Saturday, December 10, 2005

Rocket Sizing Software:

The following is a description and some code for a piece of rocket sizing software that I'm programming. It takes a bunch of input variables that the user supplies and optimizes the dv distribution, and calculates the stage masses for a multi-stage rocket.

Have you ever wondered how large a rocket would have to be to launch a specific payload? What the effects of staging are? My C program answers some of these questions by providing a preliminary mass budget for a multi-stage rocket.

pmf (propellant mass fraction) = m_inert/m_fuel. A rocket is primarily a giant fuel tank. The amount of structure is roughly proportional to the amount of fuel. The propellant mass fraction is a number (usually between 0.7 and 0.9 for liquid rockets) that defines the ratio of fuel to (fuel+structure) mass. It neglects the payload. Any lighter than 0.9 and your rocket usually doesn't have enough structure to hold the fuel tank together, or withstand the forces of launch. In space vehicles may be able to get away with higher pmfs, but not launch vehicles. The pmf coupled with the limited Isp of chemical propulsion is also the primary reason why we don't do Single Stage to Orbit. Try it for a pmf of 0.8 and an Isp of 420, and see what you get.

The Isp (specific impulse) is the fuel efficiency of the engines for a rocket stage. Typical numbers are around 340 for kerosene/LOX engines and 420-440 for hydrogen/oxygen engines.

Any extra inert mass is included in the payload of each stage. Things like extra systems or structure can be directly added in to ensure that there is enough of a mass budget to attach them to the rocket.

The dv is the total change in velocity that the rocket can make. Missions in space are usually defined by the amount of dv necessary to perform them. Launching cargo into LEO typically requires 9000+ m/sec of dv. Going to the moon requires another 3200 m/sec or so. Landing on it or takeing off requires 1600 or so m/sec dv. Anyway, all of this stuff adds for your total mission.

My program was compiled as a command line executable with DevCPP. I don't know how to do makefiles. I'm an engineer, not a unix wizard.


Blogger Bart R said...

where can i donwload your program?

Wednesday, May 29, 2013 1:51:00 PM  

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