Q&A Multi-stage rocket

[dinos55]

Hello all,

If a rocket has 3 stages + upper stage, and the details I know are the total mass of the rocket, the inclination and the Propellant Mass, the Thrust, The Isp(specific impulse) and the combustion time, for each stage. Can I find the mass of the payload and the altitude of the orbit using these details? and if yes, how? 

Thanking you in advance.

 

[Starman4308]

Not exactly. It's dependent on aerodynamics and ascent trajectory, which can be guessed at, but don't have an analytical solution anyways.

There's also the exact mass breakdown of each stage; while propellant mass is specified (twice, really, since you also know thrust, Isp, and burn time), dry mass has not been specified.

You would also need to know how the first stage performs in atmosphere, assuming any upper stages are lit above any significant atmospheric back pressure.

Finally, altitude reached is dependent on payload mass.

We could guess using rules of thumb and a guess at payload mass, but an accurate guess requires more information to basically simulate the rocket.

[Corona688]

On 1/20/2018 at 1:37 PM, dinos55 said:

Hello all,

If a rocket has 3 stages + upper stage, and the details I know are the total mass of the rocket, the inclination and the Propellant Mass, the Thrust, The Isp(specific impulse) and the combustion time, for each stage. Can I find the mass of the payload and the altitude of the orbit using these details? and if yes, how? 

Thanking you in advance.

Short answer:  No.

Long answer: Nnnnnnnnnnnnnnnno.

That still leaves a range of possible solutions, and "altitude" isn't an entirely sensible measure for this anyway.

[sevenperforce]

For a specific set of details, I can tell you the near-optimal ascent profile for a desired orbit.

[Snark]

Moving to Gameplay Questions.

[badjass]

No, BUT. You can estimate it using the following mod and the following delta v map:

You can download the mod named "kerbal engineer" It's a VERY useful mod, i can't live without it anymore. Basically it calculates your delta v (and many other parameters) You will need to know the delta v that your 3 stages have combined.
Apart from that you will need to use a "ksp delta v map" such as this one: 

As you see on that map you will need approx 3400 delta v to get to low kerbin orbit (70 000m)
So if you create a craft with a certain payload and your 3 launch stages have a combined delta v of 3400 you should be able to get to a 70 000m orbit with that mass.

-To know the height of your orbit, you can estimate that using the delta v map again. As you can see you need around 800 delta v to get from low kerbin orbit to the mun (this means your apoapsis will be the height of the munar orbit) double that (I think) to get your craft in a circular orbit that is as high as the munar orbit (=1600 delta v)
So 800 delta v for example will get your orbit halfway to munar orbit IN TERMS OF DELTA V, in absolute terms this will be much lower because your height above kerbin will increase exponentailly.


So let's show an example: let's say your payload is 20 tons and the total delta v of your 3 other stages is 4400. This means you will use 3600 delta v to get to klo (low kerbin orbit) with 1000 delta v to spare to raise your orbit. using 500 for apoapsis raising and 500 for periapsis raising. I would estimate that your orbit would be around halfway to the mun.
Hope this is clear enough and useful enough.

[dinos55]

Thank you all!