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Rendez Vous up in the air: Shuttle meets ISS

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Rendez Vous up in the air: Shuttle meets ISS

Postby GinGin » Sun Feb 03, 2019 8:50 pm

Hello, a quick sum up of my last Rendez Vous with ISS in Orbit.
A really entertaining exercise, a thousands way to get there, some more efficient than others of course :).


Starting the scenario 3 degrees behind ISS, Shuttle Orbit is 175 x 173 Nm and ISS is orbiting at 190 Nm, a 100 kfeet above us.
Aim now is to reach it with the least relative speed possible to maintain a stable position behind her and eventually dock.


More information about the main tool used for rendez vous ( Spec 34 , using lambert equations ) here in the DPS pictionnary, page

https://www.nasa.gov/centers/johnson/pdf/359895main_DPS_G_K_7.pdf

And on Thosten's Tutorial

http://www.science-and-fiction.org/science/leo_targeting.html



Sum up of the rendez vous relative plot to help you to follow the different phases and burns that will occured

Image

A bit of explanations here, with the first screen after beginning.

T1 TIG: Time of when we want to make a burn
DX ( downrange), Dz ( height), DY ( cross range) will be the relative position of the Shuttle compared to ISS at Time of Ignition ( in kilo feet)
Dx dot etc will be the relative speed at TIG

The delta position is visible in real time with the canvas on the top right corner.
For example, now we are 1104 kfeet behind ISS, 103 feet right of her and 78 kfeet below her.


T2 TIG is the time where we want to be at a specific position relative to ISS
That position is define below with DX,y z.
For example, we can say to the computer : I want to do a burn at T1 07:00, I want to be in one hour T2 08:00 at ISS position, 10 kfeet below ( Dx=0, Dy=0, Dz=-10)

Then Item 28 compute T1 will try to give us a solution in LVLH frame that we can read at the top of the Spec 34 display ( DVX,y,z), automatically transfered to OPS 202 for the burn.

All the trick here is to phase well ( adjust the catch up rate to arrive slowly but surely to ISS) then use the Spec 34 tool to pinpoint the rendez vous with coherent and usable Delta V. ( If the computer gives us a solution of thousands of Feet/s, it is theorically possible, but impossible to perform with a shuttle and its engines)


Also, a good thing is to perform the burn close to an apogee or perigee.
Quite hard to estimate them as we have an almost circular orbit and Gravity perturbations will make oscillate our orbit.

Like in the screen, you can estimate the next perigee or apogee time ( every 45 mn ish, enter that time in T1 TIG, compute T1 and see what are the relative parameters at that time


Here I entered 08: 24, Dz dot is almost null ( 0.46), so we will reach the apogee around that time , 3000 feet below ISS oribt ( Dz=-3) and 800 kfeet behind ISS ( Dx= + 870)
I made a first prograde burn to raise the apogee just below ISS orbit.

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All the tools available, looks like a mini MCC

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Small corrections to be 2000 feet below at next apogee

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Next burn will be at that apogee. The aim here is to be at next apogee ( 90 mn) 250 kfeet behind ISS for the final Rendez Vous insertion and to be able to follow Nasa " way of rendez vous" quite close:

Image


We are at 870 kfeet of ISS, in one orbit we want to be at 250 kfeet. Roughly 600 kfeet to catch up.
So, at next perigee, we want to be around 870 - 300 = 570 kfeet of ISS
That is what we are planning here:

Burn at next apogee to be at next perigee ( 09:06) 560 kfeet behind ISS and 100 kfeet below.
We have to lower a bit our perigee to increase our catch up rate ( 80 kft before)



Image


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The result:

Next perigee at 09:06 , 580 kfeet behind ISS and 100 kfeet below ISS.
Good enough

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First Sunset

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Next burn at our incoming Perigee.
As we said, aim is to be at next apogee ( 09:54) 250 keet behind ISS and 1k feet below ( Dz=-1.2)


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and the result afer burn

Apogee 09:54, 250 kfeet ish behind and almost 1 kft below.
Good
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Alright, next part is the final rendez vous adjustement.
I tried to follow the Nasa stuff above, but that is theory.
Practically, I made a bit different has I messed up a bit some burns, but result will be the same :)


New aim now.
The TI ( Terminal Initiation) in one orbit. We want to be 50 kfeet behind ISS and 1.2 kf below

For that, I first target a position at next perigee ( 10:36), 160 kft behind and 30 kfeet below ( we reduce the catch up rate by increasing the perigee from 100 kft to 30 kft)
It gives us a relatively good solution ( 25 fps of total velocity, prograde and radial to reshape the orbit)
I made a small " mistake" here, I should have reduce even further the catch up rate with a perigee around 10 kfeet. But no big deal

This burn is called NC ( final phasing burn)

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Time for the NCC burn at next perigee ( NCC stands for corrective burn )
Perigee at 10 : 37, we want to be 42 mn ish later 50 kft behind and 1 k feet below as stated above

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Single engine burn for low velocity burns

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Results, our Ti burn coming alive at 11:19 in the relative position we wanted

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At TI, we planify a rendez vous in 40 mn, at noon just below ISS ( Dz=-1)

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We gonna go a bit above ISS to come back almost at the same relative speed ( low enough for RCS translation later)




Coming from above

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Small translations to null relative speed ( Dvx,y,z in the canvas rendez vous)

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R bar established and position keeping now

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As we are a bit lower than ISS, we will have constantly to adjust our position with small RCS inputs


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Ready for Docking phase, but in another session :)
GinGin
 
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Re: Rendez Vous up in the air: Shuttle meets ISS

Postby Thorsten » Mon Feb 04, 2019 9:39 am

How much extra velocity did you have to null when the transition to the detailed ISS model occurred?
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Re: Rendez Vous up in the air: Shuttle meets ISS

Postby GinGin » Mon Feb 04, 2019 10:08 am

On the Spec 34 , it was written around 30 fps on Dx to null.
I think I made some mid course correction to decrease it a bit.

It was around 15 fps Vx, around 1 fps for Vy, and 5 on Vz before I start playing with RCS at ISS transition.
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Re: Rendez Vous up in the air: Shuttle meets ISS

Postby Thorsten » Mon Feb 04, 2019 10:32 am

It's not finished, but I am entertaining the idea of a scheme where you load ISS as model earlier, and if you'd execute 'Compute T2' after that moment, that burn could null the numerical residues.

It's not perfect in any case, but at least it's workable... :?
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Re: Rendez Vous up in the air: Shuttle meets ISS

Postby GinGin » Mon Feb 04, 2019 10:31 pm

Sweet :)
I have the feeling that when iss transition happens , the trajectory is smoothen to fit with iss orbit .
It is just a gut feeling so highly non rational , normally there is no difference in state vector before and after the iss transition at 15 kfeet ?
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Re: Rendez Vous up in the air: Shuttle meets ISS

Postby Thorsten » Tue Feb 05, 2019 7:34 am

The state vector is transitioning continuous, but derivatives are not. So for the moment of initialization ISS always remains, but dependent on where it is relative to the true J3 orbit, it might start to move according to real dynamics.

The Shuttle state vector never notices anything, it is perfectly oblivious to the presence of ISS.
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Re: Rendez Vous up in the air: Shuttle meets ISS

Postby GinGin » Tue Feb 05, 2019 3:10 pm

Alright, so if we have a very steep interception course, we will just pass next to iss and diverge from her then?
As for spec 34 , it was used quite late in real. Up to the mid course correction 4 , 2000 feet ish from
I don’t know if it the same for you, but I can’t use spec 34 below 15 kfeet ?

I can recall that cw equations were never used in reality, it was always Lambert targeted solution.

By the way, very nice work on bpv for Iss , it works as intended now :)
Do you use orthogonal braking with the -Z axis pointing towards iss? I read about it but nerves tried
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Re: Rendez Vous up in the air: Shuttle meets ISS

Postby Thorsten » Tue Feb 05, 2019 5:45 pm

Alright, so if we have a very steep interception course, we will just pass next to iss and diverge from her then?


I would guess offhand the matching works best close to the equator and worse far away. The velocity mismatch is to first and second approximation independent of the Shuttle state vector.

I don’t know if it the same for you, but I can’t use spec 34 below 15 kfeet ?


It sort of works for me.

But you must realize by now that 'ISS as coordinate position' and 'ISS as moving 3d model' are internally two completely different entities which have nothing really to do with each other (in fact, they may even be at completely different position) and that hence tracking/targeting/querying the one reveals you nothing at all about the other, so unless a function is aready implemented for both and handed over properly, it does not work.

The price of using work in progress - it always starts for somewhere, never with everything.
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