PDA

View Full Version : Why doesn't the Space Shuttle tip over?



Jens
2006-Aug-08, 02:42 AM
This is something I've wondered about from time to time, but when rockets are launched, they are actually travelling slowly in the first few seconds, and I think it takes some time before the aerodynamic effects kick in. I can't remember exactly, but I remember reading that the SS accelerates from 0 to like 70 mph in 4 or 5 seconds. But still, 4 or 5 seconds seems like a long time. Is it simply that the acceleration is fast enough considering the enormous momentum it has against tipping over, or are the engines rapidly toggled to counteract any motion?

When I was a kid, I always thought there was a kind of rail or something on the launch pad, that made sure the rocket stood up straight while it was going too slowly to be able to use fins. But clearly with the SS that isn't happening.

Nowhere Man
2006-Aug-08, 03:37 AM
I'm pretty sure it's this: The rocket bells themselves are steerable. If you watch a close-up video of the SSMEs from the time they light up through liftoff, you can see them waving about as if they'd broken loose. The Saturn V engines were giballed as well.

What I want to know is, what's the shuttle sitting on before it takes off?

Fred

Metricyard
2006-Aug-08, 03:43 AM
What I want to know is, what's the shuttle sitting on before it takes off?

Fred

I'm currious myself. I remember that the Saturn V's had huge clamps holding them down until the engines were up to speed, but what was the rocket itself sitting on? And does the shuttle have the same sort of clamping to make sure the motor is up full bore before taking off?

So many questions.

01101001
2006-Aug-08, 03:50 AM
What I want to know is, what's the shuttle sitting on before it takes off?
Mobile Launcher Platform (http://en.wikipedia.org/wiki/Mobile_Launcher_Platform), MLP.

NASA Facts: MLP (http://www-pao.ksc.nasa.gov/kscpao/nasafact/count3teaf.htm#mlp)


Eight attach posts, four on the aft skirt of each SRB, support and hold the Space Shuttle on the Mobile Launcher Platform. These posts fit on counterpart posts located in the Platform's two solid rocket booster support wells. The space vehicle disconnects from the Platform by explosive nuts which release the giant studs linking the solid rocket attach posts with the Platform support posts.

NASA: Launch Complex 39-A & 39-B (http://science.ksc.nasa.gov/facilities/lc39a.html)


There are 6 permanent and 4 extensible pedestals that are used to support the MLP at the pad.

Jens
2006-Aug-08, 04:01 AM
I wrote "toggle" in my previous message. I see the correct term is "gimbal". :doh:

And a related question is about the "roll program" is that all done by gimballing engines, or do they use the wings to do some of that?

PhantomWolf
2006-Aug-08, 04:20 AM
Also remember that the engines thrust is pointing through the CoM at all times, so its not going to fall over.

01101001
2006-Aug-08, 04:35 AM
For launch pictures, see new topic Shuttle Launch Pics (http://www.bautforum.com/showthread.php?t=45395) yielding pictures of liftoff (http://www.ktb.net/%7Ebillmeco/sts114A.html), especially this one (http://www.ktb.net/%7Ebillmeco/STS114R.jpg). I think I see at least two of the Shuttle's 3 main engines aimed slightly away from the boosters, perhaps counteracting the torque.

The SRBs are gimbaled, too, and some other recent thread, citing Wikipedia, explained that the SRBs are steerable enough to get a vertical launch even if the Shuttle main engines fail.

Gillianren
2006-Aug-08, 05:59 AM
Also remember that the engines thrust is pointing through the CoM at all times, so its not going to fall over.

Would that be "Center of Mass"?

ToSeek
2006-Aug-08, 02:25 PM
Would that be "Center of Mass"?

Yes.

Nowhere Man
2006-Aug-08, 02:29 PM
Mobile Launcher Platform (http://en.wikipedia.org/wiki/Mobile_Launcher_Platform), MLP.

NASA Facts: MLP (http://www-pao.ksc.nasa.gov/kscpao/nasafact/count3teaf.htm#mlp)


Eight attach posts, four on the aft skirt of each SRB, support and hold the Space Shuttle on the Mobile Launcher Platform. These posts fit on counterpart posts located in the Platform's two solid rocket booster support wells. The space vehicle disconnects from the Platform by explosive nuts which release the giant studs linking the solid rocket attach posts with the Platform support posts.
Thank you, zero one one zero one zero zero one. I was specifically wondering what's holding the mass of the "stack." Sounds like shuttle attached to ET, ET attached to SRBs, SRBs sitting on the platform. Yow.

Fred

NEOWatcher
2006-Aug-08, 04:13 PM
... Sounds like shuttle attached to ET, ET attached to SRBs, SRBs sitting on the platform. Yow.
I believe the wings are in there too.
if you follow 01's* Shuttle Launch Pics (http://www.bautforum.com/showthread.php?t=45395) to page 2. The picture titled "Closeup of Main Engine start!" shows the mount for the wing close up. (it's that thing that looks like an oversized door just below the "Discovery" insignia)
And you can see them in the pre-launch photos


*I hope calling you 01 is not getting to personal.

Ilya
2006-Aug-08, 04:57 PM
*I hope calling you 01 is not getting to personal.
Only if your name is Bender.

Bob
2006-Aug-08, 05:40 PM
Also remember that the engines thrust is pointing through the CoM at all times, so its not going to fall over.

It's much more complicated than that. The condition you describe is an unstable equilibrium, like a pencil balanced on its point. A stable condition exists if the center of mass is forward of the center of pressure; then aerodynamic pressure applies a restoring force if the angle of attack increases. That's why some rockets have fins - to move the center of pressure aft. Others point out that the rocket nozzles are gimbaled to keep the rocket on course and with the right attitude.

mugaliens
2006-Aug-08, 10:24 PM
In English: While the clamps are in place, which occurs between main engine start and booster ignition, it's a condition of static equilibrium, where the entire shuttle assembley, including the shuttle, the tank, and the boosters, are held by the locks and the gantry. The moment the boosters fire, however, it's a go, and the situation remains static for about half a second until the booster pressure builds to near maximum. After that the clamps let loose and it's dynamic equilibrium with the shuttle engines providing gimballed stability throughout the ascention profile, including after booster jettison.

cjl
2006-Aug-09, 02:21 AM
It's much more complicated than that. The condition you describe is an unstable equilibrium, like a pencil balanced on its point. A stable condition exists if the center of mass is forward of the center of pressure; then aerodynamic pressure applies a restoring force if the angle of attack increases. That's why some rockets have fins - to move the center of pressure aft. Others point out that the rocket nozzles are gimbaled to keep the rocket on course and with the right attitude.

While this is true, it is largely irrelevant for large vehicles. Most, if not all of the large vehicles used for space launch are aerodynamically unstable. This is not an issue because of the gimballed nozzles.
Smaller missiles and sounding rockets usually have fins to make them stable to avoid the extra weight and complexity of a gimballed system.

Bob
2006-Aug-09, 03:17 PM
I think you repeated what I said. Thank you.

Maksutov
2006-Aug-09, 03:34 PM
I believe the wings are in there too.
if you follow 01's* Shuttle Launch Pics (http://www.bautforum.com/showthread.php?t=45395) to page 2. The picture titled "Closeup of Main Engine start!" shows the mount for the wing close up. (it's that thing that looks like an oversized door just below the "Discovery" insignia)
And you can see them in the pre-launch photos [edit].The SRBs support the entire weight of the ET and orbiter (as well as the SRBs themselves). Go here for more information. (http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html)

gwiz
2006-Aug-09, 03:59 PM
I believe the wings are in there too.
if you follow 01's* Shuttle Launch Pics (http://www.bautforum.com/showthread.php?t=45395) to page 2. The picture titled "Closeup of Main Engine start!" shows the mount for the wing close up. (it's that thing that looks like an oversized door just below the "Discovery" insignia)
And you can see them in the pre-launch photos
Those "oversize doors" are not structural supports to the wing, they're for the cable and pipe connections going into the orbiter rear fuselage, which retract back into the "doors" at launch.

NEOWatcher
2006-Aug-09, 04:10 PM
Those "oversize doors" are not structural supports to the wing, they're for the cable and pipe connections going into the orbiter rear fuselage, which retract back into the "doors" at launch.
So; I am assuming those are the "Two tail service masts" Mentioned here (http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/centers.html#sts-ksc-mlp) ?

gwiz
2006-Aug-09, 04:27 PM
That's them.

Click Ticker
2006-Aug-09, 05:18 PM
No mention of the Gyroscope?

I got a toy gyroscope at the Jackson Space Center in Michigan when I was a kid. I was told they kept rockets balanced. I was a kid though, I likely misunderstood what they were for. Made sense to me, as that toy was certainly balanced well when it was spinning.

Bob
2006-Aug-09, 05:36 PM
Gyroscopes are key elements in the inertial guidance systems for rockets and aircraft. They are also extensively used for properly orienting satellites and space probes.

cjl
2006-Aug-09, 11:56 PM
No mention of the Gyroscope?

I got a toy gyroscope at the Jackson Space Center in Michigan when I was a kid. I was told they kept rockets balanced. I was a kid though, I likely misunderstood what they were for. Made sense to me, as that toy was certainly balanced well when it was spinning.

The gyroscopes are not what keep the whole rocket straight however. The gyroscopes are used as a method of determining orientation (the gyroscope stays steady while the spacecraft rotates around it, so it is in a constant orientation). The gimballed nozzles are what are used to correct for any deviation.

joema
2006-Aug-11, 07:06 PM
The SRBs are gimbaled, too, and some other recent thread, citing Wikipedia, explained that the SRBs are steerable enough to get a vertical launch even if the Shuttle main engines fail.
Just to be clear (I wrote the original statement), it's conceivable but not certain the SRBs alone have enough steering authority. Also there's some remaining doubt the orbiter/ET attach struts are strong enough, but allegedly they've been beefed up for this contingency. I wish we could get more definitive information from a NASA ascent procedures specialist.

Re the original question, the shuttle doesn't tip over due to the offset SSME thrust because it goes through the center of mass. However if you look closely at a launch from a right side camera (off the right wing), you'll see the entire stack move laterally to the right (belly wise) as it ascends after liftoff. This is caused by the offset SSME thrust.

NoAstronomer
2006-Aug-11, 07:45 PM
(I believe I read this in an article from a past issue of Air & Space, but I'm prepared to be wrong about that)

As Bob said, it's extremely complicated. One of the parts of the article described the process of releasing the shuttle from it's support and anchors on the launch platform. Mostly that's accomplished through the use of explosive bolts. The pattern of release is specifically designed to induce a slight rocking motion to the shuttle assembly as the SSMEs and SRBs are building thrust. At the point that the shuttle separates from the launch platform the rocking motion is tipping the assembly away from the gantry.

ASEI
2006-Aug-12, 08:06 PM
When the engines are ramping up during the initial burn, there is some uneven flow separation in the nozzles, causing massive side forces on the rocket. The rocket has to be clamped in place until you get zero flow separation, otherwise it could knock into supports or flip over.

neilzero
2006-Aug-14, 03:17 AM
The shuttle is below stalling speed for perhaps the first ten seconds, so it could start to tip over, etc unless the engines were gimbled to counter act the tendency to tip. Neil

joema
2006-Aug-14, 12:26 PM
The shuttle is below stalling speed for perhaps the first ten seconds, so it could start to tip over, etc unless the engines were gimbled to counter act the tendency to tip. Neil
Stalling speed is an aerodynamic term, which doesn't really apply in this case. E.g, the exact same situation would exist if a shuttle-type vehicle took off in a vacuum, where aerodynamics don't exist.

Upon takeoff, the shuttle is held upright purely by engine thrust, the thrust vector of which goes through the vehicle center of mass.

Both main engines (SSMEs) and solid rocket boosters (SRBs) are gimbaled to compensate for wind, fuel slosh, residual imbalance, or anything else pushing the vehicle off course or off attitude in flight.

The SSMEs are mounted on the side of the external tank, so it's reasonable to ask why they don't rotate the entire vehicle in a nose-down direction. If you look closely you'll see the SSMEs are canted at an angle and the thrust line angles outward, not straight down. If you follow that line inward, it goes through the vehicle center of mass. That's why the SSMEs don't rotate the vehicle on takeoff.

More info:
http://www.clavius.org/techlmstab.html
http://en.wikipedia.org/wiki/Center_of_mass