Physics
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

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

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.

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? 
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.

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

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

Some hard stuff homie.

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

Thank you all for your help, but I'm still confused, I converted miles to feet
145 miles= 765600 feet
then what? 
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.

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

*add

or did i mean subtract?

ok im leaving now hahahahaha bye

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.

what is the initial velocity? how can you find the Delta v without the initial

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

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)

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

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

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

u guys area bunch of cheaters

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

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

no 290 miles per second

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

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. 
i hate vasts

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 
Julie, could you elaborate the last step to find the change in velocity? I followed until the very end where you got 1575.4.

Nevermind, I figured it out :)

Vasts sucks balls

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