6

We know that many Space Shuttles / Rockets etc. carry their own oxidizer with them. This is partly because at the heights they operate the atmosphere is too rarefied to have enough oxygen to sustain the combustion rates they need.

But also, the liquid oxidizer means small pumps can be used instead of massive compressor stages. Which motivates the following question:

Would it be feasible for a typical passenger jet to carry along its own Oxidizer? I am looking for a back of the envelope feasibility calculation. It intuitively sounds impractical but I'm just curious to see some back of the envelope estimates of how much ( weight / volume) of Liq. O2 tanks would be needed if we hypothetically had something like a B-747 with its own cryogenic Oxygen tanks in addition to already extant massive fuel tanks.

Partially motivated by this related SE Aviation Question and a comment by User @Freeman:

How do rocket engines produce more thrust than aircraft jet engines?

Community
  • 1
curious_cat
  • 8,436
  • 7
  • 45
  • 94
  • Don't forget to allocate space and weight for the cooling system since a 747 flight is a lot longer than your average rockets "let's escape earth" run. – JustSid Dec 29 '15 at 08:39
  • @JustSid: Don't bother. You run the engines on the evaporated oxygen. The amount of evaporated oxygen depends only on the tank isolation as the enthalpy of vaporization is constant, as is the temperature of boiling oxygen under constant pressure. – MSalters Dec 29 '15 at 22:50
  • Using cryogenic fluids mean dealing with low temperatures. That's a totally different approach and resulting complexity than air compressors. Safety will need to be dealt differently too (refueling, topping after a long wait, etc). Leaking O2 near hot burners or brakes is not the same than leaking kerosene. – mins Dec 30 '15 at 13:58
  • 3
    The point in commercial aviation is not to get as much thrust as you can from your powerplant, but to get from A to B reasonably quickly at low enough cost to build in a reasonable profit margin and still offer attractive fares. So, you make the most of what's free and all around you, and don't carry anything you don't have to. – Anthony X Sep 18 '16 at 21:16

2 Answers2

5

Short answer: Yes for a short-range airliner, but it would cost a couple of seat rows. No for long-range travel.

Longer answer: To burn 1 kg of Kerosene you need 15.6 kg of air or 3.59 kg of O$_2$. Its density as a liquid is 1.14 kg/l, so it will use 2.65 times the volume of fuel. A cryogenic tank with sufficient insulation is required, though. This means it should have as little surface area per volume as possible. You could not put the LOX into the wing tanks, but need something with a spherical shape somewhere in the fuselage.

If a large commercial airliner needs 100 tons of fuel, it would need to add 359 tons of oxygen. This would be already close to its own takeoff mass, so adding the oxygen will make the airplane too heavy. If, however, you are content with a short-range version of the same airliner that consumes just 20 tons of kerosene for its trip, the needed 72 tons of LOX would require a volume of 63 m³ which is equal to a sphere of 5 m diameter. This would fit nicely into the cabin of a wide body airliner and allow for some insulation. For cg reasons the tank would be right after the wing spar carrythrough, so the airliner would have two short cabins, one ahead and one aft of the tank.

Sorry, the 747 hump cannot be used for LOX storage - the cg shift would be too big.

For running regular jet engines you would still need lots of process gas. To carry that along as well is impossible, even for a short hop. Process gas is all the nitrogen and extra oxygen which is compressed, heated and then ducted through the turbines and accelerated on the way out, but does not actively take part in the combustion. Without that mass of gas thrust would be much lower, or the exhaust speed would need to be much higher for the same thrust, turning the engine into a rocket.

Peter Kämpf
  • 231,832
  • 17
  • 588
  • 929
  • Thanks Peter. I'm confused what you mean by process gas? I grok fuel & oxidizer. Is process gas something different? The only other substance I can see is the various combustion products & any excess air that is pushed through either through the combuster or the bypass. – curious_cat Dec 29 '15 at 14:08
  • @curious_cat: Edited with an explanation. – Peter Kämpf Dec 29 '15 at 14:23
  • Another nitpickey point: Why do you insist on spherical shaped tanks? Agreed that they do minimize the surface area for a given volume but in routine land transport I see cryogenic trucks carrying Helium / Nitrogen / Air etc. that are just the regular tank truck shape. – curious_cat Dec 29 '15 at 14:39
  • If your airliner, A737 or A320 for instance, carries 40,000# of fuel, 144,000# for 72 tons of lox is right in the ballpark of your takeoff gross weight, aircraft + fuel + pax, everything. Still not easy to do! Space required, yes... Weight required, not so much. – Ralph J Dec 29 '15 at 14:47
  • On process gas: It's also used to cool the engine. In addition, bleed air from the compressor section is used for things like AC, cabin pressurization, and other things. – Steve Dec 29 '15 at 14:49
  • @curious_cat Those tankers don't mind a ton more for the tanks. That is different in an aircraft - every excess in tank wall and insulation means fewer paying passengers. I see your point, but tend to respectfully disagree. – Peter Kämpf Dec 29 '15 at 16:42
  • @Steve; You're right, but if you carry 72 tons of LOX, you don't need any stinking' process gas for cooling. OK, some for film cooling in the turbine, but much less than normal, because now you can pre-cool it with the LOX on its way to the combustion chamber. Regarding bleed air: Bleed air is wasteful, why else would the 787 do without it? It's more efficient to drive another generator and power the systems electrically. – Peter Kämpf Dec 29 '15 at 16:47
  • @PeterKämpf OTOH I see your point too. :) I mean the larger point as to why the whole idea itself is non-feasable / impractical. I am only playing devils advocate to test the smaller bits hypothetically. – curious_cat Dec 29 '15 at 18:44
  • 1
    An alternative arrangement to deal with CG issues is to have two spherical tanks at either end of the plane. – MSalters Dec 29 '15 at 22:55
  • One thing to consider is that a cryogenic tank does not have to be very thick as it isn't a pressure vessel in the true sense right? A design pressure of say a few bar ought to do fine? – curious_cat Dec 30 '15 at 05:00
  • @curious_cat: Right, but a few bar are needed because some oxygen is always cooking off. The spherical tank would minimize this, too. – Peter Kämpf Dec 30 '15 at 08:16
  • If I wanted oxidizer in the wing tanks I'd load N2O4. – Joshua Apr 11 '18 at 16:30
2

You could but why would you want to? One of the advantages to flying in the atmosphere is to use atmospheric oxygen to feed the engines, instead of needing to lug oxidizer around with you.

Finally adding cryo tanks to an aircraft can open up a can of worms worth of development and containment problems. Liquid oxygen, if not properly contained is extremely dangerous as seen with the Apollo 13 mission or the recent explosion of a SpaceX Falcon9 rocket in Florida while fueling on the pad.

It's just more trouble than it's worth.

Romeo_4808N
  • 73,674
  • 7
  • 150
  • 274