# How can I find the predicted approximate altitude of a rocket based on simply the propellant, specific impulse, and propellant mass?(solid propellant) [duplicate]

I am getting all the inputs I outlined above, including burn-time which I could not mention above(character limit), from a small scale motor test. I know there are specific specific impulses for each rocket fuel. I don't know if it is ok to use this specific impulse and all the other inputs from my test, but I am guessing it is not. If it is not, then can someone tell me how to set-up my test-fire to get around the same specific impulse in the same way they did.

First of all, is there a CFD that does all this by itself. Where I can just input solid propellant, specific impulse(if needed), burn-time at a smaller scale, and amount of propellant(mass) that I am using. If there is, than this will be very easy/simplified.

Anyways, if there is not than here is my problem. Well, it is in the title, but let me give you context on why I need this. Basically, I found a website where they gave pretty detailed and in depth equations on how to get the altitude(altitude at burnout plus altitude during coast) based on a bunch of factors, but three factors stood out to me. Motor thrust, motor total impulse, and motor burn-time. Here is the link for the website: https://www.rocketmime.com/rockets/rckt_eqn.html. What I want for my calculator I am making is to be able to input the stuff I listed before and get the motor thrust and motor total impulse. Also I am not quite sure why the person calculates motor burn-time using the crude equation of: $$t=\frac{I}{T}$$ where t is the burn-time, I is total impulse, and T is average motor thrust. I am not sure if it would be better to instead use the actual burn-time, and if it is then, if possible, it would be nice to also be able to get the burn-time from the info above.

Some ideas I had for doing this: I thought of multiplying specific impulse by mass of propellant to get the total impulse. Then since the total impulse is simply the sum of all thrust values times the time increment if we know the burn-time we can divide by the some random time-increment. Then we can divide by the burn-time divided by time-increment, so basically dividing by the number of thrust values in the equation. This should give the average thrust. I don't know if this is correct. However, the only problem in this theory is finding the burn-time, which I do not know how to do.

Also, does anyone know a CFD that can work with solid rocket motors after knowing motor thrust, motor total impulse, and motor burn-time. I just wanted to know, so that I can

• You can't do what you want knowing only those three things. You must know more about the mass of the vehicle, for example. Sep 4, 2023 at 9:48
• Have you looked at simulation tools such as OpenRocket? Sep 4, 2023 at 17:20
• Open rocket doesn't give you the altitude reached by your rocket given the things I listed. It needs thrust curve, burn-time, motor impulse, and average thrust. These are the things I need to calculate(I don't need thrust curve). My question is asking how I can calculate these things from the stuff listed in the question. I am basically getting all my information online and from small scale tests. I want to know all this information for any propellant mass I put in, so that I can simulate larger motors without needing to waste money by testing them. This is all for a rough estimation of flight Sep 4, 2023 at 18:43
• You absolutely do need things like thrust curve, and burn time; specific impulse and propellant mass are independent of those. Consider three rockets on an airless Earth, each massing 10kg, with 5 kg of fuel, Rocket A has 1 Isp 300 engine that outputs 50N thrust. Rocket B has two engines of that same design. Rocket C has three. Rocket A can't lift itself off the ground until it's burned almost all of its fuel. Rocket B, and C can. C will go higher than B, which will go higher than A. Sep 4, 2023 at 21:01
• Sep 5, 2023 at 0:18