Calculating Burn Time

[Trueborn]

I've been working on my Maneuver Node gauge for SteamGauges, and I'm having some trouble calculating how much time the burn will take. If anyone has any insights why neither the full Rocket Equation version or a simple f=m*a calculation aren't giving me good results, I would appreciate it.

deltaV = myNode.DeltaV.magnitude;                               //The burn's ÃŽâ€V
deltaVRem = myNode.GetBurnVector(FlightGlobals.ActiveVessel.orbit).magnitude;   //Remaining ÃŽâ€V in the burn
res r = calculateThrust(FlightGlobals.ActiveVessel);            //Actually calculates thrust, mass, and Isp
double mass = r.mass / Math.Pow(Math.E, (deltaVRem / r.isp));   //Mass after burn
double rate = r.thrust / (r.isp*9.81);                          //Mass flow rate
double burnTime = (r.mass - mass)/rate;                         //Mass to burn over rate should give time, but doesn't
//Try the f = m*a method for time computation...
double t2 = deltaVRem / (r.thrust / ((r.mass+mass)/2));         //Use average mass to get closer?

I'm using a custom function that just runs through each part and get's its .mass and .GetResourceMass(), and then thrust and Isp for active engines. The latter two are spot on, but my mass is always slightly off from, say, Kerbal Engineer. Thanks for the help guys!

[AncientAstronaut]

You need to know how much delta-v per minute or per second your spacecraft supplies. Then you need to know how much Delta-V the burn requires and then you'll know how many seconds or minutes the burn will take.

[Cheebsta]

I tried figuring up some total delta V examples on a page on the wiki the other day. I could only get it to work out when I used 9.816 * isp, and reduced ln(tot/dry) to 3 places past the decimal as well, otherwise I was coming up 1 or 2 off.

Not really sure how that translates to your code though as I am still a noob at this part, lol.

And yes, I know you want time, but just wondering if it's something similar to this since this was so touchy about decimal places.

Edited July 22, 2013 by Cheebsta

[EndlessWaves]

Any part with physicalSignificance set to NONE is massless for this sort of calculation so it's mass value should be ignored.

[Trueborn]

Thanks, the physicalSignificance got me the right mass. That certainly helps some.

Playing around with 9.816 vs 9.82 vs 9.81 I can get kind of close, but nothing there seems to be spot on.

I'll try rounding some values I guess...