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How Do Rockets Maintain Stability At Liftoff?  
User currently offlineComorin From United States of America, joined May 2005, 4903 posts, RR: 16
Posted (7 years 8 months 2 weeks 4 days 7 hours ago) and read 11003 times:

Just returned from a wonderful and awe-inspiring visit to NASA at Kennedy Space Center, and I have a question regarding stability at liftoff. Given that the propulsive force is happening below Center of Mass, aren't rockets inherently unstable - a slight deflection resulting in a larger deflection? Do they restrain the rocket until it 'shoots' off the pad? Any illumination on the topic is most welcome...

Thanks

30 replies: All unread, showing first 25:
 
User currently offline3DPlanes From United States of America, joined Apr 2006, 167 posts, RR: 0
Reply 1, posted (7 years 8 months 2 weeks 4 days 6 hours ago) and read 11003 times:

The rocket nozzles are gimbaled so they can vector the thrust to direct the rocket.

If you watch a shuttle launch, in the last few minutes before launch (or less), they'll usually show a shot of them moving the three main engine nozzles around through their range of motion.



"Simplicate and add lightness." - Ed Heinemann
User currently offlineComorin From United States of America, joined May 2005, 4903 posts, RR: 16
Reply 2, posted (7 years 8 months 2 weeks 4 days 4 hours ago) and read 10949 times:

Quoting 3DPlanes (Reply 1):

Thanks! I presume then that they use gyros to control the servomechanisms that position the motors. Must have been tricky in the old, pre-solid state days of Dr Von Braun...


User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 3, posted (7 years 8 months 2 weeks 4 days 3 hours ago) and read 10950 times:

IIRC, a properly-designed rocket (one with a fixed exhaust nozzle) always has the Center of Pressure behind the Center of Gravity (I may have that relationship backwards...). I was one of those kids that designed my own Estes rockets from the parts bags  Wink Good thing for large fields...


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 4, posted (7 years 8 months 2 weeks 4 days 1 hour ago) and read 10911 times:

Quoting Comorin (Thread starter):

G'day Comorin  ,

This is what happens when things go wrong, and the back of the rocket tries to overtake the front;

http://www.youtube.com/watch?v=kYUrqdUyEpI&mode=related&search=
http://www.youtube.com/watch?v=IONcgYzVFlg

Apparently, a computer glitch caused a malfunction in the guidance control system. The subsequent explosion was an automatic self-destruct mechanism.

Regards, JetMech

[Edited 2007-04-07 08:38:47]


JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineStealthZ From Australia, joined Feb 2005, 5743 posts, RR: 44
Reply 5, posted (7 years 8 months 2 weeks 4 days ago) and read 10886 times:
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An example is the Saturn V that launched Apollo to the moon,

After ignition the hold down arms would release then the acceleration of the rocket was further impeded by steel pins that were drawn through dies, effectively making steel wire! This stabilised the vehicle for the initial moments of flight.
Of the 5 rocket engines the outer 4 were steerable and the centre one was fixed, the 4 outer engines were steered outward so if one failed the others were still thrusting towards the centre of gravity.

Much more detailed information here....

http://en.allexperts.com/e/s/sa/saturn_v.htm

Cheers



If your camera sends text messages, that could explain why your photos are rubbish!
User currently offlineComorin From United States of America, joined May 2005, 4903 posts, RR: 16
Reply 6, posted (7 years 8 months 2 weeks 3 days 17 hours ago) and read 10802 times:

Quoting KELPkid (Reply 3):
IIRC, a properly-designed rocket (one with a fixed exhaust nozzle) always has the Center of Pressure behind the Center of Gravity (I may have that relationship backwards...).

I think you would need a 'righting' moment of either a fin or an aerodynamic surface to keep it on its path? Must have been a lot of fun growing up with those Estes rockets!

Quoting JetMech (Reply 4):

Who is that pleasant looking girl in your profile? Can I Bloody Hell move to Oz?  Wink

Quoting StealthZ (Reply 5):

Thanks, that was interesting reading.


I've gotten to a point where I understand finned rockets and positive stability, but still trying to wrap my head around intrinsically unstable rockets. Does the shape of the surfaces on the Shuttle enhance stability or is it irrelevant? It's also interesting to note that that the CG changes rapidly as the fuel is being used up.


User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 7, posted (7 years 8 months 2 weeks 3 days 14 hours ago) and read 10753 times:

Quoting Comorin (Reply 6):
think you would need a 'righting' moment of either a fin or an aerodynamic surface to keep it on its path? Must have been a lot of fun growing up with those Estes rockets!

Fins actually induce stability in many rockets by causing them to spin (!). Yes, spinning about the vertical (flight) axis does stabilize a rocket...this was probably Dr. Von Braun's biggest contribution to the US Space program, for this was something that Goddard had not stumbled across.

Of course, most modern rockets no longer spin for stability, the rockets like the Space shuttle and the Delta aren't even symmetrical around the vertical axis.



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineComorin From United States of America, joined May 2005, 4903 posts, RR: 16
Reply 8, posted (7 years 8 months 2 weeks 3 days 12 hours ago) and read 10711 times:

KELPkid, thanks for the post.

I believe most modern fighters are aerodynamically unstable, and are kept stable by flight control systems. I can see how servos can actuate surfaces quickly enough to do this. However, in a Launch Vehicle (picking up the jargon!), it must be difficult to gimbal the motors quickly enough to respond (and also not cause oscillations). Perhaps it's just the nozzle that's gimballed, or do they vector the thrust some other way?

Any added explanations or links welcome - thanks.


User currently offlineChksix From Sweden, joined Sep 2005, 345 posts, RR: 4
Reply 9, posted (7 years 8 months 2 weeks 3 days 12 hours ago) and read 10702 times:

The Main engines of the shuttles are gimballed around a thrust plate just above the nozzle. The engine is in this way following the gimbal motion on the other side of the plate.

Big version: Width: 519 Height: 389 File size: 62kb



The conveyor belt plane will fly
User currently offlineJerald01 From United States of America, joined Oct 2006, 161 posts, RR: 2
Reply 10, posted (7 years 8 months 2 weeks 3 days 3 hours ago) and read 10636 times:

The solid rocket boosters used on the Space Shuttle have a flexible bearing in front of the steerable exhaust nozzle. Each nozzle is vectored by two hydraulic actuators, placed 90 degrees from each other around the nozzle and each pushing/pulling longitudinaly along the rocket motor's axis. The actuators are powered by hydrazene-pressurized units which, in turn, are controlled by computers located in the nose of the booster rocket. Those computers are fed information from several sources and keep the Shuttle on the planned flight path.

The fact that these rockets are powered by solid fuel make them just a little different from those that are liquid-fueled. Liquid-fueled rockets lose weight and have tremendous changes of center-of-gravity and center-of-moment taking place very rapidly because of the burn-off of fuel. Solid rockets, on the other hand, have much less of a change of center-of-gravity and center-of-moment because the fuel is burned evenly along the full length of the rocket. This is because, during the pouring of the solid fuel into the rocket casing, a piece of tooling (called a "core") is inserted into the rocket casing and the fuel is poured around that core, leaving a hollow hole in the fuel the entire length of the rocket.

When the rocket's igniter (located at the front end of the rocket) fires, the entire face of fuel along that center hole ignites, sending hot, burning solid fuel out the nozzle. As the fuel burns, the diameter of the hole inside the fuel gets bigger, making the burning face of fuel larger, giving more power to the rocket, but keeping the CG fairly close to the original location.


When the Shuttle rises off the pad you will notice that it makes a 90-degree roll just as the wingtips clear the launch tower. This entire roll motion is the result of the booster rockets' nozzles being vectored, allowing the Shuttle to be turned toward it's intended flight path. Even though the Shuttle is travelling just over 100 MPH when it clears the launch tower, all steering is done with the booster rockets.

I hold my breath for 2 minutes and 12 seconds on each Shuttle launch, and give a sigh of relief when I see those beauties separate from the Shuttle. I know that in about three weeks I will be seeing sections of those rockets back at our plant, where we will begin their refurbishment for another launch.



"There may be old pilots, and there may be bold pilots, but there are darn few green cows"
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3597 posts, RR: 66
Reply 11, posted (7 years 8 months 2 weeks 2 days 18 hours ago) and read 10574 times:

Quoting StealthZ (Reply 5):
After ignition the hold down arms would release then the acceleration of the rocket was further impeded by steel pins that were drawn through dies, effectively making steel wire! This stabilised the vehicle for the initial moments of flight.

There was another reason for the controlled release of the Saturn V. The total vehicle weighed 6 million plus pounds while the main engines produced a total of 7.5 million pounds of thrust. Since the F-1 engines were not throttleable to the same degree as the SSME's, instantaneous release of all that thrust against the inertia of the full liftoff weight could have resulted in permanent deformation of the vehicle's structure, given the lightness of its construction.

A controlled release of the vehicle thrust ring allowed a uniform, "gradual" build up of the thrust force without inducing a sudden point load.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineComorin From United States of America, joined May 2005, 4903 posts, RR: 16
Reply 12, posted (7 years 8 months 2 weeks 2 days 18 hours ago) and read 10574 times:

Quoting Jerald01 (Reply 10):

 thumbsup  Thanks for taking the time for such a thorough explanation! It struck me while reading your post that the control systems would also need to compensate for any small differences in thrust from each of the motors.


User currently offline787atPAE From United States of America, joined Oct 2006, 143 posts, RR: 4
Reply 13, posted (7 years 8 months 2 weeks 2 days 13 hours ago) and read 10530 times:

To make things a little realistic, try balancing a pencil or a baseball bat on your finger as your finger pushes the object upwards against gravity. Just don't hurt yourself!!

Your finger is the propulsion that moves the vehicle upwards into the heavens and the blackness of space. You will find that no matter what, the pencil/bat will always lose control. Computers must be used to control the vehicle on its launch trajectory. Human reaction times are just not fast enough to deal with the dynamics of such an unstable vehicle.


User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 14, posted (7 years 8 months 2 weeks 2 days 5 hours ago) and read 10443 times:

An Estes rocket, of course, uses the launch rod for stability  Smile Certain models of rockets have minimum launch rod length requirements, and the rocket has a small tube on the side of it's body (sometimes through the middle of the body and the nose cone for multi-engined and nonsymmetric models) that you slip the launch rod through when you put the rocket on the pad. By the time the rocket clears the launch rod, it is travelling fast enough to be aerodynamically stabilized.


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineJerald01 From United States of America, joined Oct 2006, 161 posts, RR: 2
Reply 15, posted (7 years 8 months 2 weeks 2 days 5 hours ago) and read 10428 times:

Quoting Comorin (Reply 12):
Thanks for taking the time for such a thorough explanation! It struck me while reading your post that the control systems would also need to compensate for any small differences in thrust from each of the motors.

Yes, the avionics packages in the nose of the Solid Rocket Boosters (SRBs) receive input from various sensors and determine what minor corrections are needed. An electronic signal is sent to the actuator packs via hardwired circuits located on the outside of the SRB, and the actuator packs "steer" the nozzle through hydraulic rams that are connected to a compliance ring that surrounds the aft exit cone. These are HUGE rams, but they have to be, considering they are vectoring a stream of hot gases/solid fuel that is producing 3.5 million pounds of thrust.

The Solid Rocket Boosers (SRB's) are composed of four sections (the Forward, Center Forward, Center Aft, and Aft), each of which is indvidually filled with a slurry mix with the rocket section standing on end in a deep pit. The individual sections for each flight are manufactured at the same time. That is, the Forward sections of a specific flight are manufactured at the same time. The Cetner Forwards, Center Afts, and Afts are also manufactured with their counterparts from the other SRB.

The propellant mix comes from the mixer building and half of it is poured into one of the Forward sections and the other half into the other Forward section. When the pouring of the sections is complete, the sections are then cured at exactly the same time, using the same temperatures, etc. That way there is little variation between the two rocket motors.

Every process in the manufacturing sequence is tightly controlled and nothing is left to chance. Once those SRBs are started there is no way to throttle them back or shut them off. The solid fuel provides it's own combustible material and it's own oxygen, so there is no way to put the fire out once it starts. so there is no room for error OR large variations between the two SRBs.



"There may be old pilots, and there may be bold pilots, but there are darn few green cows"
User currently offlineKalvado From United States of America, joined Feb 2006, 497 posts, RR: 0
Reply 16, posted (7 years 8 months 2 weeks 1 day 19 hours ago) and read 10328 times:

Quoting 787atPAE (Reply 13):
To make things a little realistic, try balancing a pencil or a baseball bat on your finger as your finger pushes the object upwards against gravity. Just don't hurt yourself!!

That's an excellent example... but it has nothing to do with rocket lift-off, though.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17172 posts, RR: 66
Reply 17, posted (7 years 8 months 2 weeks 1 day 19 hours ago) and read 10318 times:

Quoting Comorin (Reply 8):
I believe most modern fighters are aerodynamically unstable, and are kept stable by flight control systems.

Actually quite few are even today. It's still cutting edge technology, as evidenced by the two early Gripen crashes.

By my count:
- F-16, mildly.
- Gripen.
- F-22.
- F-35?
- Typhoon.
- F-117.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 18, posted (7 years 8 months 2 weeks 1 day 12 hours ago) and read 10222 times:

Quoting KELPkid (Reply 3):
IIRC, a properly-designed rocket (one with a fixed exhaust nozzle) always has the Center of Pressure behind the Center of Gravity (I may have that relationship backwards...). I was one of those kids that designed my own Estes rockets from the parts bags



Quoting Comorin (Reply 6):
I think you would need a 'righting' moment of either a fin or an aerodynamic surface to keep it on its path? Must have been a lot of fun growing up with those Estes rockets!

Yes, you must have an aerodynamic righting moment to keep the rocket pointed in the direction of flight, which you get if you design the vehicle as suggested by KELPkid.

Then you have static stability. With controlled nozzles, you get controllability through an active guidance system.

More interesting is the question of how simple rockets without active guidance actually maintain a more or less straight path. We're talking FFARs to fireworks here. There's nothing to keep them flying in one direction, so any imbalance or manufacturing defect would cause them to follow a curved path? A misrigged free-flying model aircraft will, at best, fly circles with a rather small radius, right? So why do new years fireworks rockets, manufactured with very large tolerances, go off on a straight path?

The basic static stability keeps them more or less aligned with the direction of flight. This keeps the thrust pointed in the general direction of flight. And the thrust is orders of magnitude larger than any aerodynamical forces which could act to change the direction of flight. Thus, the rocket may well accelerate laterally but it will be completely insignificant compared to the acceleration in the direction of flight, and more or less straight flight is maintained even though the pin of the rocket may be all crooked.

If you have the thrust misaligned with the stability axis, things get more interesting of course. However, this tends to induce rotation as well, giving a straight spiralling path rather than a curved path.

I'm really no rocket scientist, just a simple aero engineer, so this is strictly speaking outside of my aeroea (har har har) of expertise, but I consider it interesting enough and enough frequently overlooked to be worthy of mentioning anyway. Consider the above an attempt to get you thinking about something which you may, or may not, have overlooked rather than an attempt to describe it all absolutely accurately and down to every detail.
Cheers!
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineComorin From United States of America, joined May 2005, 4903 posts, RR: 16
Reply 19, posted (7 years 8 months 2 weeks 1 day 8 hours ago) and read 10162 times:

Good points, all, and many thanks to the a.net Brains Trust!

User currently offlineRwessel From United States of America, joined Jan 2007, 2411 posts, RR: 2
Reply 20, posted (7 years 8 months 2 weeks 1 day 7 hours ago) and read 10151 times:
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Quoting Jerald01 (Reply 10):
The fact that these rockets are powered by solid fuel make them just a little different from those that are liquid-fueled. Liquid-fueled rockets lose weight and have tremendous changes of center-of-gravity and center-of-moment taking place very rapidly because of the burn-off of fuel. Solid rockets, on the other hand, have much less of a change of center-of-gravity and center-of-moment because the fuel is burned evenly along the full length of the rocket. This is because, during the pouring of the solid fuel into the rocket casing, a piece of tooling (called a "core") is inserted into the rocket casing and the fuel is poured around that core, leaving a hollow hole in the fuel the entire length of the rocket.

When the rocket's igniter (located at the front end of the rocket) fires, the entire face of fuel along that center hole ignites, sending hot, burning solid fuel out the nozzle. As the fuel burns, the diameter of the hole inside the fuel gets bigger, making the burning face of fuel larger, giving more power to the rocket, but keeping the CG fairly close to the original location.

While that's true of core burning solids (which the SRBs most certainly are), end burning solids often have worse CG changes than liquids. On liquids the fuel tanks are commonly stacked, and the fuel CG change is closer to the average of the two tank lengths, rather than the whole chamber as on an end burning solid.

A considerable range of tradeoffs between thrust and burn time is possible on solids by controlling the size of the burning surface. Core burners being at the maximum thrust/minimum duration end of the spectrum, (flat) end burners being at the minimum thrust/maximum duration end.

Quoting Jerald01 (Reply 10):
When the Shuttle rises off the pad you will notice that it makes a 90-degree roll just as the wingtips clear the launch tower. This entire roll motion is the result of the booster rockets' nozzles being vectored, allowing the Shuttle to be turned toward it's intended flight path. Even though the Shuttle is travelling just over 100 MPH when it clears the launch tower, all steering is done with the booster rockets.

There are two aspects to the roll after liftoff. First, since the Shuttle is rather asymmetrical, they want it pointed in the correct direction for the orbit they're trying to hit. That would be possible either right side up or upside down. The reasons for rolling inverted are for aerodynamic load management (apparently it's lower upside down), and for visibility in case an emergency separation is needed. So the total amount of roll is dependent on which way they want to go plus the need to roll inverted. The pads are oriented somewhat oddly given that requirement - you'd expect them to be aligned to have the Shuttle in the "correct" orientation for an "average" launch, instead they're aligned for a launch due north. I suspect that's left-over from Apollo.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17172 posts, RR: 66
Reply 21, posted (7 years 8 months 2 weeks 1 day 7 hours ago) and read 10150 times:

Quoting Rwessel (Reply 20):
The pads are oriented somewhat oddly given that requirement - you'd expect them to be aligned to have the Shuttle in the "correct" orientation for an "average" launch, instead they're aligned for a launch due north. I suspect that's left-over from Apollo.

As usual, the Russians are much more into the KISS principle. The Soyuz pads are simply oriented in the correct direction for launch. No need for any fancy high tech roll crap. Big grin



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2577 posts, RR: 25
Reply 22, posted (7 years 8 months 2 weeks 1 day 7 hours ago) and read 10147 times:

Quoting Starlionblue (Reply 21):
The Soyuz pads are simply oriented in the correct direction for launch. No need for any fancy high tech roll crap.

Soyuz, like Apollo/Saturn V, wouldn't needed to roll for orientation due to its symmetry.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offline787atPAE From United States of America, joined Oct 2006, 143 posts, RR: 4
Reply 23, posted (7 years 8 months 2 weeks 1 day 7 hours ago) and read 10144 times:

Quoting Rwessel (Reply 20):
reasons for rolling inverted are for aerodynamic load management

Try again. The only reason why the vehicle is "heads down" is because of communication between air and ground at Bermuda and similar sites. This is the same reason why the first shuttle launch was at an inclination of about 41 deg off the equator. NASA has been doing it this way ever since STS-1, it is certified, and it makes no sense to do it otherwise. Nowadays, we have the TDRS satellite system for comm, therefore, late in the shuttle's ascent, NASA takes advantage by having the shuttle's computers command a 180 deg roll about 350 sec into flight.

Quoting Starlionblue (Reply 21):
Soyuz pads are simply oriented in the correct direction for launch. No need for any fancy high tech roll crap

In a way, the US system is similar in that principle. The shuttle's tail is pointed due south so the Range Safety folks know how it is lifting off. This has been the convention since STS-1. I hope I never hear of a story where the RSO says during a liftoff "is it supposed to be pointed that way?" and then made their decision to pop the switch That will a bad day....

Quoting Kalvado (Reply 16):
That's an excellent example... but it has nothing to do with rocket lift-off, though.

This has everything to do with a rocket liftoff. Do a free body diagram of both and compare the figures. You will find they are very similar. The propulsive force will be in exactly the same as the "finger force", just about 10000000 orders of magnitude different.   The main difference will be in the aero of each. For a quick example to demonstrate stability, that doesn't matter.

[Edited 2007-04-10 02:35:23]

User currently offlineKalvado From United States of America, joined Feb 2006, 497 posts, RR: 0
Reply 24, posted (7 years 8 months 2 weeks 1 day 6 hours ago) and read 10120 times:

Quoting 787atPAE (Reply 23):
For a quick example to demonstrate stability, that doesn't matter.

for an excellent example of how not to do it - maybe...
rockets are generally stable at the time of launch; how it's achieved is a very different story..


25 Post contains images 3MilesToWRO : You are not quite right. Human reactions are too slow for a pencil or 1m long stick, but they are OTOH too FAST for a stick of 3m. What you are descr
26 FredT : I'm afraid the thrust of the rocket will stay aligned with the rocket rather than remain acting straight up if the rocket tilts, as the lifting force
27 Rwessel : That would be great if the thrust axis was exactly aligned with the CG and nothing ever disturbed the vehicle. Otherwise that sucker is tipping over.
28 Post contains images KELPkid : One of the simplest, least expensive Estes kits used to be like this...the engine, on the pad, hung down below the rocket body (with the top of the e
29 Rwessel : I remember several spin recovery rockets, and several gliders, although I don't remember one with a moving engine. There were several that actually e
30 Post contains images KELPkid : I want to say it was the "Alpha", but I can't remember...(this was when I was 12 years old...). It was the smallest rocket kit Estes made at the time
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