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Old 05-07-2018, 11:24 AM   #16
BrianJ
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Quote:
Originally Posted by IronRain View Post
 Hi Brian,

I have some difficulties getting the spacecraft on a Mars trajectory. Everything seems to go well, but after jettison of the Star stage, it begins to wobble.
Hi,
I think it is due to the combination of how IMFD plans the burns (impulsive calculation, except IMFD Delta Velocity) and the fact that Star48 motor has no attitude control.

IMFD wants to correct the burn attitude at the end, but can't.

Try this:
Over-burn the 2nd stage by ~30m/s (if I remember correctly, might be more or less)
Reset the IMFD Eject time forward so that the dV required matches the dV available from the Star48. Hopefully that will be more than 1/2 the Star48 burn-time, in the future.
Re-align the 2nd stage to the correct attitude, jettison 2nd stage.
Start the Star48 burn 1/2 burn-time before IMFD Eject time.
This will minimise any corrections needed at the end of the burn.

Any better?

Cheers,
BrianJ

P.S. In your video, it's the IMFD required velocity "crosshairs" that move off axis, not the Star48 wobbling.

Last edited by BrianJ; 05-07-2018 at 05:14 PM.
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Old 05-08-2018, 01:11 PM   #17
IronRain
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Originally Posted by boogabooga View Post
 IronRain,

Don't use Vel. Frame. That assumes a fixed pitch and yaw relative to your velocity vector. When you want to point to a fixed position in space, such as during spin stabilization, try P30LVLH (I think that is what it's called) instead.
That seems to go a lot better. Still need some testing (my solution wasn't valid anymore, so I only did a quick run), but it looks promising.

Quote:
Originally Posted by BrianJ View Post
 Hi,
I think it is due to the combination of how IMFD plans the burns (impulsive calculation, except IMFD Delta Velocity) and the fact that Star48 motor has no attitude control.

IMFD wants to correct the burn attitude at the end, but can't.

Try this:
Over-burn the 2nd stage by ~30m/s (if I remember correctly, might be more or less)
Reset the IMFD Eject time forward so that the dV required matches the dV available from the Star48. Hopefully that will be more than 1/2 the Star48 burn-time, in the future.
Re-align the 2nd stage to the correct attitude, jettison 2nd stage.
Start the Star48 burn 1/2 burn-time before IMFD Eject time.
This will minimise any corrections needed at the end of the burn.

Any better?

Cheers,
BrianJ

P.S. In your video, it's the IMFD required velocity "crosshairs" that move off axis, not the Star48 wobbling.
I will also try this method later!
I didn't mean that the Star48 itself was wobbling (poor choice of words), but that it wasn't aligned with the crosshair anymore due jettison from the second stage for example. With P30LVLH it's staying rock-solid-centered!

I'm always amazed how much dV that little stages gives you.
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Old 05-08-2018, 04:45 PM   #18
BrianJ
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I just noticed a bug with the Phoenix lander fuel - fuel level not correctly set at vessel creation. Now fixed:
Update v.180508 now on OH.

Quote:
Originally Posted by boogabooga View Post
 Don't use Vel. Frame. That assumes a fixed pitch and yaw relative to your velocity vector. When you want to point to a fixed position in space, such as during spin stabilization, try P30LVLH (I think that is what it's called) instead.
I never knew P30 LVLH did that! Thank you!

Although, I still get similar results using either Vel.Frame or P30LVLH.
I still need to over-burn the 2nd stage, then re-plan the burn for the Star48 about 60s in the future, align 2nd stage, jettison and start the burn early. I end up with a ~10m/s error.

Is there a simpler/better way?

Cheers,
Brian
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Old 05-09-2018, 09:24 PM   #19
boogabooga
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Quote:
Originally Posted by BrianJ View Post
 Although, I still get similar results using either Vel.Frame or P30LVLH.
Make sure that you also do off-axis instead of realtime.
Vel. Frame and P30LVLH do about the same thing when you use realtime.

---------- Post added at 05:24 PM ---------- Previous post was at 04:11 AM ----------

Got the error down to 1.3 m/s:



Technique:

What BrianJ said, but...

- Use an off-axis burn in P30LVLH mode

- way over-burn the second stage by ~200 m/s (almost to exhaustion), not 30 m/s. Stop the auto burn. Go back to orbit eject and advance the eject time forward until the required deltaV matches what you have available. Should be like 90 sec in the future and not 30.

-Hit autoburn again so that IMFD will point you to the new correct attitude.

-10 seconds before the second burn is about to begin, hit G to spin up the Star48. Engine should fire just after it detaches, and the attitude should stay very accurate because IMFD was holding the attitude precisely during the spin-up.
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Old 05-09-2019, 10:30 AM   #20
4throck
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On the requests forum Mars Polar Landed was mentioned.
Could Phoenix be used to simulate that mission, just by changing the scenarios ?
Anything date specific for the EDL ?
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Old 05-09-2019, 12:34 PM   #21
BrianJ
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Quote:
Originally Posted by 4throck View Post
 On the requests forum Mars Polar Landed was mentioned.
Could Phoenix be used to simulate that mission, just by changing the scenarios ?
Anything date specific for the EDL ?
Hi,
I don't think there is anything date specific in the code etc.(only altitude specific, when you're into EDL at Mars).


Main differences to Phoenix I see are; the two Deep Space probes "Scott" and "Amundsen", solar arrays and high gain antenna.


The Deep Space probes probably wouldn't be too difficult to add.
A new static HGA would be minimal work.
Changing solar arrays would be quite a lot of work.


If anyone wants to have a go - please do!
Otherwise, I'll get around to it at some point ;-)


Cheers,
Brian
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