Science Forums 2

[CraigD]

Anyone who would like to can run XGRAVSIM4.

In order to do so, you’ll need a MUMPS language interpreter. A free, non-expiring, single user copy of the one I use can be had at Free Caché Downloads. The vendor, Intersystems, requires you register an email address with them to get it, but in my experience won’t spam you unless you request it, or give the address to any other company.

Pay no attention to the name “Cache”. Intersystems makes a great effort to hide the fact that Cache is, in fact, a (much extended) implementation of the MUMPS language, but it’s one of the best.

Once installed, you need only click on the icon it installs in your system tray, or follow the start menu path to its “Terminal” application, and paste the following into it

 f  r R q:'(R)  s I=((R,";",(R,";")),":") i (I) s @I=R

(making sure to end with exactly 1 Enter keystroke) followed by the “MUMPS code for XGRAVSIM4” in post #6, followed by a blank line (the Enter key), followed by

x XGRAVSIM4

. For documentation, enter ? at the XGRAVSIM4> prompt.

You can then paste any of the command lines in my examples above into it to run it, or make up your own.

To do anything but manually adjust the acceleration of bodies, you’ll need .X property code, which you must know MUMPS to write. Post #6 has a little .X program that allows you to set a specific impulse of the motor (ship.X(“SI”)) the empty (no propellant) mass of the ship (ship.X(“M0”)) and a propellant flow rate (ship.X("Mf")) which can be pasted into the sim to see how simple rockets in a vacuum would behave. Note that in my example, I forgot to multiply the SI of 250 seconds by 9.8 m/s/s, so my 1000 kg rocket is under-powered by a factor of nearly 10 vs. a real solid fuel rocket – though it still flys

A nice feature of XGRAVSIM4 is that you can enter $.all to output the “all simulation data” property, which is all the properties necessary to run the sim. These can easily be copied and pasted into text files, then pasted into the simulator with slight changes to run another simulation – just note that all output starts with a # comment character, which must be removed before pasting it actually does anything, and any line wrapping must be fixed manually (or avoided by capturing the text using File, Log in Terminal, or something similar)