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Old 08-16-2018, 04:25 PM   #16
rmcallaway
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Quote:
Originally Posted by martins View Post
 One of the reason Orbiter adds a separation velocity upon undocking is to avoid unintentional immediate redocking. Currently, docking occurs if the docking reference points are below a threshold distance, and the separation is closing.
Makes total sense. I just wonder if it would be simple to allow an addon to control this velocity exclusively.

For NASSP there would never be a zero velocity undocking (save for the "soft undock" in which the vehicles would revert from hard dock to "attached" I believe which could pose problems of it's own)

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Originally Posted by martins View Post
 Note that for attachments (as opposed to docking connections), connections are never established automatically on proximity, so there is a provision for user-defined separation velocity.
Yeah we use this (I believe) for our soft dock condition.

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Originally Posted by martins View Post
 I guess I could add an extended undocking function that allows to specify the separation velocity, but this would always have to be (sufficiently) positive.
This could potentially be useful. What would be a "sufficient" velocity? For a hard dock to undock, I was looking at no smaller than 0.5 feet/sec (probe extension imparted velocity and zero tunnel pressure)

EDIT: I could also ignore reattaching I believe under certain conditions, that would be more realistic on our end (ie only re attaching if the capture latches were engaged)

Last edited by rmcallaway; 08-16-2018 at 04:44 PM.
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Old 08-16-2018, 05:07 PM   #17
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I guess this feature request from 2015 is relevant to this thread: https://www.orbiter-forum.com/project.php?issueid=1204
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Old 08-16-2018, 06:04 PM   #18
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A little side question here:

As a read through this thread I tend to think of a Force parameter that could be optionally added to the Vessel::Undock() call.

I then thought of the Space Shuttle undocking and wondered, how did the real Shuttle handle the problem with the offset center of gravity (of the shuttle) during undocking?
If the springs would just apply a force straight into the docking direction I would assume that a little rotational component would be incorporated in that action (as the force vector does not go strait through the COG...)

Anyone having infos on that?
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Old 08-16-2018, 06:08 PM   #19
rmcallaway
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Quote:
Originally Posted by kuddel View Post
 A little side question here:

As a read through this thread I tend to think of a Force parameter that could be optionally added to the Vessel::Undock() call.

I then thought of the Space Shuttle undocking and wondered, how did the real Shuttle handle the problem with the offset center of gravity (of the shuttle) during undocking?
If the springs would just apply a force straight into the docking direction I would assume that a little rotational component would be incorporated in that action (as the force vector does not go strait through the COG...)

Anyone having infos on that?
The AddForce is what we plan on using for this, actually. We are crunching the numbers to add it to the undocking code. And through some experimenting, it seems that Orbiter always imparts 0.2fps, divided between the spacecraft proportional to the masses.

And I honestly don't know how the shuttle worked with this, but it could be similar to the Apollo soft undock I mentioned earlier, where it undocks to a point where it doesnt impart a velocity, so the RCS can take care of separation? You have me curious now too! Research time!
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Old 08-16-2018, 06:50 PM   #20
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Quote:
Originally Posted by rmcallaway View Post
 And I honestly don't know how the shuttle worked with this, but it could be similar to the Apollo soft undock I mentioned earlier, where it undocks to a point where it doesnt impart a velocity, so the RCS can take care of separation? You have me curious now too! Research time!
The ODS/PMA was held together by a series of hooks (ODS) and latches (PMA). When the Undock button was pushed the hooks and latches would begin to drive open, a process that took around 2 minutes. Once the hooks and latches were fully open a set of compressed springs in the ODS would gently push the two vehicles apart. These springs had been compressed since hard-dock occurred. Soft-dock was contact and capture of the PMA with the extended ODS docking ring.

The attitude control systems on both spacecraft were inhibited for the undocking leaving both in free drift to prevent any recontacts. Once the orbiter had passed the 50' mark, the orbiter RCS was re-enabled.

---------- Post added at 08:50 PM ---------- Previous post was at 08:38 PM ----------

This schematic shows the layout of the ODS: https://www.dropbox.com/s/7oiabunx6n...ayout.jpg?dl=0

Last edited by DaveS; 08-16-2018 at 06:42 PM.
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Old 08-16-2018, 07:07 PM   #21
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Thanks Dave,
that was my knowledge so far. But still I am wondering how much rotational component that introduced.
As the shuttle is in free drift, no RCS is used (the shouldn't so close to the station, anyhow).
Let me research whether the shuttle had flywheels for attitude control, I'm not sure but I don't think they had.
That would be a "plume free" option
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Old 08-16-2018, 07:13 PM   #22
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Quote:
Originally Posted by kuddel View Post
 But still I am wondering how much rotational component that introduced.

Not that much - its 700 lb(f) by all springs, on a heavy orbiter and the ODS is much closer to the COG than the RCS. Also not sure how long the spring force acts on both spacecraft.
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Old 08-16-2018, 08:01 PM   #23
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I found something for an "automatic undocking without crew etc."

Here they seem to use the up-firing RCS tail jets:


---------- Post added at 20:01 ---------- Previous post was at 19:21 ----------

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Originally Posted by Urwumpe View Post
 and the ODS is much closer to the COG than the RCS
Yes the ODS being positioned in the payload bay moves it closer to the CG (of the Shuttle), but still what about situations where there's a payload (e.g. MLPM) in the bay at undocking.

That surely also adds a lot of inertia, so rotation would not be "so fast".

[1] The MLPM on STS-131 for example had a down-mass of 9242 kg!

Last edited by kuddel; 08-16-2018 at 08:07 PM.
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Old 08-16-2018, 08:02 PM   #24
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Quote:
Originally Posted by Urwumpe View Post
 Not that much - its 700 lb(f) by all springs, on a heavy orbiter and the ODS is much closer to the COG than the RCS. Also not sure how long the spring force acts on both spacecraft.
According to the APAS Reference Guide, the springs provide a force between 551 lbf and 595 lbf.
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Old 08-16-2018, 08:05 PM   #25
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@DaveS: SCOM says 700 lb in sum of all springs.



Quote:
Originally Posted by kuddel View Post
 I found something for an "automatic undocking without crew etc."

Yes, also note the undocking attitude of ISS and Shuttle to understand what happens.
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Old 08-16-2018, 08:08 PM   #26
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Quote:
Originally Posted by kuddel View Post
 Yes the ODS being positioned in the payload bay moves it closer to the CG (of the Shuttle), but still what about situations where there's a payload (e.g. MLPM) in the bay at undocking.

[1] The MLPM on STS-131 for example had a down-mass of 9242 kg!
That doesn't change the c.g much. The orbiter is designed to have the c.g well aft even without a payload, thanks to lead ballasting. That's why the heaviest payloads are always mounted in the aft-most sections of the payload bay, to control the c.g.
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Old 08-16-2018, 08:48 PM   #27
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This might be a very stupid comment but what about deleting the docking port, apply the force and then after a bit of time recreating it? Using DelDock should cause no deltaV at all between the vehicles.
I don t know if you can have issues deleting docking ports with vessels docked to it, but I think I tested it a while ago for other purposes and it worked.
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Old 08-17-2018, 06:10 PM   #28
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Originally Posted by fred18 View Post
 This might be a very stupid comment but what about deleting the docking port, apply the force and then after a bit of time recreating it? Using DelDock should cause no deltaV at all between the vehicles.
I don t know if you can have issues deleting docking ports with vessels docked to it, but I think I tested it a while ago for other purposes and it worked.
I was thinking about that too, but then I read the documentation on Vessel::Undock()
Code:
/**
 * \brief Delete a previously defined docking port.
 * \param hDock dock handle
 * \return \e false indicates failure (invalid dock handle)
 * \note Any object docked at the port will be undocked before the
 *   docking port is deleted.
 * \sa CreateDock, ClearDockDefinitions, DockCount, Dock, Undock
 */
bool DelDock (DOCKHANDLE hDock) const;
If "will be undocked" means "the usual way", this might not help.
But one can try and see what happens

---------- Post added 17-08-18 at 15:12 ---------- Previous post was 16-08-18 at 21:49 ----------

It seems that Vessel::Undock() will not apply any further forces!
Code:
v = vessel.get_focusinterface()
dh = v:get_dockhandle(0)
v::del_dock(dh)
This keeps the two vessels at their position without applying any further forces.


So something like this (untested) might be another thin to try
Code:
v = vessel.get_focusinterface()
dh = v:get_dockhandle(0)
-- Get docking parameter (for apply force and later re-enabling)
pos,dir,rot = v:get_dockparams(dh)

-- Delete the docking port and apply some force where it used to be...
v::del_dock(dh)
v::add_force(dir,pos) -- this needs to be checked/changed/calculated ;)

-- Re create the docking port
-- This should be done after some time and/or distance...
proc.wait_simdt(7)
newdh = V:create_dock(pos,dir,rot)


---------- Post added at 18:10 ---------- Previous post was at 15:12 ----------

Note: That script is not realistic! As it only applies a force to one vessel (self)
To make it better one needs to get the parameters of the other vessel as well and apply forces into both docking ports in opposite directions.
The possibly different accelerations of the two vessels (due to possible different masses, CG etc...) should then be handled by Orbiter core, I guess.
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Old 08-17-2018, 10:04 PM   #29
kuddel
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Default Lua Script Undock Test Scenario

I couldn't resist doing a little proof of concept script here.
I had to push the shuttle with 200 kN however...is the dir vector really a unit-vector?

Doing the simple (not realistic as mentioned before) thing was quite easy in Lua I must admit.

Edit: Attachment removed (see Post#31)

Last edited by kuddel; 08-20-2018 at 01:01 PM. Reason: Attachment removed
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Old 08-20-2018, 11:53 AM   #30
rmcallaway
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Regarding Orbiter itself, from experimenting it looks like it defaults to adding 2m/s at undocking, and this is divided among the spacecraft based on mass. Not a super realistic approach but of course it prevents undocking.

I think what we can do in NASSP is just simply compensate for this known docking velocity in our code and add what we want as the velocity. Since it is constant, it should be able to be removed constantly. Does this theory sound like a safe work around?

EDIT: Units

Last edited by rmcallaway; 08-20-2018 at 12:11 PM.
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