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Old 12-22-2018, 08:25 PM   #1
vchamp
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Default Updating TI Pegasus Ares

I'm currently updating Total Immersion vessels to make them compatible with Orbiter 2016 and D3D9Client. Pegasus Ares is the first to get the update.

I'm going to make this vessel as advanced as I can, instead of copying again the behavior of the original Pegasus Ares addon from francisdrake.

Due to time restrictions I don't know how long will it take to finish this project, so I decided to start this thread to post updates, questions and get a feedback.

If you want to try this update, first you have to install the original addon from francisdrake and the VC extension for Orbiter 2010. Links:
Pegasus Ares 01

Total Immersion - Pegasus Ares

Then overwrite PegasusAres.dll with the latest build from https://gitlab.com/vchamp/ti/blob/ma...egasusAres.dll.

What's new compared to the old addon:
- Works in Orbiter 2016
- Works with D3D9Client
- Removed UMMu because sadly it is not updated for 2016
- New touchdown points specs
- Added exhaust stream for atmospheric flight
- Altitude indicator follows terrain, i.e. ground relative
- Disengage thrust auto hold when 'Full thrust' or 'Kill thrust' is pressed
- (WIP) More complex aerodynamics, cross sections change according to the status of heat shield and parachute, variable drag element for parachute, airfoil for auto stabilization in direction of airflow

Planned next changes:
- More changes to aerodynamic to make it more realistic
- Update mass and other parameters depending on heat shield and parachute status
- Functioning heat shield (it's only a visual now) and damage model
- Indicator of terrain slope below vessel, maybe some instrument to find the best place to land (most horizontal and flat, in crater or outside), if Orbiter SDK allows it
- Instruments for choosing landing target and displaying relative position to it
- Autopilots

Last edited by vchamp; 12-23-2018 at 07:36 PM.
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Old 12-23-2018, 07:33 PM   #2
4throck
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It's better, but chute aerodynamics need work.
It's on your planned list, and it will make a difference.
Also, some external cameras would be great, specially downward facing!

But great job, keep working on it, I appreciate it!
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Old 01-10-2019, 10:03 AM   #3
vchamp
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Trying to figure out how to make the capsule orient its z axis along the airflow during reentry.

The first question is: Do real capsules have stable orientation so that their heat shield is perpendicular to the speed vector just by having that characteristic shape? Or this orientation must be kept forcibly with thrusters? Would real PAres rotate back to "z-axis along airflow" position if rotated with RCS given enough dynamic pressure? I assume it would because the air will press against the side walls.

So this orientation shouldn't be possible in a free fall:


The second question is how to simulate it. It seems that I need two things for this: create an airfoil with vertical lift and add a drag element, like I did with parachute. I'd like to understand how airfoils are related to drag elements and why adding only an airfoil or a drag element is not enough for my purpose.
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Old 01-10-2019, 10:13 AM   #4
Urwumpe
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Quote:
Originally Posted by vchamp View Post
 The first question is: Do real capsules have stable orientation so that their heat shield is perpendicular to the speed vector just by having that characteristic shape? Or this orientation must be kept forcibly with thrusters? Would real PAres rotate back to "z-axis along airflow" position if rotated with RCS given enough dynamic pressure? I assume it would because the air will press against the side walls.

Both. First of all, the coarse orientation is not kept by shape, but rather by moving the CoG away from the Center of Pressure, where the aerodynamic forces act: For a rotation symmetric capsule, this means the CoG is further outside and not on the centerline of the capsule. That is also why the lack of moon rocks in Apollo 13 was a small issue, the CoG needed to be trimmed, because it was near the limit.



Next, there are ALWAYS small oscillations, so Yaw and Pitch thrusters are always firing a bit to keep the orientation within limits. The CoG "trim" just ensures that less fuel is needed.



If you just want to do a ballistic reentry, remember also: The aerodynamic forces on a capsule are not turning you around into proper orientation automatically and instantly. If you fly nose first, like Soyuz 5 did, it would be stable in that orientation. It started to swing around once it oscillated far enough away from the other stable orientation that the correct orientation was reached. But the shape is also ensuring that the correct orientation has a bigger zone of stability.



The Space Shuttle for example would actually prefer to fly engines first during reentry, if the FCS would not prevent this by keeping the AOA right in a tiny zone where only small control inputs can keep the nose up at one end and prevent cartwheeling at the other end.
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Old 01-10-2019, 12:27 PM   #5
vchamp
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If I understand correctly capsules tend to orient themselves to the needed orientation, however not because of their shapes but because of their CoG position. It seems that this is not simulated in Orbiter and the only way to achieve this is by changing aerodynamic properties. Please correct me if I am wrong.

I think the right way is to change cm parameter in the AirfoilCoeffFunc depending on aoa and M.

Drag element could also be used for self-orientation in airflow but it would add a constant drag which is not desired.
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Old 01-10-2019, 12:55 PM   #6
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Quote:
Originally Posted by vchamp View Post
 If I understand correctly capsules tend to orient themselves to the needed orientation, however not because of their shapes but because of their CoG position. It seems that this is not simulated in Orbiter and the only way to achieve this is by changing aerodynamic properties. Please correct me if I am wrong.

I think the right way is to change cm parameter in the AirfoilCoeffFunc depending on aoa and M.
It is. And you write already how. cm describes essentially the effect of the airfoil / capsule itself, if it would have no mass.

And then, you could also simply place the airfoil reference to a different location. That way, you could start with a symmetric capsule lift function (f(aoa) = -f(-aoa)) and reuse it for different capsule or different mass distributions (For example, think of the Curiosity rover, which ejected metal weights during reentry for different trim angles)
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