoping for the best.//   

       No, but there are rockets which use excess fuel or oxidiser to form a boundary layer to protect the nozzle, rather than using liquid cooling. They're not hoping for the best - they have the fluid dynamics pretty closely worked out. But it's less efficient (for a large rocket) than active nozzle cooling and stoichiometric combustion.
MaxwellBuchanan, Jan 23 2013
  

       // whatever pressure you have in the combustion chamber //   

       Very approximately, the pressure on the top of the combustion chamber will be at least equal to the product of the overall mass of the rocket and its acceleration.   

       // NO2 looks interesting for a rocket oxidiser //   

       It's no more difficult to use HNO3, which technique is WKTE.
8th of 7, Jan 23 2013
  

       //Amazing number of rocket scientists we have on here.//   

       Well, hey - it's not brain surgery.
Loris, Jan 23 2013
  

       Yes, except that when a tank of NO2 explodes everybody laughs. When a tank of nitric acid explodes, not so much.
MaxwellBuchanan, Jan 23 2013
  

       Isn't that half the fun?
Loris, Jan 23 2013
  

       //fun// sp. "face"
MaxwellBuchanan, Jan 23 2013
  

       More like rocket surgery, I should think.
UnaBubba, Jan 23 2013
  

       I am thinking about 40,000 feet and an air-thirsty jet. The jet gets only the air entering the front end. At altitude there is less air. Maybe the front end could have a telescoping cone. The cone would telescope out (or back in on descent) to maintain the same air resistance at altitude as at sea level and by doing so ensure the same amount of gas was pushed into the engine. This might allow the jet to operate at even higher altitudes.
bungston, Jan 23 2013
  

       Aren't you just describing the SR-71?
DIYMatt, Jan 23 2013
  

       Sounds a lot like it.
UnaBubba, Jan 23 2013
  

       There's the two orders of magnitude difference in power between an SR-71 engine and a Space Shuttle solid booster... of course on the bright side there's only one order of magnitude power:weight difference.
FlyingToaster, Jan 24 2013
  

       well, you'd need about 200 SR-71 engines to sub in for the 2 SSSB engines, and they'd weigh 10 times as much.   

       Okay, having picked a seed out of my teeth, scramjets are supposed to be about the same as a rocket (when they get operational) in those metrics.
FlyingToaster, Jan 24 2013
  

       I think the original idea behind this (ie, take an existing cheap technology and adapt it simply for spaceflight) is dead, alas. I don't doubt, though, that the concept of an engine that transitions from turbojet to ramjet to rocket is doable - that's how the Reaction Engines machine works. But it's definitely rocket science.
MaxwellBuchanan, Jan 24 2013
  

       I wouldn't say "dead", so much as it'd be a niche application. I think you'd have to optimize heavily for one or the other method. Maybe for a LEO application it'd be happiest as a jet engine that could do rocket somewhat inefficiently, while geosynch, t'other way'round.   

       The Idea "Supercharged Ramjet" (yup that's my horn I'm blowing) is similar in concept: a ramjet that features an electric compressor section that tucks away into the cone when not in use: probably hideously inefficient while operating at sub-ramjet speeds, but enables an aircraft that operates at ramjet speeds to get there and back without having a completely separate engine.
FlyingToaster, Jan 24 2013
  

       I like the 'supercharged ramjet' idea. But in that case why not drive the supercharger from a fan on the back end? In which case it becomes a turbojet. Then the inlet and outlet blades would either retract or feather when it reached ramjet speeds.   

       More broadly, though, someone mentioned using a regular(ish) jet engine as the first stage of a conventional rocket, and I quite like that. Given the thrust-to-weight ratios of jets (and I'm thinking here of the so-called model jet engines; the big buggers are even better), I think they'd make a pretty good first stage.
MaxwellBuchanan, Jan 24 2013
  

       Motorjets were used on a few aircraft 60'ish years ago: jet engines which compressor is full-time IC-engine driven instead of exhaust-turbine driven. I just changed the aircraft mission parameters to one exclusively operating in the ramjet speed-range, using the "supercharger" just to get up enough speed for the ramjet to operate without help. I'm pretty sure (for halfbaked purposes anyways) that it could be stowed in the diffuser (nosecone) of a ramjet so wouldn't really affect ramjet operations.   

       But jet engines' p:w really pales in comparison with rocket engines' (Wikipedia: "power to weight ratio", last table on the page).
FlyingToaster, Jan 24 2013
  

       //jet engines' p:w really pales in comparison with rocket engines'//   

       Wow - it certainly does. However, power isn't the only consideration. Especially for a rocket (or a jet climbing vertically), fuel consumption is a huge factor, since most of the launch mass is typically fuel (and oxidiser, in the case of a rocket).
MaxwellBuchanan, Jan 24 2013
  

       I think it's a matter of "can't be arsed": a turbojet will get you to a maximum of 2,500mph at 20mi altitude... if you want to get to 200mi altitude you need to be going 17,500mph to be in orbit, 50x the KE and 100x the PE. The ISS is at 250mi altitude.
FlyingToaster, Jan 24 2013
  

       I know, but I was thinking about small launchers. 20 miles up gets you above 99% of the atmosphere, and atmospheric drag is a very, very major factor for small launchers.
MaxwellBuchanan, Jan 24 2013
  

       Sure, look at SpaceShip One.
FlyingToaster, Jan 24 2013
  

       By which I mean SS1 is a rocket that uses a turbojet launch platform. If you actually could combine a turbojet and a rocket then it could be much smaller than the combined two vehicles, but certainly bigger than the SS1 if for no other reason than fuel.
FlyingToaster, Jan 24 2013
  

       //but certainly bigger than the SS1 if for no other reason than fuel.//   

       That depends a great deal on payload. For a ground-launched rocket, the lower mass limit is quite high, because as you get smaller air resistance dominates.   

       But if you're launching at 20-30km, things scale pretty nicely.   

       Rockoons are an attractive option for very low mass rockets. What I'm wondering is whether a jet first stage (with the advantage of predictable performance and use of atmospheric oxidiser) isn't an equally viable option for very low mass rockets. You've got the disadvantage of cost compared to a balloon, but you've got a modest advantage in terms of starting velocity for the rocket, and also in terms of guidability of the first stage.
MaxwellBuchanan, Jan 24 2013
  

       Well that's the Rutan design, innit. Fly a jet up to x feet and release the rocket.
FlyingToaster, Jan 24 2013
  

       Actually I think the real advantage of a jet first stage would be that you wouldn't have to fart around with a rocket engine which has an atmospheric bell: go straight to the space rocket motor design. Maybe: is 20mi rare enough to use a vacuum bell ?   

       On the save-weight note, perhaps LOx could be made on the way up.   

       Ramjets are lightweight (compared to turbojets) and operate at a much higher speed. It could be launched from a (relatively) short rocket or railgun sled to get up fast enough for the rj to kick in. Or from a B58, which was designed to fly with a weapons pod that's bigger than the SS1 (albeit wingless).
FlyingToaster, Jan 24 2013
  

       Ramjets are already launched with the aid of a booster rocket. The problem is, the Air Force keeps launching them, and they keep blowin' up before they get to top speed.
DIYMatt, Jan 24 2013
  

       //perhaps LOx could be made on the way up. //   

       That would be cool.
MaxwellBuchanan, Jan 25 2013
  

       This idea is a good one, you can think about it like a full- flow staged-combustion rocket. You run ALL of the LOX and kerosene propellant through one of those, the older kind you only transfer a fraction of fuel in the turbopumps itself. Even in the full-flow variant there is a much smaller volume of oxidizer being pressurized. A jet turbine is an upscaled gas turbopump used on hot gas instead of cryogenic fuels. There is a certain amount of conversion required in order to get a rocket turbopump to stuff the same quantity of atmosphere into it as your typical jet turbofan engine. It needs to be bigger in order to support the volumes atmospheric air is at. Impellers are swapped with fans packed with blades. One approach to the air problem being pursued is an oxygen pre-cooler, which runs liquid hydrogen fuel alongside the input air, which attempts to liquify oxygen and flood the turbopump inlet, which is still not going to approach the efficiency of a tank full of LOX sitting ready, even if you do get free oxidizer for the first minute or two of flight. And why? Why bother? Why not just build two engines dedicated to burning two different oxidizers, air and LOX. The thrust-to-weight problem is what we're really trying to solve.   

       I think the better plan here is to abandon the turbine completely and figure out how to blast a rocket engine through a tube and get some sort of venturi effect, burning additional fuel in the afterburner while it's within the atmosphere. That's what the SR-71 was and that's what current hypersonic research aircraft consist of. You'd just add an additional "pure-rocket" mode where the air- breathing parts get out of the way, I think in most incarnations this strap-on air-breathing engine would be a staged "booster" rocket that lands on it's own once it's job is complete. If you built this it would quickly adapt to become a more efficient platform, perhaps using a cluster of 4 large rocketjet engines at the base instead of rocket nozzles.   

       Or, why not just build a jet turbine that uses LOX to "boost" it's performance, maybe you can double the thrust-to- weight ratio at atmospheric pressure over a conventional turbine.   

       I think the best shot at improving propulsion is going to be some sort of electronic acceleration, it's already 10 times more efficient, now it just needs to be a few orders of magnitude more powerful, perhaps by using a nuclear reaction. We know that nuclear reactions are going to be the best payoff, it's anyone's guess why it's out of reach to civilization. A beam of energy from a ground station is staring us in the face. This is the "light-rail" of space transportation. This method has already been tested and proven to work, it just needs higher beam energies. You could theoretically provide as much or more energy to a point-source than with the nuclear idea.
zardinuk, Mar 28 2015
  

       I think you'd be better off putting the lox on your bagel and putting your burned fuel through a nozzle. Accelerating it to supersonic speeds will improve efficiency.
kevinthenerd, May 08 2017
  
      
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