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How does a rocket stay stable during liftoff?

by Davidthefat
Tags: liftoff, rocket, stable, stay
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Davidthefat
#1
Jan1-12, 08:44 PM
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Because torque is force times distance, wouldn't it only take a relatively small force to make a rocket unstable if the force was applied near the base or the top? How is it kept stable? I just saw SpaceX's future plans to make a fully recyclable launch vehicle; I think it is far fetched TBH.
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rcgldr
#2
Jan1-12, 09:07 PM
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Some rockets have movable nozzles that allow thrust vectoring. Others use small rocket engines that generate thrust pependicular to the main rocket. I don't know if any rocket combined both methods.
torquil
#3
Jan2-12, 06:14 AM
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Don't they also start to spin slowly as soon as possible after launch, to increase stability?

rcgldr
#4
Jan2-12, 06:21 AM
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How does a rocket stay stable during liftoff?

Quote Quote by torquil View Post
Don't they also start to spin slowly as soon as possible after launch, to increase stability?
You're probably thinking of the space shuttle which rolled upside down during launch (there were also pitch and yaw adjustments) for aerodynamic load issues on the shuttle, (it also allowed the ground to be used for references by the on board crew, but I'm not sure how useful that was). I'm not sure why the shuttle wasn't simply attached to a launch tower so the roll manuever wasn't required, unless the setup was historical (not an issue for previous rockets), and too costly to redo. Most rockets don't spin during launch.
D H
#5
Jan2-12, 07:26 AM
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I'll answer the question raised in the title first, "How does a rocket stay stable during liftoff?"

A rocket has passive elements (features inherent in the design) and active elements (controls) that maintain stability. The key passive element is that the center of pressure needs to be below the center of mass. Active elements include gimbaled thrusters and vernier thrusters used to control the orientation of the vehicle.

Quote Quote by Davidthefat View Post
I just saw SpaceX's future plans to make a fully recyclable launch vehicle; I think it is far fetched TBH.
Other than the reusable aspect, SpaceX's design is rather old school. Thrust from the bottom, payload at the top, axially symmetric. Those thrusters at the top are not used nominally. They are the launch abort system and are activated only when something goes very wrong with the rocket such as an explosion or loss of control. The capsule detaches from the main body and the launch abort system takes the capsule away from the malfunctioning rocket.


Quote Quote by torquil View Post
Don't they also start to spin slowly as soon as possible after launch, to increase stability?
Gack! No! You are apparently talking about the roll program, which rcgldr already discussed to some extent. Once the roll program has the vehicle in the desired attitude the rotation ceases.


Quote Quote by rcgldr View Post
You're probably thinking of the space shuttle which rolled upside down during launch (there were also pitch and yaw adjustments) for aerodynamic load issues on the shuttle, (it also allowed the ground to be used for references by the on board crew, but I'm not sure how useful that was). I'm not sure why the shuttle wasn't simply attached to a launch tower so the roll manuever wasn't required, unless the setup was historical (not an issue for previous rockets), and too costly to redo. Most rockets don't spin during launch.
The main reasons for the roll program were to alleviate structural loading problems and to improve line of sight for S-band communications. That it helped the pilot see the ground was a secondary benefit. Once the roll program was completed the Shuttle stayed in that attitude throughout the rest of the launch.

So why didn't the Shuttle start out in the right orientation in the first place? The answer is that NASA reused the Apollo launch towers for the Shuttle. The already-built flame trenches required the Shuttle to be oriented in the wrong direction. It was easier to correct this problem post-launch than it was to redesign/rebuild the launch towers.
ColinW
#6
Jan2-12, 09:18 AM
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Look up "inverted pendulum". The control principles are similar. Plenty of examples on Youtube.
D H
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Jan2-12, 09:20 AM
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Quote Quote by ColinW View Post
Look up "inverted pendulum". The control principles are similar. Plenty of examples on Youtube.
Not all that similar. An inverted pendulum is unstable. A properly designed rocket is stable.
ColinW
#8
Jan2-12, 09:40 AM
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Quote Quote by D H View Post
Not all that similar. An inverted pendulum is unstable. A properly designed rocket is stable.
Make your mind up! One post you say they use gimbaled thrusters to maintain stability, next post you say they don't need them.
D H
#9
Jan2-12, 09:58 AM
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Quote Quote by ColinW View Post
Make your mind up! One post you say they use gimbaled thrusters to maintain stability, next post you say they don't need them.
A rocket doesn't stay in one direction. It has to turn as it climbs. Plus there are winds to worry about, max Q, and a host of other problems. Most importantly, the rocket has to attain the orbit that the mission planners want it to be in. A rocket without vernier controls will end up far from where the planners want it to be. Stability isn't the only concern, and just because a rocket is nominally stable doesn't mean control problems won't arise.

A rocket with thrusters atop such as the Apollo launch abort system (LAS) is more akin to an inverted pendulum. The SpaceX Dragon LAS avoids this problem to some extent by having the thrusters near the bottom of the capsule. They can't be directly below the capsule for obvious reasons, so I suspect there are still some controllability issues.
Filip Larsen
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Jan2-12, 10:14 AM
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It seems to me that an abort systems not necessarily has to be controllable (but of course still stable) during its short burn as the main purpose of that subsystem is to separate the manned section from the main hazards during a launch vechicle break-up. I would not be surprised to learn that such systems are design to work safely only in cases where the abort is initiated from normal attitudes.
jetwaterluffy
#11
Jan2-12, 10:35 AM
P: 227
Try to keep the center of pressure behind the center of mass by more than the diameter of the rocket and the air resistance will keep it straight. That's what I was told! (But that was for a small- scale rocket which only goes up 200m)
D H
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Jan2-12, 11:35 AM
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Quote Quote by Filip Larsen View Post
It seems to me that an abort systems not necessarily has to be controllable (but of course still stable) during its short burn as the main purpose of that subsystem is to separate the manned section from the main hazards during a launch vechicle break-up.
It's the other way around. Stability is a nice-to-have but is not essential. Controllability is absolutely essential.

I would not be surprised to learn that such systems are design to work safely only in cases where the abort is initiated from normal attitudes.
The Shuttle was not safe all the way from pre-launch to on-orbit. New human-rated vehicles must be, at least on paper.


Quote Quote by jetwaterluffy View Post
Try to keep the center of pressure behind the center of mass by more than the diameter of the rocket and the air resistance will keep it straight. That's what I was told! (But that was for a small- scale rocket which only goes up 200m)
Yes, a rocket is nominally stable if the center of press is well behind the center of mass. Stability is not the only concern. A rocket that is going to put something into space had better come close to the planned orbit. It is much cheaper fuel-wise to correct errors as they occur than it is to correct errors after the fact once the vehicle has attained orbit.
Antiphon
#13
Jan2-12, 12:01 PM
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I think the basic thing on the OPs mind is: why is a rocket stable if the center of thrust is below the center of mass? It seems counterintuitive because you can pull a dolly with a rope but you cant push it with a stick.
Filip Larsen
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Jan2-12, 12:37 PM
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Quote Quote by D H View Post
It's the other way around. Stability is a nice-to-have but is not essential. Controllability is absolutely essential.
I was specifically referring to launch abort systems like the Apollo LES where the engine thrust is not controlled in any feed-back fashion; the thrust direction of each engine is fixed and the command to ignite or not to ignite the pitch control engine is based on "simple" initial conditions at the abort time (like those given by a sequence controller or altimeter).
AlephZero
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Jan2-12, 12:51 PM
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Quote Quote by Antiphon View Post
I think the basic thing on the OPs mind is: why is a rocket stable if the center of thrust is below the center of mass? It seems counterintuitive because you can pull a dolly with a rope but you cant push it with a stick.
The analogy doesn't hold, because, ignoring any thrust vectoring systems, the direction of thrust of the rocket is fixed relative to the rocket.

If the thrust is slightly off-center, a stable rocket will tend to fly in a large circle, not tumble end-over-end. That's the difference between "stable" and "unstable".

The same applies to "rear engined" aircraft, whether jet fighters or civilian airliners with engines mounted on the rear fuselage.
Davidthefat
#16
Jan2-12, 02:17 PM
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Quote Quote by D H View Post


Other than the reusable aspect, SpaceX's design is rather old school. Thrust from the bottom, payload at the top, axially symmetric. Those thrusters at the top are not used nominally. They are the launch abort system and are activated only when something goes very wrong with the rocket such as an explosion or loss of control. The capsule detaches from the main body and the launch abort system takes the capsule away from the malfunctioning rocket.
Isn't the old school design the safer design? I mean there really is no way to bail out of a space shuttle, at least with the traditional design, there is a way to survive. Since the human cargo is at the top, there is nothing to "fall" on it to disable it like how the Columbia mission epic-ly failed.
Antiphon
#17
Jan2-12, 02:17 PM
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Very clear to me now. Also explains footage of rocket tests where the thing starts doing slow cartwheels.
D H
#18
Jan2-12, 04:25 PM
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Quote Quote by Davidthefat View Post
Isn't the old school design the safer design?
I was not being denigrating when I called the SpaceX design "old school". Besides, it's not completely "old school". They are doing many new things with respect to reusability. And with respect to ops. And logistics. And manifesting. One of the biggest lessons learned from the Shuttle program is that the cost of the vehicle itself is a tiny part of the total program cost. Operations, the logistics train, and payload integration can swamp the vehicle costs.

I mean there really is no way to bail out of a space shuttle, at least with the traditional design, there is a way to survive. Since the human cargo is at the top, there is nothing to "fall" on it to disable it like how the Columbia mission epic-ly failed.
Yep. Side mount creates a whole lot of problems.


Quote Quote by Antiphon View Post
Very clear to me now. Also explains footage of rocket tests where the thing starts doing slow cartwheels.
That's a different problem. Those incidents typically happened shortly after launch, when the vehicle's air speed is near zero. A vehicle is stable if the center of pressure is below the center of mass. When a vehicle is barely moving there is no center of pressure, making the vehicle a lot less stable. Note also that a similar problem exists (no center of pressure) once the rocket is outside the atmosphere.


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