View Full Version : Rocket equation

Jairo

2005-Jul-07, 03:16 PM

I was trying to calculate the delta V of the ascent stage of lunar module by myself, to check for some people that it could actualy leave the Moon.

I know the full (4,691kg) and dry (2,333kg) mass, and I know the specific impulse of the engine (3,050Ns/kg). Using the rocket equation, this gives me a delta V of 2,120m/s, wich is indeed higher than the speed required for orbit (1,845m/s).

But I suppose this equation aplies only to absence of gravity. Is it wrong? How do I calculate that the right way?

Thanks.

papageno

2005-Jul-07, 03:22 PM

I was trying to calculate the delta V of the ascent stage of lunar module by myself, to check for some people that it could actualy leave the Moon.

I know the full (4,691kg) and dry (2,333kg) mass, and I know the specific impulse of the engine (3,050Ns/kg). Using the rocket equation, this gives me a delta V of 2,120m/s, wich is indeed higher than the speed required for orbit (1,845m/s).

But I suppose this equation aplies only to absence of gravity. Is it wrong? How do I calculate that the right way?

Thanks.

Is the rocket used at full throttle?

Argos

2005-Jul-07, 03:27 PM

Delta V calculator (http://www.strout.net/info/science/delta-v/intro.html).

Saluki

2005-Jul-07, 03:55 PM

If you calculated your minimum orbital speed correctly, it should include the effects of gravity. Therefore, if your ΔV exceeds the orbital speed, you should have proven that the ascent module could reach the specified orbit.

Jairo

2005-Jul-07, 05:10 PM

If I were using an ion engine, I think I would have an even greater specifc impulse and delta V, but I surely wouldnīt be able to leave the ground. Right?

Thatīs why I think that still isnīt a definitive proof.

Ravaun

2012-Feb-19, 04:14 AM

The 'basic rocket equation' does not account for the Thrust required to lift the mass which you're moving. For simplicity's sake, use 33.85 MN thrust/3097.891 kg/3,057.044086031 m/sec. That's 1 kN thrust/1,000 kg GROSS MASS/279.775165753 m/sec of Δ-V.

(Basis used is Saturn V rocket stack, using 199,000 kg payload, 1996 kg I.U. Final Δ-V to LEO = 8,081.130749123 m/sec.)

That should correct your thrust calculations, so you can select the proper thruster unit to utilize.

Ravaun

"The reason we haven't figured out what makes gravity is because the Earth sucks!"

Garrison

2012-Feb-19, 08:56 PM

The 'basic rocket equation' does not account for the Thrust required to lift the mass which you're moving. For simplicity's sake, use 33.85 MN thrust/3097.891 kg/3,057.044086031 m/sec. That's 1 kN thrust/1,000 kg GROSS MASS/279.775165753 m/sec of Δ-V.

(Basis used is Saturn V rocket stack, using 199,000 kg payload, 1996 kg I.U. Final Δ-V to LEO = 8,081.130749123 m/sec.)

That should correct your thrust calculations, so you can select the proper thruster unit to utilize.

Ravaun

"The reason we haven't figured out what makes gravity is because the Earth sucks!"

Ravaun the last post in this thread was almost 7 years ago, so I don't think you are going to get a response from Jairo

DoggerDan

2012-Feb-23, 09:42 AM

Ravaun the last post in this thread was almost 7 years ago, so I don't think you are going to get a response from Jairo

Oh, let him exercise his brain a little! He's not hurting anything or anyone by crunching the numbers. If anything, he's improving his number-crunching.

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