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:
1. Hope the nose cone pulls it out.
2. Use a piston to shove it out.
Also, are you using shear pins to retain the nose cone until the charge
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
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
The charge calculator in VCT, downloadable here:
will calculate an estimate of separation velocity.
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,
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
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
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
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.
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
/ NC \
|| EBAY ||
| CHARGE |
|| | ||
|| | ||
|* STOP *|
| | |
| 'CHUTE |
| | |
| | |
| | |
| | |
| | |
NC shoots out of tube, pulls piston and recovery gear out behind it.
Niall Oswald wrote:
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
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
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.
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.
A large factor is differential drag.
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
If you do NOT assume a near apogee deploy, the forces get really big
I have posted several drag force vs velocity charts here in the recent
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
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.
Jerry Irvine, Box 1242, Claremont, California 91711 USA
Opinion, the whole thing. <mail to: firstname.lastname@example.org>
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