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Gliding on Mars [RSS/RO]


Mr.  Sandman
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Hello , I'm trying to land very heavy payloads on Mars and for this I'm using a gliding technique (Pretty much like the Space Shuttle).

My first payload , the Mars Ascent Vehicle (201 tons) lands safely on Mars with Gliding+Parachute+Retro rockets

bU0WzJI.png

As you can see the CoT is below the CoM, which is below the CoL and I can tilt it at 40 degrees without problems (so it's pretty stable).

But for some reason..

GzsSdxq.png

This is the MDV.It flips in the air if it's more than 25 degrees inclined(I need around 30-40 to slow down)

In the Mars Descent Vehicle the CoT is above the CoM and CoL.I have no idea why this happens and how to fix it.Is this why it flips?Thanks for the answers.

Edited by Mr. Sandman
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2 hours ago, Streetwind said:

You might want to ask this in the FAR thread, since it's a modded aerodynamics issue.

Yeah I'll repost this there too, thanks for the tip

2 hours ago, Weywot8 said:

Just guessing but probably your front still isn't heavy enough vs the first one? Try shifting the front boosters further to the back/reduce the thrust on the boosters.

You can't reduce thrust in RSS/RO and the engines are not the thrust there (I think that's the aerodynamic thrust)

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27 minutes ago, Mr. Sandman said:

You can't reduce thrust in RSS/RO and the engines are not the thrust there (I think that's the aerodynamic thrust)

Didn't realise you can't set thrust values in VAB for RSS/RO. Thought that they were only un-throttle-able in-flight. Was under the impression that CoT (purple) took into account all forms of thrust on the ship, regardless of staging and throttle (but that's irrelevant here of course). IAlso, isn't Aerodynamic Thrust = Lift?

Anyhows, ignoring any retro-rockets with regards to the flipping problem (no pun intended), the differences in internal weight while retaining the same external shape means that the same amount of force is acting on less weight at the nose, and the CoM (yellow) appears to be shifted backwards/to the right in the second design, increasing the ability of any force at the nose to flip the ship. Also, your CoL(blue) looks like it is closer to the CoM in the second one, decreasing the ability of Lift to stabilise the ship.

TL;DR answer : The front end of your ship is still to light.  +/-  not enough lift at the back somehow

 

 

 

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2 minutes ago, Weywot8 said:

Didn't realise you can't set thrust values in VAB for RSS/RO. Thought that they were only un-throttle-able in-flight. Was under the impression that CoT (purple) took into account all forms of thrust on the ship, regardless of staging and throttle (but that's irrelevant here of course). IAlso, isn't Aerodynamic Thrust = Lift?

Anyhows, ignoring any retro-rockets with regards to the flipping problem (no pun intended), the differences in internal weight while retaining the same external shape means that the same amount of force is acting on less weight at the nose, and the CoM (yellow) appears to be shifted backwards/to the right in the second design, increasing the ability of any force at the nose to flip the ship. Also, your CoL(blue) looks like it is closer to the CoM in the second one, decreasing the ability of Lift to stabilise the ship.

TL;DR answer : The front end of your ship is still to light.  +/-  not enough lift at the back somehow

 

 

 

Booting up KSP now , I'll try make the nose heavier and see what I can do with the lift on the back.I say that the thrust doesn't come from the engines because when the fairing is gone , they point straight down , making the thrust vector perfectly alligned with the CoM and CoL.

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29 minutes ago, Weywot8 said:

Didn't realise you can't set thrust values in VAB for RSS/RO. Thought that they were only un-throttle-able in-flight. Was under the impression that CoT (purple) took into account all forms of thrust on the ship, regardless of staging and throttle (but that's irrelevant here of course). IAlso, isn't Aerodynamic Thrust = Lift?

Anyhows, ignoring any retro-rockets with regards to the flipping problem (no pun intended), the differences in internal weight while retaining the same external shape means that the same amount of force is acting on less weight at the nose, and the CoM (yellow) appears to be shifted backwards/to the right in the second design, increasing the ability of any force at the nose to flip the ship. Also, your CoL(blue) looks like it is closer to the CoM in the second one, decreasing the ability of Lift to stabilise the ship.

TL;DR answer : The front end of your ship is still to light.  +/-  not enough lift at the back somehow

 

 

 

Ok so I'm trying to bring the Thrust down and I noticed that the Thurst problem is the fairing itself (I moved the entire fairing up and down , the thrust moved with it)

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1 hour ago, Mr. Sandman said:

Ok so I'm trying to bring the Thrust down and I noticed that the Thurst problem is the fairing itself (I moved the entire fairing up and down , the thrust moved with it)

That's a first for me. Never seen that  happen before. The fairing 'thrust' should be considered as lift. You should probably head over to the FAR thread and let them scratch their heads over it. The underlying solution to your problem is still weight distribution and, if you are amenable to it, sticking a few SAS modules to help you keep your heading. 

Alternatively, the movable tail fins can be stuck to the back. Max out their deflection settings and activate the flaps for effectiveness in thin Duna air. 

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20 minutes ago, Weywot8 said:

That's a first for me. Never seen that  happen before. The fairing 'thrust' should be considered as lift. You should probably head over to the FAR thread and let them scratch their heads over it. The underlying solution to your problem is still weight distribution and, if you are amenable to it, sticking a few SAS modules to help you keep your heading. 

Alternatively, the movable tail fins can be stuck to the back. Max out their deflection settings and activate the flaps for effectiveness in thin Duna air. 

Sadly in RSS/RO Mars has a realistic atmosphere (600 pascals, so pretty much thin) and SAS wheels are useless if only not for small probes (again, realistic).I will try to distribute better theweight and see what happens.I solved the thrust problem, but the damn thing flips again.

Edited by Mr. Sandman
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4 minutes ago, Mr. Sandman said:

RSS/RO Mars has a realistic atmosphere (600 pascals, so pretty much thin)

Fins should help some - It's the same thin air rushing over the fairing producing the flipping force. So large enough moveable fins should also generate force of a similar magnitude, but smaller of course. Potentially enough for an extra 10-15 degrees of stable pitch maybe? And they help produce more drag for the aerobraking  too.

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Just now, Weywot8 said:

 

Fins should help some - It's the same thin air rushing over the fairing producing the flipping force. So large enough moveable fins should also generate force of a similar magnitude, but smaller of course. Potentially enough for an extra 10-15 degrees of stable pitch maybe? And they help produce more drag for the aerobraking  too.

Yeah that should help.After all I'm traveling in thin air at 3400 m/s.

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1 hour ago, Weywot8 said:

 

Fins should help some - It's the same thin air rushing over the fairing producing the flipping force. So large enough moveable fins should also generate force of a similar magnitude, but smaller of course. Potentially enough for an extra 10-15 degrees of stable pitch maybe? And they help produce more drag for the aerobraking  too.

Got it to fly!The problem was pretty simple : as you said , the nosecone was way too light and the CoM was way too down the vehicle.I just measured  the exact half of the vehicle (fairings included) and put the CoM a little bit down from there.This gives stability and the  torque to keep it inclined 30-40 degrees.

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