I've been reading up about the rocket equation in Wikipedia, and I have a set of questions.
The first and more focused one:
How would the rocket equation look like for a design where some of the stages overlap? For example, the space shuttle uses the SRBs and the SSMEs together and then only SSMEs. So presumably the first stage is SSME + SRB and the second is SSME. What's the equation for this kind of design?
Wikipedia replies with an intriguing "more complicated", but I could have figured that one out on my own. It's tempting to say that one should simply add the exhaust velocities of the two components together and solve normally. However that can't be right... let's say you have three identical engines as the first and only stage... to solve the rocket equation you use the exhaust velocity of the engine, not three times the exhaust velocity (errr, right?). If I understand the equation correctly, this is because it deals with how much propellant you need to get a certain dV, and not how quickly you get to that dV (a thrust issue?).
Second - Let's assume we want to design a rocket, which is probably a true statement for many of us. We start of with a set of given specs. For example, it must be TSTO, use a Kerosene/LOX engine as the first stage and get 20 tons into LEO.
What would be the general procedure for that? Edit: I'm talking theoretical of course (not including aerodynamics, structural requirements etc.).
Presumably, you start with the rocket equation to get the required propellant mass in % total mass, then work back to find the total mass. Ok, so now you know how much propellent you need burned to give the spacecraft the required dV. Correct me if I'm wrong, but that still doesn't mean your bird will ever leave the pad, because it might be too heavy to lift off? Consider an Ion engine attempting to launch from earth - high Isp but low thrust. So the next step would be to some how calculate required thrust, but how and based on which requirements?
I would greatly appreciate any help, or advice as to where I might find the answer.
DanP
The first and more focused one:
How would the rocket equation look like for a design where some of the stages overlap? For example, the space shuttle uses the SRBs and the SSMEs together and then only SSMEs. So presumably the first stage is SSME + SRB and the second is SSME. What's the equation for this kind of design?
Wikipedia replies with an intriguing "more complicated", but I could have figured that one out on my own. It's tempting to say that one should simply add the exhaust velocities of the two components together and solve normally. However that can't be right... let's say you have three identical engines as the first and only stage... to solve the rocket equation you use the exhaust velocity of the engine, not three times the exhaust velocity (errr, right?). If I understand the equation correctly, this is because it deals with how much propellant you need to get a certain dV, and not how quickly you get to that dV (a thrust issue?).
Second - Let's assume we want to design a rocket, which is probably a true statement for many of us. We start of with a set of given specs. For example, it must be TSTO, use a Kerosene/LOX engine as the first stage and get 20 tons into LEO.
What would be the general procedure for that? Edit: I'm talking theoretical of course (not including aerodynamics, structural requirements etc.).
Presumably, you start with the rocket equation to get the required propellant mass in % total mass, then work back to find the total mass. Ok, so now you know how much propellent you need burned to give the spacecraft the required dV. Correct me if I'm wrong, but that still doesn't mean your bird will ever leave the pad, because it might be too heavy to lift off? Consider an Ion engine attempting to launch from earth - high Isp but low thrust. So the next step would be to some how calculate required thrust, but how and based on which requirements?
I would greatly appreciate any help, or advice as to where I might find the answer.
DanP