4
$\begingroup$

Would it be possible (though not practical) to create a jet that runs on liquid nitrogen that is heated, creating the expansion effect? I believe nitrogen takes up 674 (about) times as much space in gaseous form as in liquid, but if it were somehow evenly heated, could it provide a similar effect to combustion without the carbon emissions? enter image description here

$\endgroup$
11
  • $\begingroup$ Thinking of a turbofan and nitrogen replacing the fuel in the combustion chamber... $\endgroup$
    – oeste
    May 28, 2017 at 14:16
  • 2
    $\begingroup$ But why take the trouble using nitrogen, why not water? That expands as dramatically as nitrogen does when heated to the boil. $\endgroup$
    – Koyovis
    May 28, 2017 at 14:24
  • 2
    $\begingroup$ How would you cool the nitrogen? It has a boiling point of - 195 C. $\endgroup$
    – Koyovis
    May 28, 2017 at 16:30
  • 3
    $\begingroup$ That's named cold gas thruster. About efficiency you need to compare the specific impulse (76s) of N2 and the specific impulse of a turbofan (3000s). The lower the sp, the lower the efficiency. $\endgroup$
    – mins
    May 28, 2017 at 16:38
  • 3
    $\begingroup$ How would you heat the nitrogen? If with some sort of fuel, why bother? It's more efficient to use the fuel directly in a conventional jet engine. Only thing I can think of that might work is using it as reaction mass in some sort of NERVA-style nuclear engine: daviddarling.info/encyclopedia/N/NERVA.html $\endgroup$
    – jamesqf
    May 28, 2017 at 19:17

4 Answers 4

4
$\begingroup$

At best, the mechanical energy you can release by evaporating liquid nitrogen would be about its heat of vaporization, which is 5.56 kJ/mol or roughly 200 kJ/kg. (Converting this energy to useful thrust probably requires you to use the expanding nitrogen to drive a turbine that drives a fan or propeller, but that's a Simple Matter of Engineering, at least comparatively).

The chemical energy density of jet fuel is about 46 MJ/kg.

Even if we suppose the efficiency of your nitrogen engine could be a bit better than that of hydrocarbon-burning engines, this still mean that you would need to carry about a hundred times more nitrogen (by weight) than jet fuel for the same result.

$\endgroup$
1
  • $\begingroup$ Some of the nitrogen needs to be boiled off for auto-refrigeration as well, so a part of the 200 kJ/kg is used for keeping the fuel cool. $\endgroup$
    – Koyovis
    May 30, 2017 at 2:44
3
$\begingroup$

You run into several major problems that make your entire assertion fundamentally flawed:

  1. The liquid nitrogen needs to be constantly cooled and kept in heavy containers to remain liquid, greatly increasing the mass of your fuel tanks AND requiring some form of active cooling system that needs a source of energy to run on.
  2. You then need to heat up the nitrogen as it enters the engine proper (through thick and heated pipes to prevent the pipes from getting quickly coated with a layer of heavy water ice, requiring more heating systems that require more energy), this requires a source of energy and thus more mass and fuel.

In the end you're going to need a large generator running some kind of fuel to produce the electricity needed to cool the liquid nitrogen, heat it up in the engine, and prevent icing on the nitrogen pipes. This would likely have the same mass, size, and fuel requirements roughly as the jet engine you're thinking to replace with a nitrogen expansion vessel. And on top of that your fuel system for the main engines just got a lot more complicated, prone to failure (cryogenic systems are very prone to failure as all but the most exotic materials tend to not last very long at cryogenic temperatures, certainly when compared to normal temperatures), and heavier.

All these problems were encountered during the 1990s and early 2000s when several manufacturers (including Airbus and I think Boeing) started drawing up plans for aircraft fueled by liquid hydrogen. Turned out that just the tanks and cooling/heating systems would take up the entire cabin space of say a 747 while giving the range and performance of something smaller than a 737 (while leaving it with no room for passengers or cargo).

$\endgroup$
3
  • $\begingroup$ Why cannot evaporation process cool the nitrogen? Some part of the fuel evaporates and goes into the engine, that remains stays liquid. $\endgroup$
    – h22
    May 29, 2017 at 12:42
  • $\begingroup$ @h22 not enough, not by far. It helps, sure, but the incoming heat from the surrounding environment is far more than that unless you have incredibly large isolation vessels. A vessel that can keep a liter or so of liquid nitrogen in liquid state for a few hours weighs 5-10 kilos empty and has a volume of 5 liters or more. Now think what it'd take to keep several hundred tons of the stuff in liquid form... $\endgroup$
    – jwenting
    May 30, 2017 at 5:52
  • $\begingroup$ The logic seems to be somewhat self-contradicting. If you don't isolate the tanks, you get a fairly high flow of nitrogen gas (from evaporation). That's the source of your trust (possibly with some further heating). You can heat the nitrogen steam directly as it exits the tank, preventing the pipes freezing. I think the main problem is that the tank ends up covered in ice, not the pipes. This is a real issue for rockets. $\endgroup$
    – MSalters
    Jun 1, 2017 at 11:27
3
$\begingroup$

Liquid nitrogen could be heated by the coming air (-50 celsius at flight altitude is not the best temperature but still way above the temperature of liquid nitrogen) and then expand. The engine would need lots of air to heat the nitrogen, so may actually need the air intake, compressor (compressing air raises the temperature a lot). Higher flight speed may be optimal, as such an engine would likely want higher intake temperature.

There is no need for such an engine to be pure rocket, it can have also bypass, using fan to accelerate some air bypassing the core, same as most of jet engines do.

Storing liquid nitrogen is also not a problem, as the process of evaporation (that must be even accelerated by the engine) should cool it enough.

Some thrust seems possible. Mathematical analysis by specialist in physics is required to figure out if the thrust would be sufficient to sustain the flight.

$\endgroup$
4
  • $\begingroup$ I can see the half truths being injected here. Yes compressing incoming air does heat it up, but where does the energy to do this come from. Thrust generation: whether pure expansion nozzle or with external fan does not really matter, just a few percentage points. But even if the fan is used, your still not tying oxygen atoms to the nitrogen and including that into the specific thrust.. Boiling off nitrogen to cool it makes it even less efficient. At what point does "impractical" turn into "forget about it"? $\endgroup$
    – Koyovis
    May 30, 2017 at 6:37
  • $\begingroup$ The energy comes from the temperature difference. Same way, if you have the electricity generating thermo-pair (one end in the liquid nitrogen, another in the air outside the aircraft), it would generate the electricity because there is a temperature difference for it to work. The fan may be required to extract enough heat from the rather cold surrounding air. At which point I need to say think again? $\endgroup$
    – h22
    May 30, 2017 at 7:25
  • $\begingroup$ Yes a Stirling engine works. Yes a cold thruster creates thrust. And all of the other smoke pots that together create the smoke screen. But now look at the total energy contents of liquid nitrogen compared to that of kerosine and you'll find your "can't be done". If an expanding has engine was so great, why did the Space Shuttle not simply expand its LOX? $\endgroup$
    – Koyovis
    May 30, 2017 at 8:06
  • $\begingroup$ @Koyovis: The idea of a jet engine is to have a compressor in front and a turbine in the rear. That's where the energy comes from. This produces a net propulsion as long as the turbine extracts only a fraction of the power of the exhaust stream. In this scheme it would work as the compressed air flow is increased by the added nitrogen stream. However, this idea probably fails when it suggests a bypass. The turbine probably won't provide enough power for that. $\endgroup$
    – MSalters
    Jun 1, 2017 at 11:39
3
$\begingroup$

You could if you would cool the N$_2$ on the ground and then let it expand, you would then not need the compressor amd the turbine in your turbofan drawing, just an exhaust nozzle.

In a way you would use the liquid nitrogen as an energy storage source, like a battery. Possible, yes, practical, no, like stated in the question. A practical problem being the fuel tank that needs to be kept below -159 deg C for the nitrogen to remain fluid.

Another practical problem is that it would be a rocket engine, not a combustion engine. Rocket engines bring all their propellants on board, while a jet is an air breathing engine: most of what streams out of the jet exhaust is air, which was floating around in the atmosphere just where the plane was, now bound to carbon atoms from the fuel. That's what you need all the compressor and turbine blades for, to first compress outside air and then extract energy to run the compressor.

Rocket engines have an issue with how long the fuel lasts. All of the mass you're propelling to create thrust with, you carried on board and took off with. It could be an amusing exercise to compute how many passengers would be able to sit around the huge nitrogen tank with very cold feet from the -159 C fluid inside, ready for a flight of 3 minutes.

There was a clever Frenchman who sold licenses for compressed air vehicle motors some time ago, a wonderful scheme to extract money from people with more money than sense. It hasn't worked in a car yet due to it all being a bit inefficient - can't see it work in an aeroplane where weight is so important. But perhaps I'm just overly pessimistic, these could one day be just practical design problems that were overcome.

Edit The question turned into a discussion that is more interesting than what I initially expected. A string of half-truths may look like a proper necklace at first glance.

$\endgroup$
8
  • 1
    $\begingroup$ Why would you use blades? It's a pure reaction thruster. Else efficiency is even lower. $\endgroup$
    – mins
    May 28, 2017 at 16:46
  • 1
    $\begingroup$ If that's the case why is a standard combustion turbofan any different? $\endgroup$
    – oeste
    May 28, 2017 at 17:08
  • $\begingroup$ Yes why would there be blades. $\endgroup$
    – Koyovis
    May 28, 2017 at 23:32
  • $\begingroup$ @oeste Standard combustion releases internal chemical energy, and ties air mass to the propelling gases. $\endgroup$
    – Koyovis
    May 29, 2017 at 0:24
  • 1
    $\begingroup$ @jwenting: Creating that tank of liquid nitrogen takes that energy. The challenge is, can we convert that energy back into mechanical energy with sufficient efficiency, using a light-weight and compact system ? As it turns out, jet engines are pretty good at that. The real problem is what Henning Makholm noted: it doesn't actually take that much energy to create liquid nitrogen. Or, alternatively, you need quite a bit of liquid nitrogen to store a given amount of energy. $\endgroup$
    – MSalters
    Jun 1, 2017 at 11:45

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .