I'm getting ready to run a ground test of the deployment system of my L1 rocket -- a couple of questions I'd like to ask.
1) What ejection pressure is considered ideal for a 4" airframe? I've seen
15psi batted around but wonder if that isn't a little on the high side.
2) In calculating chamber volume, does one deduct the volume of parachute, harness, wadding, nomex, etc?
TIA for any input, about these questions or any pitfalls of ground testing (beyond the obvious like neighbors, too big a charge, and the "simulations aren't reality" factor).
There isn't an ideal pressure. You want to determine the amount of force needed, then determine the pressure that will produce that force, then calc the amount of BP that will produce that pressure.
How do you plan to deploy the parachute? The usual options are:
Hope the nose cone pulls it out.
Use a piston to shove it out.
Also, are you using shear pins to retain the nose cone until the charge fires?
No shear pins. In this case the layout (aft -> forward) is | bulkhead | charge | kevlar blanket | recovery harness / chute | payload | nose cone. Essentially a loosely packed "shell" of nylon wrapped in kevlar in a black powder cannon.
I have speculated that separation velocity may be a preferred method of determining charge size. The velocity calculation takes into account the relative masses of the components, and the barrel length (i.e., the NC shoulder length), all of which are factors in how 'energetically' the NC separates.
To elaborate on this a bit. You can use any of the charge calculators to find the pressure an amount of BP will produce given the volume. However most have seen that just using the pressure is inadequate, because it doesn't take into account the area of the base of the NC, which determines the total force applied. So you use that area, and take a swag at how much force is 'good'.
However, given two rockets, with the same internal pressure and the same NC base area, the separation dynamics can be radically different, due to other factors of the rocket that are ignored in simple pressure/force calculations.
When you ground test an ejection charge, you typically test until you get a separation that you judge to be 'energetic' enough; e.g.: "Blew that sucker clean across my back yard!" What are you judging here? Distance, yes, but to get that distance required a sufficient separation velocity; get a 10 lb. NC up to 30 fps, and it'll go about the same distance as a 5 lb. NC at 30 fps.
So why isn't your charge calculator calculating for velocity? A velocity calculation takes into account the barrel length (i.e., the NC shoulder length), which directly affects the velocity because that is the distance over which the NC is accelerated by the pressure and force on the base. A NC that has a shoulder twice as long will be ejected with about twice the velocity as an otherwise similar NC. Likewise, the calculation needs to take into account the masses of the NC and body, because those masses determine the magnitude of the accelerations.
One of the things you will find, when fiddling with these dynamics, is that a ground test is not quite equivallent to an in-flight free body separation event. The difference will, again, depend upon the relative NC/body masses, barrel length, etc. of the particular rocket.
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will calculate an estimate of separation velocity.
Thinking about what you've said about separation velocity, I had a (half-baked perhaps) idea - I don't think its especially practical, more of a 'would that work' thing.
The idea is a kind of 'backwards piston'. This is assuming you have electronics in the nosecone (first impracticality!), firing a charge located at the base of the NC.
Below the NC, in the tube, would be a piston, with a bulkhead at the top (NC end), attached to the NC with a short length of kevlar or steel cable (something heat-resistant). Inside the main BT would be a stop - perhaps a short length of coupler bonded into the tube. Below the piston is the recovery system, attached to the piston by the shock cord.
When the NC charge fires, the piston cannot move downwards because it's against the stop, and the NC is projected out of the tube fast by the charge. Almost immediately after leaving the tube, the piston-NC strap becomes taut, and the piston is then yanked out of the tube by the nosecone's inertia, dragging the recovery system out with it.
Ignoring any arguments about whether there's actually be any use for this, do you ('you' being the world in general I guess :) )think it would work?
One possible reason why you *might* want to do something like this is to convert a 'nosecone at apogee' type rocket to fly on hybrids, without cutting up tubes and such. PML do a nosecone intended to hold electronics for hybrids.
Don't know enough to comment in full, but it comes to my mind that when the piston hits the stop, there is still (probably) some 'gas pressure' pushing the piston down. Thus, the nose cone (etc.) would have to overcome not only the inertia of the piston/recovery system, but it would have to 'fight' against the gas pressure that is continuing to try and push the piston down. Granted, the gas pressure will dissipate quickly (but I don't know how quickly), but so will the 'tug' from the separating cone.
I think you are forgetting action-reaction. The nose cone does not get blown off while the rest of the rocket is stationary. The "rest of the rocket" is also acted upon by the expanding gas. The two pieces are moving away from each other. The rest of the rocket does not stand still while the nose cone flies away.
I'm not forgetting it at all. What you're forgetting is inertia. The body tube of the rocket will continue moving forward, but the PISTON will be pushed downward. Once the piston has reached the end of its travel, then and only then will the reaction force be pushing against the rocket (to push it backwards). However, there will still be higher-pressure gas remaining within the body tube, and since we've just pushed all of the air out of the back of the rocket (which, by the way, assumes that there is a vent hole to even allow this to happen in the first place), the movement of the piston FORWARD will be problematic, because we have a partial vacuum in the back of the piston, and higher pressure at the front of the piston (until the higher pressure has completely bled off). Thus, the reaction force will get translated into yanking the nose assembly backwards (my guess) more than pulling the piston forwards.
Writing this out has made me think that perhaps one doesn't want a vent hole, as the downward motion of the piston would compress the air behind it, and would act like the 'spring' someone else commented about once the higher pressure in the front of the piston has dissipated.
My whole point in joining this discussion was to accentuate that this is an interesting idea, but that there are a whole lot of variables involved in it that one needs to fully consider.
Our recent work in this area was to fly several 6" rockets with paper slide couplers and motor eject with varying and large downward speed deploys.
If you do NOT assume a near apogee deploy, the forces get really big really fast.
I have posted several drag force vs velocity charts here in the recent past.
I have always been more interested in dynamically testing the answers in the software to disclose major issues with the modeling strategy.
The ejection calculator NEEDS to factor in deployment speed.
It could be approximated by time from apogee and rocket diameter and a rough CD estimate.
That way you can see the range of pressures needed at various deployment speeds.
In practice I see people setting altimiter deploy as apogee eject, motor as backup to that (in my therory, that might be a double size charge).
Then secondary deploy at low altitude and it seems like that rarely fails. More often there is accidental deploy at apogee if break bolts break on initial eject force.
I appreciate your input, the idea was really just a 'brain fart' :)
The idea is that the piston is unable to move backwards in the tube,since it's sitting against the stop (which would be placed just far enough in that the nosecone, attachments etc can fit in above it).
So all being well the pressure behind the piston would be atmospheric when deployment happened. I can see where you're coming from with the pressure above the piston, I don't know how quickly the tube would depressurize above the piston after the nosecone shoulder cleared the tube. As you say,as the piston was yanked out of the tube the pressure behind it would drop, which could well result in non-deployment.
Another big problem I see would be friction between the piston and the tube,
As a retrofit, perhaps a better idea would be (and bearing in mind that this would only work with limited types of electronics - acceleration-triggered timers would probably be the most suitable) to have the e-bay inside the piston, with the charge either above or below - above to cannon the nose cone and chute out, with the piston below, or with the charge below the piston to push the whole lot out.
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