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Question about MSL Sky Crane

by CitronBleu
Tags: crane
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CitronBleu
#1
Dec14-12, 07:51 PM
P: 13
Hello and thank you for allowing me to post a question.

I am not very familiar with systems engineering and have a question about the frame joints connecting the main body of the MSL (Mars Science Laboratory) powered descent stage with its 8 Aerojet MR-80B 800 lbs thrusters (1).









I am not sure what the spacecraft chassis or joints are made of.

The descent stage weighs 2,400 kg with an extra 390 Kg of propellant. The rover itself weighs 900 Kg. This comes out to a total of 3,600 Kg, or 3 1/2 tons (2). Gravity on Mars is 0.38 that of Earth (3). When the powered descent engine starts, the Sky Crane and rover are both plunging towards the Martian surface at breakneck velocity 125 m/s (4).

What type of forces are acting upon these frame joints when the descent vehicle separates from the back shell and the thrusters are at full throttle? Are they solid enough to maintain their structural integrity under such forces?


(1) http://matthewwturner.com/uah/IPT200...7-5481-979.pdf
(2) http://en.wikipedia.org/wiki/Mars_Science_Laboratory
(3) http://en.wikipedia.org/wiki/Mars
(4) http://trs-new.jpl.nasa.gov/dspace/b.../1/10-1775.pdf
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Aero51
#2
Dec14-12, 08:38 PM
P: 546
What type of forces are acting upon these frame joints when the descent vehicle separates from the back shell and the thrusters are at full throttle? Are they solid enough to maintain their structural integrity under such forces?
During the decent the primary loads will be thermal. When it activates its rockets the the majority of the loads will be bending and torsion. The second question is kind of silly. That's like asking if a bridge designed to hold people will maintain its structural integrity if people are held. Unless you meant "Are they designed the support the craft when it lands?", in which case I think so.
CitronBleu
#3
Dec14-12, 09:13 PM
P: 13
Quote Quote by Aero51 View Post
During the decent the primary loads will be thermal. When it activates its rockets the the majority of the loads will be bending and torsion. The second question is kind of silly. That's like asking if a bridge designed to hold people will maintain its structural integrity if people are held. Unless you meant "Are they designed the support the craft when it lands?", in which case I think so.
Thank you for anwering Aero51.

My question does not relate to what happened, but what would happen considering the mechanics involved.

With all the parameters and considering the width and size of the frames holding the thrusters, are they what an engineer would build to withstand the weight of the whole vehicle plunging to Mars, including the force required to slow the vehicle down?

AlephZero
#4
Dec14-12, 11:04 PM
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Question about MSL Sky Crane

I don't understand what yuo are asking here.

Are you suggesting the engineers who designed this didn't know what they were doing, but just got lucky? Or what?

Actually, those structures look quite similar to how jet engines are attached to aircraft. In particualar, they are designed so there will be NO thermal loads transferred through them as the varioos parts expand and contract. The joints can rotate a bit to compensate for thermal expansion and contraction.
CitronBleu
#5
Dec15-12, 08:02 AM
P: 13
Hello AlephZero,

Thank you for your reply. I am not suggesting anything. I am fascinated by the MSL mission and in particular the powered descent stage. I want to understand how it functions and when I looked at the joints connecting the thrusters with the main body I asked myself how they could sustain the forces involved. The joints seem excessively thin. I was trying to figure out how much energy is applied to those joints when the vehicle is airborne and is slowing down, and which type of material is needed to resist the resulting bend and torque from the combined force of the thrusters, vehicle mass, and deceleration.

Thank you for your explanation of thermal load and pointing me to the structures in jet engines. Do you have any link that could clearly help show this similarity you describe and help me understand how the system works?

Thank you so much and please forgive me for my ignorance!

CB
CitronBleu
#6
Dec15-12, 10:21 PM
P: 13
Anyone?

Is there not one person who can help me calculate the forces involved?

I am not sure how to calculate this, please help...

Vehicle mass is 3,600 Kg.

Vehicle is falling at speed 125m/s at altitude 1620 m.

Mars pull of gravity is 3.71 m/sē


http://trs-new.jpl.nasa.gov/dspace/b.../1/10-1775.pdf

What is the force acting on the frame joints connecting the main body of the vehicle with the thrusters?
Travis_King
#7
Dec17-12, 11:13 AM
P: 851
If you've got the acceleration due to mars's gravity, the decelleration due to the thrusters, the number of thrusters, and the mass of the craft. You can pretty easily come up with a ballpark figure for the load on one of those joint structures.
CitronBleu
#8
Dec17-12, 12:10 PM
P: 13
Quote Quote by Travis_King View Post
If you've got the acceleration due to mars's gravity, the decelleration due to the thrusters, the number of thrusters, and the mass of the craft. You can pretty easily come up with a ballpark figure for the load on one of those joint structures.
Thank you Travis_King!

Mass of craft = 3,600 kg x 0.38 (gravity on Mars) = 1368 (Kg?)
Mars acceleration due to gravity = 3.71 m/sē
8 thrusters 800 lbf of thrust each / two thrusters per frame / four frames total
The craft is diving at 125 m / s at 1620 m altitude and slows down to 20 m /s at 105 m altitude.

According to the above diagram the deceleration from 125m/s to 20m/s is achieved in 35.00 -3.40 = 31.60 s and over 1620 - 105 = 1,515 m

Do you have some type of formula I can use?


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