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#1200 by Zoe on 07 Aug, 2008 22:19
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Why does Discovery get a 2 engine TAL call to a specific site "Discovery Houston two engine Istres/Moron/Zaragoza" and Atlantis only get a generic "Atlantis Houston two engine TAL" call?
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#1201 by Jim on 07 Aug, 2008 22:45
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Why does Discovery get a 2 engine TAL call to a specific site "Discovery Houston two engine Istres/Moron/Zaragoza" and only gets a generic "Atlantis Houston two engine TAL" call?
It has nothing to do with the orbiters. It has to do with the specific mission. -
#1202 by NASAAN101 on 08 Aug, 2008 02:46
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Hay guys,
the TAL or the Transoceanic Abort Landing site. How do they pick them. and during what part of the launch can they do it..
Nikki -
#1203 by Jim on 08 Aug, 2008 05:03
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Hay guys,
the TAL or the Transoceanic Abort Landing site. How do they pick them. and during what part of the launch can they do it..
Nikki
They pick site that are near the flight path of the orbiter. When setting up a mission, they determine at what energy levels (velocity and Altitude) they can reach certain sites, if different numbers of engine shutdown.
The part of the launch trajectory varies for each mission.
here is some info:
http://spaceflight1.nasa.gov/shuttle/reference/shutref/sts/aborts/index.html#intact -
#1204 by Zoe on 08 Aug, 2008 05:26
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They pick site that are near the flight path of the orbiter. When setting up a mission, they determine at what energy levels (velocity and Altitude) they can reach certain sites, if different numbers of engine shutdown.
Is that why they sometimes have a different site in the single engine call? I have never heard "single engine Moron" if Moron is the site in the two engine call I later hear them get told to select Istres or Zaragoza. -
#1205 by pr1268 on 08 Aug, 2008 19:26
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Orbital mechanics wants to drive the trajectory into an ellipse.
I'm curious about that: Assuming the Shuttle is 100 m distant from the ISS, i.e., in a 100 m higher orbit, then this means that the Shuttle has to travel 2*PI*Δh (628 m) further each orbit than the ISS (also assuming both Shuttle and ISS are in a perfectly circular orbit).
So, my intuitive thinking says that the Shuttle would travel more slowly (Kepler's laws) than the ISS if it weren't using the RCS thrusters (other than to move it into the higher orbit to begin with).
But, how does the elliptical motion occur? Is this a motion of the Shuttle and ISS relative to each other? Do the Shuttle and ISS exert an infinitesimally small (but measurable) gravitational force on each other? Links/pointers to useful info related to this would be appreciated. Thanks! -
#1206 by Jim on 08 Aug, 2008 19:28
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Is this a motion of the Shuttle and ISS relative to each other? Do the Shuttle and ISS exert an infinitesimally small (but measurable) gravitational force on each other?
Yes, no -
#1207 by Jorge on 08 Aug, 2008 20:38
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Orbital mechanics wants to drive the trajectory into an ellipse.
I'm curious about that: Assuming the Shuttle is 100 m distant from the ISS, i.e., in a 100 m higher orbit, then this means that the Shuttle has to travel 2*PI*Δh (628 m) further each orbit than the ISS (also assuming both Shuttle and ISS are in a perfectly circular orbit).
So, my intuitive thinking says that the Shuttle would travel more slowly (Kepler's laws) than the ISS if it weren't using the RCS thrusters (other than to move it into the higher orbit to begin with).
Wrong initial conditions. The flyaround starts with the orbiter along the ISS velocity vector, not above or below ISS, and the initial velocity to start the flyaround is in the radial-up (away from Earth) direction.QuoteBut, how does the elliptical motion occur? Is this a motion of the Shuttle and ISS relative to each other? Do the Shuttle and ISS exert an infinitesimally small (but measurable) gravitational force on each other? Links/pointers to useful info related to this would be appreciated. Thanks!
The mutual gravitation of the two spacecraft is negligible. The apparent "force" causing the orbiter to want to move in an ellipse is an artifact of examining the relative motion in a rotating frame.
If you look at the motion in the inertial (non-rotating) frame, ISS is in a circular orbit and the orbiter is in a slightly elliptical equiperiod orbit. Since the orbiter's orbit has the same period, it would return to the same spot relative to ISS after each orbit.
If you solve the equations of motion in an ISS-centered rotating frame, you wind up with what are called Hill's equations or the Clohessy-Wiltshire equations:
http://academy.gsfc.nasa.gov/2003/interns/collange/module/Hill.htm
This example uses a different frame than JSC uses (RSW vs. LVLH) but will suffice for our purposes.
The flyaround is in-plane, so ignore the Z equation.
We are interested in unperturbed (i.e. where the orbiter would go if the crew took hands off the stick) so set fx and fy to zero and solve for x-double-dot and y-double-dot.
You wind up with an x-term (the "Rbar term"), an x-dot term and a y-dot term. The x-dot and y-dot terms cause the elliptical motion. They're essentially the Coriolis terms.
The analytical solution is also on that page. Take the x(t) and y(t) equations and solve them for these initial conditions:
omega = 1.15e-3 (orbital rate for ISS, mrad/s)
x0 = 0
y0 = 125 m
xdot0 = 0.3 m/s
ydot0 = 0
and you'll see the ellipse that would result if the crew went hands-off after starting the flyaround. -
#1208 by pr1268 on 09 Aug, 2008 22:08
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Thank you to both Jim and Jorge for the responses and the link to the Hill's Equations page. Physics is fun!!! (Especially orbital mechanics, even though I don't understand most of it.)
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#1209 by Danny Dot on 10 Aug, 2008 02:07
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Hay guys,
the TAL or the Transoceanic Abort Landing site. How do they pick them. and during what part of the launch can they do it..
Nikki
The one engine out TAL requires good weather for a launch to occur. If no TAL sites that support one engine out performance have good weather the launch is scrubbed.
There is no weather requirement for 2 engine out TAL sites and they are typically closer to the ground track. After the shuttle reaches single engine out capability to orbit, the 2 engine out TAL site is selected.
Danny -
#1210 by NASAAN101 on 11 Aug, 2008 13:32
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Hay guys,
I have a Question fir you guys. What does this mean? The shuttle's mobile launch platform was "hard down" at the pad?
I had a friend ask me ans i wasn't to sure..
NIKKI
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#1211 by Jim on 11 Aug, 2008 16:11
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Hay guys,
I have a Question fir you guys. What does this mean? The shuttle's mobile launch platform was "hard down" at the pad?
I had a friend ask me ans i wasn't to sure..
NIKKI
It means it was lowered on to the pedestals at the pad. This just differentiates the difference between arriving at the pad vs being secured on the pedestals -
#1212 by tva on 12 Aug, 2008 08:31
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Different descent profile ?
I wonder if a different (theoretically) way to slow down the orbiter would be possible hence diminish the thermal load on the TPS.
The OMS tanks hold 0,5 mT proppellant. The max. ascent payload is approx. 20mT. How big velocity reduction could be achieved by turning this 20 mT into OMS oxidizer and propellant ?
Will it help at all ? If you add together the mass of oxidizer and propellant in the ET and the two SRB's, you get almost 1900 mT. That is hundred times more !
Does it mean that you needed several hundred tons of (weigthless) propellant to make any significant difference ?
I have the impression that the peak thermal load on the TPS appears at en altitude of 75-55 km ASL when the orbiter still travelling nearly 22000 km/h.
Wish we could reduce it to Mach 4-5 while the orbiter is still above EI !
Is it just wishfull thinking without any basis in physics ?
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#1213 by kimmern123 on 12 Aug, 2008 08:53
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I'm definitely no expert, but I'd think slowing the orbiter down that much would make the entry a LOT steeper and would be even more stressful to the orbiter than the current descent profile.
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#1214 by Jim on 12 Aug, 2008 10:50
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Different descent profile ?
I wonder if a different (theoretically) way to slow down the orbiter would be possible hence diminish the thermal load on the TPS.
You could land a vehicle on earth, just like a lunar lander. But it would take as much propellant as was used to launch it. (Actually it would need more propellant for launch since it has to haul up all the propellant for landing)
There is a thermal load on the TPS for a reason. It is an elegant way of eliminating the need for propellant. -
#1215 by elmarko on 12 Aug, 2008 11:56
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Yeah, that's not going to happen when you have to bring up all of that fuel in the first place at launch, is it?
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#1216 by Jorge on 12 Aug, 2008 22:34
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Different descent profile ?
I wonder if a different (theoretically) way to slow down the orbiter would be possible hence diminish the thermal load on the TPS.
The OMS tanks hold 0,5 mT proppellant. The max. ascent payload is approx. 20mT. How big velocity reduction could be achieved by turning this 20 mT into OMS oxidizer and propellant ?
Will it help at all ? If you add together the mass of oxidizer and propellant in the ET and the two SRB's, you get almost 1900 mT. That is hundred times more !
Does it mean that you needed several hundred tons of (weigthless) propellant to make any significant difference ?
I have the impression that the peak thermal load on the TPS appears at en altitude of 75-55 km ASL when the orbiter still travelling nearly 22000 km/h.
Wish we could reduce it to Mach 4-5 while the orbiter is still above EI !
Is it just wishfull thinking without any basis in physics ?
It is wishful thinking.
I don't know where you got the 0.5 mT figure for the OMS propellants but it is wrong. A full OMS load is 25,000 lbm, or a little over 11 mT. Using the entire payload capacity for additional OMS propellant would only reduce velocity by about 2,000 fps or so. Compared to the initial velocity of 25,000 fps it will not make much of a difference in entry heating.
And what use is a shuttle flight with zero payload anyway? -
#1217 by elmarko on 13 Aug, 2008 06:38
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And what use is a shuttle flight with zero payload anyway?
Yeah, that as well, I don't know why I didn't think of that obvious point. -
#1218 by tva on 13 Aug, 2008 07:13
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And what use is a shuttle flight with zero payload anyway?
I still whimpering about the loss of Columbia, you know... -
#1219 by rdale on 13 Aug, 2008 08:39
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And what use is a shuttle flight with zero payload anyway?
I still whimpering about the loss of Columbia, you know...
Columbia HAD a payload...
