# First stage of a rocket

i was wondering why not use the air in the surrounding to oxidise the fuel in the first stage of a rocket? it definitely will burn with the fuel(assuming bi propellant ) and will also reduce the wet mass of the rocket? is there any problem with this ?

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D H
Staff Emeritus
There are huge problems with that. DoD and NASA have spent lots of money, lots and lots and lots of money, on this concept, starting in the 1950s. It has never worked. Given the vast sums of money spent, I'd argue it cannot work. It's past time to put those monies elsewhere (and that's pretty much what has happened.)

there is not enough oxygen in the surrounding area to feed that much hydrogen it roughly has an expansion rate of 2000 times the size of itself in liquid form compared to itself in gaseous form, and you need a large amount of oxygen to fuel it and the only way to supplement it is in the liquid form , the equation goes 2h2+o2 yields 2h20 , so you need half the amount in oxygen then hydrogen to do it , the atmosphere cant feed it quick enough also , the energy released when 2 water molecules are formed (correct me if im wrong) is about 400,000 joules of energy. and yes the energy is from combustion but to put it in more specific terms its from the electrons of the oxygen and hydrogen going to an outer energy level to a lower energy level and thats where the energy is released at

mfb
Mentor
The first stage starts with a velocity of 0, so the initial oxygen supply would be really low. In addition, air has just 20% oxygen, which reduces the burning temperature and therefore the exhaust velocity significantly.

As an example, a Saturn V had 1 300 000 liters of liquid oxygen. The same amount of oxygen can be found in ~5 million cubic meters of air. The first stage burned 150 seconds and had a terminal velocity of ~2400 m/s. Approximating the acceleration as uniform, it travelled 180 km during this time. To get 5 million m^3 air, we would need an inlet of ~30m^2 or roughly the cross-section of the rocket. That is completely impractical (you cannot build a hollow rocket!), and ignores all the other issues mentioned.
Problematic aerodynamics would be another issue.

chacka said:
the energy released when 2 water molecules are formed (correct me if im wrong) is about 400,000 joules of energy.
That is roughly 24 orders of magnitude off. Do you mean moles instead of molecules?

A rocket engine that burns atmospheric oxygen is just jet engine, and the problems with these, as I understand it, are that
1) they are not powerful enough compared to their weight to work as vertically lifting engines.
2) they cannot function through a wide enough speed range to make them practical in a rocket.

Point 1 is perhaps due to the reasons already discussed in this thread. One way to work around this would be to fire the engine horizontally and let wings do the lifting. However, that only further shifts the problem towards point 2.

There ARE continuous attempts at solving point 2 in an effort to make something of a space-plane a practical concept. The best contender as far as I know is the SABRE engine which is currently under development and making impressive progress.

In fact, I will do some light reading to catch up on latest developments right now!

SteamKing
Staff Emeritus