Does Rocketry Work beyond Earth's atmosphere?

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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread postby nokidding on Tue Jul 09, 2019 2:57 am

Just got to the end…may I add something from personal experience? I burnt off lot of gas at high rates from drilling rigs. I was interested in the thread because a few years back I tried to calculate the thrust produced by a horizontal gas flare - someone had angled the flare tip up and I was concerned that the down thrust would exceed the design utilisation for the boom suspension. This was a gas well being flowed full bore for production testing. Like people here I assumed the gas flare itself pushed back.

When you watch a high rate gas stream exiting an open pipe under pressure and burning off you see the ignition point commences a few metres out and appears to race back unstably. The gas needs to draw in Oxygen from the surrounding air to burn. This runs counter to the idea that pressure is built up by resistance of the air. However between the tip and where it starts to burn there is clearly a high pressure jet.

Beyond the ignition point you have what looks much like a rocket launch - imagine 2000 m3 / min shooting out of a 6 inch pipe and igniting (you are perhaps 20 - 30m away – the noise is extreme). The flare is obviously dissipating huge energy but does it push back? Any force has to be pressure in the burning gas stream - and this is only seen at the nozzle / pipe exit. Air pressure may make this greater than in a vacuum but only by 1 bar or 15 psi (at sea level).

What relevance has a gas flare to rockets in space or in air? The rocket engine produces a stream of gas, much like the oil rig flare boom. It may be extremely hot and the product of a chemical reaction, but it is still just gas. The nozzle is an open pipe much like the flare tip, but designed to maximise the velocity of the gas (in speed and direction).

How does pressure build up in a pipe/nozzle with an open end and a void beyond? Because gas is pushing from behind against the inertia of the moving gas forcing it to increase its speed. Force = Mass x Acceleration (not Mass x Velocity as someone stated here). Acceleration is the key to it. I could model the mass flow rate etc using process engineering software so could then put a figure on the thrust.

Both air and vacuum provide a void for the gas to expand into. The pipe/nozzle stays pressurised as long there is sufficient gas supply to accelerate the expanding gas out of the open end into the void. The gas goes one way and the rocket goes the other, or stays put (hopefully) in the case of the flare boom.

To those in doubt just consider - this is the same Law that makes the image of the ‘plane’ going into the WTC so unphysical.

nokidding
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Re: Does Rocketry Work beyond Earth's atmosphere?

Unread postby Altair on Tue Jul 09, 2019 8:01 am

Very good explanation, nokidding. In fact it's quite puzzling that all pop physics explanations about how rocketry works are centered around Newton's 3rd law, blindly assuming that if you eject a given amount of matter at a given speed from a rocket, it will generate an equal and opposite force upon the later. While from an engineering perspective, if you want to accelerate a body, you must exert a pressure against it, so all calculations should be centered around where and how much of this pressure is applied.

Extrapolating from your rig example, we could say that the rocket would indeed be subject to some force, but this would be only generated by the fuel being accelerated by the pumps (that would apply pressure upon the pumps), but combustion wouldn't add much to this, as pressure in the combustion chamber wouldn't be significantly higher. Amateurs build 'pressure water rockets' (search in YT) that would work in a vacuum, as a liquid would transmit a force to the rocket's body, but of course you cannot store the needed energy in that way.
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