Posted by Rory on .
The Deorbit Burn
The Shuttle must reduce its velocity at a precalculated point in its orbit in order to return to Earth. In order to reduce the velocity and change the orbit of the Shuttle, a maneuver called the deorbit burn is performed. For this maneuver, the Shuttle is turned in a direction such that the Orbital Maneuvering System (OMS) nozzles point in the direction of the Shuttle's velocity back toward Earth. The OMS engines fire and give the Shuttle a velocity in the opposite direction, thus slowing the spacecraft.
The Shuttle must perform the deorbit burn to change its orbit so that the perigee, the point in the orbit closest to Earth, is inside of Earth's atmosphere. Deorbit maneuvers are done to lower the perigee of the orbit to 60 miles (or less). An altitude of 60 miles is important because this is where the orbiting spacecraft is recaptured by Earth’s gravity and reenters Earth’s atmosphere.
Calculate the minimum change in velocity (delta V or ∆V) required for the Space Shuttle to decrease its altitude to 60 miles if it’s orbiting with an apogee of 236 miles and a perigee of 205 miles above the surface of Earth.
Use the rule of thumb that below an altitude of 500 miles, for every 2 feet per second (ft/s) change in the orbiting space craft’s velocity its altitude will change by 1 mile.
and I have to answer in feet per second

Physics 
bobpursley,
change of altitude: 20560=145
rule of thumb: deletV=145mile*2/sec.mile
290ft/sec 
Physics 
Lafonda,
I see that someone else is in the VASTS program that didn't understand this problem...

Physics 
Lafonda,
But I might add that that would be the necessary velocity to do it, but the question asks for the CHANGE in velocity. The change in velocity would be 290ft/sec  (the initial) 2ft/sec, so 288 ft/sec.

Physics 
sam,
isn't that wrong because of the units?
you would have to convert 145 mi to 765,600 ft.
then divide by 2 (because its 2 ft/s) and get 382,800.
then subtract 2 ft/s to get the change, so 382,798.
is that right or did i do something wrong? 
Physics 
Brooke,
I too am in VASTS... that's what I thought. The units would have to be converted first to feet to get an answer in ft/sec.

Physics 
Levi,
I am also in the program, but I was wondering did you guys use the program to help you at all?

Physics 
Brooke,
My computer is really slow and I've only been able to get it to work once...

Physics 
Raequan,
Some hard stuff homie.

Physics 
Poop,
How can you find Delta V without having the initial Velocity?

Physics 
Rory,
Thank you all for your help, but I'm still confused, I converted miles to feet
145 miles= 765600 feet
then what? 
Physics 
Brooke,
If you divide 765600 by 2 ft/sec you're going to still have a huge number. I don't think that's right somehow. I'm going to get a teacher's help today.

Physics 
Anonymous,
if you take 1 and at the answer minus 1 it is correct

Physics 
Anonymous,
*add

Physics 
Anonymous,
or did i mean subtract?

Physics 
Anonymous,
ok im leaving now hahahahaha bye

Physics 
Brooke,
The first one I believe is right except for the units. Since 2 ft/sec is just rule of thumb, it wouldn't count towards the change. That's just the information given so that 2ft/sec = 1 mile. If you have 145 miles, for every mile you're going 2 ft/sec. So you would multiply 145 times 2.

Physics 
Levi B.,
what is the initial velocity? how can you find the Delta v without the initial

Physics 
Kayla,
I'm so glad I'm not the only one lost in VASTS. What a relief!

Physics 
T,
Eh, from what it looks like, it should just be the change in height that you need to do, which will be in units of miles right? and then you multiply that by the ratio of delta V to altitude change, which I believe is in units of (ft/s)/(change in miles)

Physics 
Joey,
Ughhh this is confusing.  In the VASTS program. I'm just glad I'm not the only one having trouble :P

Physics 
Muhammad,
I am also in the VASTS program.My data is different, but they have the same concept.

Physics 
Monika,
This is the same question that HAS is using. I'm happy that I'm not the only one who needs help with it.

Physics 
red,
u guys area bunch of cheaters

Physics 
Michael,
I'm struggling with this ... Can anyone help me?

Physics 
Anonymous,
I'm pretty sure it would be 290 feet per second

Physics 
Anonymous,
no 290 miles per second

Physics 
Jordan,
You would convert 290 miles into 1531200 ft which then the answer would be a531200 ft/sec

Physics 
Nobody,
I am in the WAS program and we have a similar question with different values. How I think it works is much like the first answer. You find the change in perigee needed. Its 205 right now but needs to be 60. So 20560 is 145 MILES. This is the distance that needs to be changed. Because the rule is 2 ft/s for every mile we do not need to change the units, we have 145 miles and need to change 2 ft/s for each of those miles. So we multiply by 2 and get 290 ft/s. The final thing to consider is that the change is slowing the craft ad not speeding it up so the delta V would be negative and not positive. Making your final answer 290 ft/s.
I hope this helps/is correct and doesn't confuse further. 
Physics 
anon,
i hate vasts

Physics 
Julie,
R = 6378000 meters
a₁ = R + (212 miles + 246 miles)/2
a₁ = 6746540 meters
a₂ = R + 60 miles
a₂ = 6474561 meters
GMₑ = 3.986e14 m³ sec⁻²
Energy per unit mass in present orbit.
E₁/m = −GMₑ/(2a₁)
E₁/m = −29541068 J
Energy per unit mass in destination orbit.
E₂/m = −GMₑ/(2a₂)
E₂/m = −30782010 J
ΔE/m = 1240942 J
½ (Δv)² = 1240942 J
Δv = 1575.4 m/s 
Physics 
Emmaleigh,
Julie, could you elaborate the last step to find the change in velocity? I followed until the very end where you got 1575.4.

Physics 
Emmaleigh,
Nevermind, I figured it out :)

Ass n Titties 
Ryan,
Vasts sucks balls

Physics 
mememachine,
lol ryan wtf, do you understand this cuz i dont, does the apogee even matter? cuz noone has mentioned it above