question abt re-entry (1 Viewer)

[shkspeare]

ok for low orbit satellites it increases in velocity as it goes lower and lower because of the forces of friction caused by the molcules of the atmosphere and the atmosphere becomes more dense as it gets lower too so it will accelerate rite?

i dont understand why for re-entry dotpoint, why does jacaranda say that a spacecraft will "decelerate" as it re-enters

the heading is "decelerating g forces"

and it says "greater angles of re-entry mean greater rates of deceleration which means a faster rate of heat buidup as kinetic energy is convereted"

why would it decelerate!? greater rates of deceleration means it will become slower at a faster rate as time goes on rite?

blarrrr LOST

 

[Avicenna]

orbital velocity= sqrt.GM/r

now as the satellite comes closer and closer to the Earth's surface and thus centre its orbital velocity increases, but due to the increased air friction (as you said greater air density) it will experience drag. Now during this process some of its KE is converted into heat energy due to friction with air molecules. So what happens when it comes in at a large angle of re-entry. The rate at which it slows down increses so it is decreasing in speed at very large rates, thus the KE it had is now being converted into heat energy very quickly

 

[shkspeare]

quoting from 3 pages earlier in jacaranda

"as a satellite slows, it loses altitude and begins a spiral downwards. As it descends, it encounters higher density air and higher drag, speeding up the process"

and using the orbital velocity = sqrtt GM/r

say M = 4, r = 2 (of course this isnt real values)

so as it begins it's re-entry phase, it will have a orbital velocity of sqrt 2

getting close to the earth, say when r = 1, the orbital velocity qill b sqrt 4 = 2

as u can see 2 > sqrt 2.... thus it must be increasing in speed? tell me where i'm wrong >.<

 

[Avicenna]

Originally posted by shkspeare
quoting from 3 pages earlier in jacaranda

"as a satellite slows, it loses altitude and begins a spiral downwards. As it descends, it encounters higher density air and higher drag, speeding up the process"

and using the orbital velocity = sqrtt GM/r

say M = 4, r = 2 (of course this isnt real values)

so as it begins it's re-entry phase, it will have a orbital velocity of sqrt 2

getting close to the earth, say when r = 1, the orbital velocity qill b sqrt 4 = 2

as u can see 2 > sqrt 2.... thus it must be increasing in speed? tell me where i'm wrong >.<

I agree with you that the velosity increases at low altitudes but thats only when it is in a stable orbit. Remeber that you're satellite is orbiting the Earth in a spiral fashion an its altitude is constantly changing, so it is no longer in a stable orbit and is accelerating towards the Earth's surface with a new velocity which is unlike its CIRCULAR orbital velocity.

i hope this can clarify your doubts

 

[shkspeare]

ohh

how come when a spacecraft re-enters they sometimes "retrofire" or aim their rockets ahead of them so they 'slow down' instead of speed up

wots the point of that if its already supposedly decelerating as it goes lower?

 

[Dash]

Spacecrafts retrofire their rockets before entering the Earth's atmosphere to reduce G forces...
By slowing down the spacecraft before it starts to decelerate I imaging that you would be reducing the G forces that the crew would experience...
But I don't quite remember this being in the syllabus, but consider the case below as the basis for that answer.
Say if one person was in a car travelling 100km/h and another person in a car travelling 50km/h.
Both cars had to come to a complete stop within a fixed distance.
In the spacecraft's case, this would be the distance from re-entry point to the surface of the Earth.
Well if we consider the car senario, then its obvious that the person in the car travelling at 100km/h will have more force acting on them.
So yeh, guess you can make the link now...
As I said before, I don't remember this being in the syllabus.
I just remember the jacaranda mentioning the retrofiring process.
Check with your teacher

 

[shkspeare]

mm i dont quite get the car link >.<

 

[shkspeare]

blar now im wondering whether or not satellites in low earth orbit actually decelerate instead of "accelerate" ...........

 

[Dash]

LOL you don't get the car link?!?!
Okay... um I'll tryin explain it again
Lets start from the top...
Stay we have two identical cars: Car A and Car B.
Car A is traveling at 100km/h and Car B is traveling at 60km/h.
Once the driver in each car hits the brakes, they have to break within 30 metres.
When a brakes a passenger experiences a forward force from the momentum of the car...
The passenger in Car A would experience a significant forward force opposed to the passenger in Car B.
So linking this back to the spacecraft scenario...
If we enter the atmosphere at a slower speed by retro firing their spacecrafts,
then the crew will have less G forces acting upon due to the example above...
Notice that I included at minimal stopping distance... (done in my previous post)
The spacecraft also has to come to a stop within a certain distance (re-entry point to Earth's surface)
The atmosphere will provide the resistance and act as the braking force in the car scenario.
So simply, spacecrafts retrofire their rockets to slow down the craft to reduce G forces...
And if you don't understand anything I've said, jus remember the line above this one