Flight Question How to make a accurate burn in delta mission mod?

[NOMAD]

Once I played the delta mission mod and tried to launch the Phoenix to

Mars.When the third stage burn out,I noticed that the delta V doesn't close

to zero,the remaining delta V was about 190m/s,which means that I didn't

have enough fuel to make course correction burn! Therefore,when should I

stop the 2nd stage burning and start the 3rd stage burning? To make a

accurate burn ,what should I do?

Thanks.

 

[BrianJ]

Hi there,
from the included readme.txt....

LAUNCH NOTES
I usually use IMFD or TransX to set the time, direction and dV for the final escape burn.

The 3rd stage burn takes 87 seconds (using max. fuel) and should be started 50 seconds before the nominal start time given by IMFD or TransX.

Make sure you start the final 2nd stage burn in plenty of time to allow for this (about 250 seconds before nominal start time of burn).

I get better results by over-burning the 2nd stage by 20~30m/s and delaying the nominal start time of the 3rd stage burn until the required dV is about 5m/s less than the 3rd stage available dV (shown on the HUD display).

So, in a nutshell:
1. Stop the 2nd stage burn when the remaining dV required(for the burn) is equal to the dV available from the 3rd stage (shown on HUD)

2. If necessary, reset the burn start time on IMFD or TransX to 100 seconds in the future. Adjust attitude of 2nd stage and make any adjustment burn so that dV required is equal to dV of 3rd stage.

3. Make sure 2nd stage is pointed correctly for the burn. Jettison 3rd stage and start burn 50 seconds before nominal start time on IMFD/TransX



You need to bear in mind that:
a) The 3rd stage dV is fixed, so you must use the 2nd stage to set up the orbit such that the 3rd stage dV is exactly what is required for the remainder of the burn.

b) Both TransX and IMFD(Target Intercept/Orbit Eject) regard all maneuvers as impulsive (the burn is instantaneous and takes no time).

(Although IMFD will make corrections as the burn progresses, the DeltaMod 3rd stage has no attitude control and the dV is fixed, so it won't make any difference)

So, by starting the 3rd stage burn 50 seconds before the nominal start time you are minimising the error caused by the finite length of the burn.


Is that any help?

You should be able to get the dV error at the end of the burn down to < 20m/s

Let me know how you get on

Cheers,
Brian

 

[NOMAD]

Thanks BrianJ.With the help of the step,I finally made it to Mars!
I also notice that when the 3rd stage spin up and jettison,the delta V increase ,why?

 

[BrianJ]

Thanks BrianJ.With the help of the step,I finally made it to Mars!

Cool!

I also notice that when the 3rd stage spin up and jettison,the delta V increase ,why?

Yes, the remaining dV capability of the 2nd stage will increase when the 3rd stage is jettisoned - because the 2nd stage now has less mass to carry.

Cheers,
Brian

 

[NOMAD]

Thanks BrianJ.