100 years of liquid-fueled rockets

Cap'n Jack said:
100 years ago (actualy, 100 years ago yesterday), the first liquid-fueled rocket took flight. Here's a replica:

JAK_4094 by Jack Silver, on Flickr

They have come a long way from that crude example!
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I always thought Goddard was very clever for putting the center of thrust ahead of the center of mass and the center of drag. Self stabilizing rockt without gyros or any sort of controls.
 
Goddard was a really smart fella and did some really good ****, but... :-)

It just seems odd that he overlooked what would seem to be obvious (perhaps just the benefit of hindsight?) But, putting the motor up front does absolutely nothing for stability. The engine is bolted (welded?) in place and the thrust line just points along the longitudinal axis of the rocket. Ain't never gonna make it turn.
Later he experimented with pendulums for control - don't work in airplanes, don't work in rockets.

Once you are in free flight the actual forces you "feel" are from the aerodynamics and thrust - not gravity. Gravity accelerates both ends of a pendulum (and everything it is attached to) at the same rate so there is no tendency for it to point down.
 
Cap'n Jack said:
That was actually a mistake...
en.wikipedia.org

Rocket - Wikipedia

en.wikipedia.org en.wikipedia.org
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Interesting. And counter intuitive, though that's certainly not my first experience of that in the real world. I don't understand from the Wikipedia article _why_ it's not stable. I get why it can't turn...
 
jsstevens said:
Interesting. And counter intuitive, though that's certainly not my first experience of that in the real world. I don't understand from the Wikipedia article _why_ it's not stable. I get why it can't turn...
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I'll try to break it down, as I agree most explanations about this don't contribute to an understanding of why it doesn't work.

- If the path of the rocket is perturbed by a rotation, the rocket will make thrust in that new direction and it goes into a skidding turn if there is enough momentum to continue to rotate the rocket. There isn't enough "correction factor" with only the body of the rocket. It is the same with an airplane- we still need vertical and horizontal stabilizers (or control surfaces that do the same thing).

- Once the rocket is flying, gravity doesn't act to stabilize it- it is under its own acceleration. Think of a bottle rocket where the motor is at the top of the device. One can tilt the launch bottle and it will fly in that direction. In fact the Mysorean rockets (invented in India, copied by Congreve, used in the Battle for Baltimore; "...the rockers red glare...") flew wherever they were pointed. They were essentially giant bottle rockets with a long bamboo stick stabilizer. Are bottle rockets your "...first experience of that in the real world." ? Note that the stick acts like the fins on a rear-fired rocket and this could be considered a conventional rocket with a very simple fin.

Goddard's last rocket looks very modern, he moved the motor to the rear and uses fins. Directional control use paddles to direct the exhaust, visible in the image below, in a similar fashion used in the V2 rocket.

JAK_4095 by Jack Silver, on Flickr
 
Thanks for the explanation. I understand now. If the rocket was still accelerating, would it tend to correct itself because the center of mass was behind the center of thrust?

I have used bottle rockets and was aware of the stick's role in stability. They can also be taped to paper and balsa gliders if you're careful to get them close to center line. Reporting bottle rockets happen to be hard on the airplane though...

I wasn't thinking of the pendulum effect, only the source of thrust being ahead of the center of mass (which only matters when accelerating) and the center of drag.

My "first experience of that in the real world" was referring to things that were (at least to me) counter intuitive in general.
 
jsstevens said:
I understand now. If the rocket was still accelerating, would it tend to correct itself because the center of mass was behind the center of thrust?
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If you consider that at the moment of firing on the earth's surface, the rocket is accelerating at 1 g, you have just described the "pendulum effect" in your post. Another point of view is that you are correct- if the rocket is rotated, it accelerates in that new direction with the rest of it following along, but no longer on the same path as before. If the perturbation is large enough, it continues to turn. There still isn't enough "righting moment" to stop the spin.
 
jsstevens said:
If the rocket was still accelerating, would it tend to correct itself because the center of mass was behind the center of thrust?
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Nope. The thrust acts along a line through with the center of mass. There is no sideways force generated to cause (or prevent) a rotation.
 
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