Flight Question TransX Burn Accuracy

[Tniffoc]

Oh my god! I just realized that you are using 1/10 time slowdown. That is not good because there is a lag time involved in processing the inputs. It is not necessary to go to 1/10th time, in fact I never use it for burns. It is really important the you FEATHER the acceleration with the throttle, not slow time down!

I didn't use 1/10th time for anything during the burn, I used on my 4th (or so) attempt at burning in my Eject values. I just used it about 30s prior to the burn to line up the green x as best I could. It took WAY longer for me to do it than it did for flytandem to do it, and he was on 1x time. Even after this seemingly very precise alignment of the green cross, it drifts around during the burn, whereas it stays completely still in flytandem's video.

EDIT: As for the feathering thing, as I approach the end of the burn, i.e. dV < 1.5, I use the translation thrusters to provide for that extra degree of accuracy. (Oh, the irony ):blush:

 

[Mister Mxyzptlk]

Once X is centered do you use translation to keep it there?

 

[Hlynkacg]

First off, welcome to the forums :thumbup:

I was in you shoes about 2 months ago so here are my "Lessons Learned" from my first interplanetary flights: (Yes, I was actually taking notes)

Be gentle, get lined up well before you make the burn and be sure to kill you relative motion before starting the burn. As with docking the crosshairs will start to drift as you get closer to your target velocity, simply throttle back the engines and adjust gently. Use your linear thrusters (translation RCS mode) for fine adjustments +/-.

Timing, try to perform your burn as you cross your target's orbital plane (ascending or descending node) If your ECC is close to 0 (as it should be) this will help more than waiting for PeR.

Don't Rush, When it comes to orbital mechanics I have found it that it is fairly easy to correct for an overshoot or late burn so take your time and err accordingly. Better to be on course and 10 seconds late, than to be on time and off course. 0.5 degrees over the course of several million kilometers adds up fast.

I also second everything Mister Mxyzptlk said.

 

[Tniffoc]

Once X is centered do you use translation to keep it there?

Not until near the end of the burn, I kept trying to chase the green x around with rotation thrusters. Near the end of the burn, I did use the translation thrusters.

Is it just me or did it not look like flytandem was using any translation thrust to keep it aligned until the very end?

Be gentle, get lined up well before you make the burn and be sure to kill you relative motion before starting the burn. As with docking the crosshairs will start to drift as you get closer to your target velocity, simply throttle back the engines and adjust gently. Use your linear thrusters (translation RCS mode) for fine adjustments +/-.

What exactly do you mean by kill relative motion before the burn. Motion relative to what? I've just been using the kill rotation autopilot after aligning the x, and then burning.

 

[Mister Mxyzptlk]

Yes, don't use rotation, use translation throughout the burn. Use fine tuned translation(ctrl along with trans.) to keep the X centered If it is really close to center. Also the burn doesn't have to be continuous. Towards the end turn thrust off, recenter your X and then give it another shot. as long as you don't take over five minutes to finish your burn you will be fine.

 

[Tniffoc]

Alright! I was able to get dV down to a reasonable amount by using translations to keep the green x aligned during the burn. Unfortunately, my orbit still appears to be quite different from that of the hypothetical burn.

Could this much of a difference be caused by a 3 second late burn?

 

[Tex]

Actually I use rotation through out the burn to keep the green X centered. Using the Ctrl key in conjunction with your rotational keys will allow you to make small corrections. Pressing 5 on the numpad to kill thrust frequently as you keep that green x in the center helps to stop some of that drift. It mostly just takes practice, but I don't switch to translation until the end to simply clean up the burn.

 

[Hlynkacg]

What exactly do you mean by kill relative motion before the burn. Motion relative to what? I've just been using the kill rotation autopilot after aligning the x, and then burning.

Assuming that you've zeroed out your RInc that is more than enough for a simple moon shot. However once you start straying out into the wider solar system you'll have to worry about conflicting orbital planes. A stable orbit around the Earth with 0 Ecc is all well and good but geting to jupiter means going through all the calculations to clear the earth-orbit then doing them all again relative to the the Sun, and finding the time/place where the two are aligned.

The reason that people reccomend IMFD in place of transX is that it is much better at handling "Off-Plane" orbits.

...my orbit still appears to be quite different from that of the hypothetical burn.

Could this much of a difference be caused by a 3 second late burn?

I don't think so, but transX has issues with multiple gravity fields so turn of HTO and check your work. If your CPA is under 100k you'll be fine. Besides, what do you think the mid-course correction is for?

 

[Mister Mxyzptlk]

I don't know about IMFD and offplane orbits but I do know that using TransX in conjunction with Align Plane MFD allows me change planes at the node with no problems at all.

 

[Hlynkacg]

I don't know about IMFD and offplane orbits but I do know that using TransX in conjunction with Align Plane MFD allows me change planes at the node with no problems at all.

Agreed, for beginners going to the moon or simple planetary transfers TransX and the Align Plane MFD are more than adequate. That said, having a "multi-plane" function built in helps a lot when dealing with multiple bodies. For instance, moon-hopping around Jupiter and Saturn.