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  #21  
Old 11-07-2014, 07:19 AM
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Quote:
Originally Posted by MarkB.
Guys,

Could somebody talk me through the physics of the advantage of a slight delay between staging on our rockets?

I understand why some full-size sounding rockets do it to, for instance, to avoid melting fins or noses from friction heating or to generally minimize Max-Q, but this reasoning does not apply to model rockets.

Old G. Harry in the Handbook discusses why you would not want a delay; essentially max altitude is generated by maximum final burn-out velocity which would seem to argue against a coast phase of any kind.

What am I missing?

For a given amount of fuel, your car will go much farther at 55 mph than at 80 mph due to drag. Translate that to rocket fuel and altitude and you get the same results. More altitude due to less drag.

In the case you are speaking of, a higher velocity burnout could coast farther with sufficient mass (inertia), but burnout will occur at lower altitude due to that same mass and higher drag, negating any advantage. I don't know if Stine was talking about sounding rockets or models. In the case of sounding rockets, they burn out above most of the atmosphere, making drag a minor issue. In the case of model rockets, the rockets would have to burn out at nearly the same altitude for the higher velocity rocket to do much better than the lower velocity rocket....or would have to be traveling a heck of a lot faster to begin with. Small models don't have much mass, so there's not much inertia to make up for the added drag of high velocity.
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  #22  
Old 11-07-2014, 08:33 AM
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Quote:
Originally Posted by Doug Sams
Understood, JR. I was pointing that out because at least one plugged motor - can't recall for sure which one - had a -0PT suffix IIRC. That is, while it had the 0 in the suffix, it was not suitable for staging

Also, I recall an early motor which had a delay, but was plugged. B4-4P maybe? So, while it didn't have an ejection charge, it did have a smoke trail.

That is, between pure boosters and ordinary motors, there were a couple of other (rare) configurations which some might confuse with boosters.

Doug

.

Oh, ok... never heard of those... learn something new every day...

I was posting more for folks who might read it and wonder what a "-P" motor was and its proper use versus "-0" motors...

Later! OL JR
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  #23  
Old 11-07-2014, 09:02 AM
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OK

I'm with you so far. But . . . .

The car analogy is not quite on because it assumes constant thrust for the given distance. My pick up gets much better mileage at 50 than at 70, but I stay on the gas for a given distance. Assuming flat or even uphill terrain, if I start coasting at 50, my truck does not roll as far as it does when I start coasting from 70. And that's the devil in the details here. We're talking coasting.

Assuming we're talking about BP 18 and 24mm motors, the difference in peak altitude is less than 1000 feet so drag due to air density is for practical purposes going to be a constant. Also, its highly unlikely we can exceed the velocity at which the construction of the rocket fails so were keeping the velocity below about 350mph (speed of balsa).

Lets use B6-0/B6-6. A flight profile would be boost to a given velocity. Relatively instant ignition to a final velocity and then a coast to V = 0.

With say a B6-2/B6-6, it would be boost to a given velocity; coast(deceleration); ignition to a final velocity and then coast to V = 0.

I'll dig out the book but it would seem that anything that would lower the final burnout velocity reduces peak altitude and that 2 second coast phase is deceleration. As noted for the coasting truck, burnout velocity is everything.

I'll see if I can find the Handbook under all these rocket parts . . . .
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  #24  
Old 11-07-2014, 03:08 PM
Peter Olivola Peter Olivola is offline
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Sticking with the automotive analogy for a moment, the question is, how much fuel will your truck use and how far will it go in total between the two instances?

0-70 MPH at full throttle taking X seconds/immediate coast down.
0-50 MPH at same throttle position as above reducing throttle to sustain at 50 MPH for a total of X seconds/immediate coast down.

Eliminating all other factors as variables, the second example should use less fuel and the truck should go further in total.

Try it.



Quote:
Originally Posted by MarkB.
OK

I'm with you so far. But . . . .

The car analogy is not quite on because it assumes constant thrust for the given distance. My pick up gets much better mileage at 50 than at 70, but I stay on the gas for a given distance. Assuming flat or even uphill terrain, if I start coasting at 50, my truck does not roll as far as it does when I start coasting from 70. And that's the devil in the details here. We're talking coasting.

Assuming we're talking about BP 18 and 24mm motors, the difference in peak altitude is less than 1000 feet so drag due to air density is for practical purposes going to be a constant. Also, its highly unlikely we can exceed the velocity at which the construction of the rocket fails so were keeping the velocity below about 350mph (speed of balsa).

Lets use B6-0/B6-6. A flight profile would be boost to a given velocity. Relatively instant ignition to a final velocity and then a coast to V = 0.

With say a B6-2/B6-6, it would be boost to a given velocity; coast(deceleration); ignition to a final velocity and then coast to V = 0.

I'll dig out the book but it would seem that anything that would lower the final burnout velocity reduces peak altitude and that 2 second coast phase is deceleration. As noted for the coasting truck, burnout velocity is everything.

I'll see if I can find the Handbook under all these rocket parts . . . .
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  #25  
Old 11-07-2014, 03:29 PM
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Edit: Not at all sure this is right! See http://www.oldrocketforum.com/showp...00&postcount=41

I did a quick and dirty sim the other day in OpenRocket, optimizing for maximum altitude allowing the time delay between booster burnout and sustainer ignition to vary. In the case I was looking at it came up with an increase in altitude of a couple percent or so with an ignition delay of about 0.8 seconds.

I believe it's correct that the delay would make no difference for a dragless rocket, but when there's drag there's some advantage in reducing the average velocity by delaying the sustainer. I haven't worked out the equations of motion or anything, though.
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Last edited by Rich Holmes : 11-08-2014 at 05:17 PM.
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  #26  
Old 11-07-2014, 07:44 PM
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Quote:
Originally Posted by MarkB.
OK

I'm with you so far. But . . . .

The car analogy is not quite on because it assumes constant thrust for the given distance. My pick up gets much better mileage at 50 than at 70, but I stay on the gas for a given distance. Assuming flat or even uphill terrain, if I start coasting at 50, my truck does not roll as far as it does when I start coasting from 70. And that's the devil in the details here. We're talking coasting.


The car analogy is a very good example. Remember that we are looking at peak altitude performance which includes both thrust and coast phases, not how far the rocket will coast. You are forgetting that you have to burn all your fuel before you start coasting to simulate a rocket flight. Start with a full tank on both your 50mph and 70mph runs, then coast to a stop when you run out of gas. I'd use a small auxiliary tank unless you just want to explore several hundred miles of I-70 across the midwest. If you want to simulate a rocket's thrust profile, you are welcome to go full throttle until you get up to speed, then let up and hit the cruise control. Thanks to reduced drag, you will drive considerably farther before you start coasting. Therefore, you will get much farther from your "launch pad" before you coast to a stop.
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  #27  
Old 11-07-2014, 07:58 PM
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I get the coasting at a slower speed is less drag but nobody seems to take the weight of the booster and motor into consideration.

I would think that during a delayed staging event, the excess weight would more than counteract the gain in altitude from lower drag.

EDIT: Now that I think of it, you don't even get lower drag because you have the drag from the booster and it's fins during the first coast phase.
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  #28  
Old 11-07-2014, 08:07 PM
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Quote:
Originally Posted by Bob H
I get the coasting at a slower speed is less drag but nobody seems to take the weight of the booster and motor into consideration.

I would think that during a delayed staging event, the excess weight would more than counteract the gain in altitude from lower drag.

That's moving mass, i.e. momentum. Think about boosted darts. Think about throwing a golf ball vs throwing a ping-pong ball.
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  #29  
Old 11-07-2014, 08:23 PM
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Quote:
Originally Posted by tbzep
That's moving mass, i.e. momentum. Think about boosted darts. Think about throwing a golf ball vs throwing a ping-pong ball.
OK, I'll give you the momentum argument, for now, but how about the additional drag from holding on to the booster during the delay?
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  #30  
Old 11-07-2014, 08:57 PM
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Reading Rockets of The World kind of explains the answer why rockets coast. It explains that the coast phase was to get above the dense atmosphere. This stopped problems with max Q and allowed the rocket to reach higher altitudes.

Anyway that is what I gathered from reading about sounding rockets, there probably is more on this, might be wrong, but from what I read, think this might be part of the reason for the coast.

Mike
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