Question Need help making an efficient TMI burn with a low impulse craft

[Matrix Aran]

So I've been experimenting with a Frankensteinian monster of a stack that in theory has enough deltaV budget to make an Earth ejection burn towards Mars that I've plotted out in TransX. Now the problem is that the stack only has an initial acceleration of 1m/s, and so to make the 3500ish m/s burn, it takes nearly an hour. I could just eyeball the burn, but the margins on the booster's fuel are rather stretched as is, so I'd rather not cut into fuel that I'd rather have for MCC burns down the line.

I've tried fiddling around with IMFD to make the actual burn, but while all the documentation suggests that the MFD has multiple burn guidance modes, I seem to never be able to get orbit eject or the manual DeltaV tool to use anything but Realtime mode, which goes horribly wrong.

After some experimenting, I've found that starting the burn early with the prograde autopilot and then handing off the guidance to IMFD later on in the burn seems to make the realtime autopilot happier, but I still have the nagging feeling that this is either inducing a guidance error, or wasting too much fuel.

I've poked around the forums and there are a few tools out there for Ion drive vehicles, though they seem to deal with extremely low impulse vehicles, and I'm not sure if they'd be useful for this application, or just overkill.

Are there any other MFDs that might make this easier, have I missed something completely in IMFD, or is there something that I can do in general to ease the troubles I'm having with making this burn?

 

[dgatsoulis]

One of the ways to deal with this problem is to cut the burn into two or more periapsis kicks. (In this case I'd go for 7 x ~500 m/s). Use burntime mfd to see how much time one 500 m/s burn will take you and start the first burn at half that time before the trajectory periapsis. Repeat every time you get back at periapsis.

Sometime ago I had a stack that required 11 perigee kicks. Here are a couple of pics of all the burns combined:

Don't forget to begin the first burn a couple of days before the ejection time.

Hope this helps

 

[Matrix Aran]

Thank you, this sounds like a much more sensible approach to the problem, and it also now gives me the option to toy around with some staging between kicks. Any rules of thumb about how many days in advance I should start the sequence, or rather, how critical is my timing? Would TransX suffice for judging the trajectory periapsis?

 

[dgatsoulis]

Any rules of thumb about how many days in advance I should start the sequence, or rather, how critical is my timing?

Not too critical but it really depends on how many kicks you'll need. The first ones will only change your orbit slightly, but -usually- the one before the last gets you to an ApA of about 1/2 to 2/3 of a lunar one, so two to three days should be enough.

Would TransX suffice for judging the trajectory periapsis?

Yes. Let's say that a 500 m/s burn take you 500 seconds.
Start the burn 250 seconds before you reach "T to Pe" and end it 250 seconds after "T to Pe". For better results, when you get to the last burn, set it up as a manouevre.

 

[Cras]

dgatsoulis, how did you make those screen shots? Did you just mock them up afterward, or is that actually drawn within Orbiter?

 

[dgatsoulis]

dgatsoulis, how did you make those screen shots? Did you just mock them up afterward, or is that actually drawn within Orbiter?

Within Orbiter. I made a quicksave after each burn was finished and then made a scenario with 11 ships and copied the trajectory data from the quicksaves.

 

[Cras]

very cool

 

[insanity]

Within Orbiter. I made a quicksave after each burn was finished and then made a scenario with 11 ships and copied the trajectory data from the quicksaves.

That's a really good idea. I hadn't thought of that before!