Ejection velocity and ground impact velocity

Hi all,

I'm messing with some rocket designs to get a feel for things. I've looked around for these values, but strangely have come up empty. I wonder if there are just too many variables that I'm not considering.

In any case, generally speaking, for a typical low-powered rocket on parachute recovery, how fast is too fast for the velocity at ejection? Obviously, the ideal would be zero so that the ejection charge fires right at apogee. At what velocity does one start to worry about damage to the rocket?

And a related question....on a parachute descent for a typical low-powered rocket that is descending towards a grassy field, at what velocity do you consider the rocket falling too fast and risking damage to the fins?

Are there good rule-of-thumb values for these?

about 48 fps and 15 fps deploy and landing.
Rex

Rex R said:

about 48 fps and 15 fps deploy and landing.
Rex

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Really--15 fps for landing?

From my airborne days, we hit the ground at about 18 fps with the old T10 chute. Dang those balsa fins are delicate!

The_Lone_Beagle said:

Really--15 fps for landing?

From my airborne days, we hit the ground at about 18 fps with the old T10 chute. Dang those balsa fins are delicate!

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But the rockets are much lighter. No offense. :wink:

D'oh!


We were also carrying a bunch of gear, but anyway, yes, we were all "healthy" young boys, that's for sure!

The_Lone_Beagle said:

Really--15 fps for landing?

From my airborne days, we hit the ground at about 18 fps with the old T10 chute. Dang those balsa fins are delicate!

Click to expand...

And if you didn't PLF, your legs would break.

well he did ask about Upper limits...12 - 13 fps is about right for most lpr rockets.
Rex

It really doesn't matter what weight it is, fps is the same for all. Consider what you are hitting at the bottom of the fall makes a lot of difference. Concrete or asphalt are a lot harder than 14" high grass

We were told that a PLF was like free falling from 8' platform I tried it and survived but the velocity was a lot more than 15fps. I wish I knew better 40 years ago. 15fps is 3.5'

Yes, after a good night's sleep and a cup of coffee, thinking out loud, when we landed, we didn't act like rigid bodies...we basically "deformed," i.e., we would land with our knees slightly bent and then use that somewhat as a shock absorber to redirect our fall to the full side of our bodies, which would absorb the main impact.

The rocket coming down is just a rigid body, and if a fin hits first, it is going to take all the weight of impact at almost a point. Which would not be good for the rocket...

Rex R said:

about 48 fps and 15 fps deploy and landing.
Rex

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Really? 48 fps for ejection? That sounds high for a low power rocket, but I could be mistaken.

MichaelRapp said:

Hi all,

I'm messing with some rocket designs to get a feel for things. I've looked around for these values, but strangely have come up empty. I wonder if there are just too many variables that I'm not considering.

In any case, generally speaking, for a typical low-powered rocket on parachute recovery, how fast is too fast for the velocity at ejection? Obviously, the ideal would be zero so that the ejection charge fires right at apogee. At what velocity does one start to worry about damage to the rocket?

Click to expand...

That's a tough question to answer simply because there are so many variables. First and foremost would be construction methods used. If the rocket is built rather stout out of strong materials it can stand up to some abuse. Also, would be how quickly does the chute open, are you using elastic shock cords (Estes dent anyone?) or Kevlar cords, etc. etc.

MichaelRapp said:

And a related question....on a parachute descent for a typical low-powered rocket that is descending towards a grassy field, at what velocity do you consider the rocket falling too fast and risking damage to the fins?

Are there good rule-of-thumb values for these?

Click to expand...

Again, this all depends on how stout the vehicle in question is built. A rocket with long pointy fins made from thin balsa is obviously more susceptible to damage over something like a tube fin rocket.

Then there is the field itself. You say, "grassy" but that could mean a lot of different things. How hard the ground is makes a difference. I let my rockets do about 25 fps at my home field especially if it's muddy because I can get away with it. However, if I were to ever fly someplace like a desert playa I would never let them get much above 12 fps. I've seen grassy fields be hard as a rock and some are very soft. NSL 2009 at Bong for example there were areas of that grassy field outside of the runway which were rock hard believe it or not. If you are unsure err on the side caution and keep the descent around 15 fps or slower. Won't be long and you will get a good feel for what your rockets can handle.

iirc, delays can vary as much as plus or minus 1.5 seconds from the stated time(not counting user error ). this works out to about 48 fps, this is obviously not ideal but, life happens.
Rex

Back to the OP's comment about the ideal being zero so it fires at apogee, keep in mind that the rocket's flight is parabolic and it never stops. It doesn't go straight up, stop, and fall straight back down. But the key is to try not to have it fire during acceleration, and preferably not while it is pointing downward. If it fires while still accelerating, you will zipper your airframe. If it fires while pointing down, you may get a tangled chute.

2 seconds past apogee should theoretically get you at about 64 f/s, 3 seconds will take you to about 96 f/s. This doesn't include whatever forward velocity your rocket may still have at ejection; unless you are absolutely vertical, there will be some amount. If you're not really sure about your ejection delay, use a nylon chute and double the shock cord length; that will prevent it from shredding if you eject late. You might still get a zipper, but at least it will come down in one piece.

The following table summarizes this discussion.

1.) For every second before or after apogee you deploy your recovery system, you rocket velocity increases by 32 fps per second.
2.) You must expect that your ejection charge can be off by +/- 1.5 seconds and that your estimation of apogee is off by +/- 2 seconds.
3.) Round up and it's easy to be off by +/- 4 seconds which gives a deployment velocity of 128 fps is probable.
4.) Fortunately most low power kits can survive this.

5.) Dropping a rocket from 4' in any orientation to the surface you land on simulates a landing speed of 16 fps. The other speeds are shown in the table.

Bob