On 5/12/2018 3:31 PM, Terry McKiernan wrote:
I'll start off by making it clear that I have no standing in any
official way with ROC, nor have I ever done a high-power multi-stage
project myself. Furthermore, I'm attempting to keep my normal
'snarkniness' meter set to zero for the course of this conversation.
I AM, however, an occasional RSO at ROC launches, a Cal Pyro 3 permit
holder for many years, and have been doing rocketry semi-continuously
since the sixties. I'm trying to make my comments here more in the
nature of 'what I think things ought to be' rather than 'what may
actually be allowed and legal'.
These days, if it were up to me, I would probably insist that the
sustainer stage be fired by a more sophisticated unit that ensures the
rocket is pointed up and actually moving, rather than a timer of some
sort. Not sure where, exactly, the line should be drawn, but here's the
reality -- I've witnessed a number of flights where multi-stage
high-power rockets launch, the sustainer doesn't fire 'optimally', and
the results are either 'almost disastrous' or 'actually disastrous'. By
that I mean that the rocket is off vertical a significant amount, or the
forward velocity has decreased significantly, or (worst case) the rocket
is pointed past horizontal and is at some angle pointed down (sometimes
straight down). In every single one of those cases, what's going
through my mind is, "Please, Great Flying Spaghetti Monster, don't
ignite the sustainer now." In some cases, but not all, my prayer has
been answered -- and in the cases where it has not been answered, the
results have generally not been pretty.
So I go back to what I would call the common-sense answer (and others
are welcome to disagree) -- since altimeters are READILY AVAILABLE that
allow for multi-stage flights to be done far more safely than they used
to be, and because we've seen the damage that can be done by accidental
downward firing of a sustainer, we 'ought' to insist upon it.
Since we do things in both a formal and informal manner at ROC, what
that means is that I, personally, would probably not allow a timer-only
launch if I were RSO, and if I were the only Cal Pyro 3 person on site,
I would likely assert my prerogative to disallow the flight -- however,
it is possible that another RSO would think it's perfectly fine, and
another Cal Pyro 3 holder would allow it without question.
I should add that I've seen launches (over many years, and not naming
names) by people I greatly respect in our club that have failed, so it's
not that just inexperienced people can have a failure. The issue boils
down to seeing the damage that a rocket coming in ballistic can do --
the damage that a rocket coming in ballistic UNDER POWER can do just
makes me feel that anything 'reasonable' that can be done to mitigate a
failure 'should' be a requirement.
So, lacking any ruling from a higher authority, my suggestion really
does boil down to: if there is a reasonable way to mitigate a probable
method of failure, one should pursue that remedy. My guess is that the
only disagreement we'll be having are the definitions of 'reasonable'
and 'probable' in the preceding sentence.
That may not answer your question (and it is likely that it would keep
you from asking me to RSO your rocket in this instance <G>), but I hope
you take it under consideration.
David Erbas-White
Hello all,
Thanks to everyone who provided suggestions on my 2-stage rocket / coupler problem. Now I have a second question I hope you can help with.
Is a MissileWorks PET2+ an OK device to use for the sustainer airstart? When I come out for ROCstock in June will I be allowed to launch?
The PET2+ is certainly marketed as a way to do airstarts -- both on the MissileWorks site and on retail sites like Apogee. It includes wiring diagrams showing how to do exactly what I have planned for my 2-stage rocket.
However, today I found some postings on www.rocketryforum.com where some people were saying that a "simple timer" was no longer allowed for airstarts and no RSO would approve it. This was followed by a long discussion about what a "simpler timer" means, and is this really a rule or just a best practice of some TRA prefectures, and what really are the rules anyway etc. Some people seemed to think it was perfectly OK and had used PET2's even for L3 rockets and some said no, you have to have an advanced altimeter like the Raven or TeleMega 3 that will check the altitude and make sure you rocket is going up and sufficiently vertical before firing the sustainer. But no one seemed to have a hard and fast answer that was documented in a TRA or NAR rulebook.
Note that the PET2+ does use "X gees for Y seconds" to detect main engine ignition (MEIG), main engine cutoff (MECO), and similar sustainer events (SEIG and SECO). It also does some X/Y/Z orientation checks at power on (i.e. on the pad) and won't initialize if the rocket is pointed funny. It's not just a timer that starts counting down when you apply power.
My plan (repeating myself from my previous posting) is to have both the PET2+ and an EggTimer TRS in the payload bay of the upper stage. The PET2+ will be set to ignite the sustainer at either MEIG + booster motor burn time + 0.5 sec (or so) or at MECO + 0.5 sec (or so). I am thinking about actually putting 2 igniters in the sustainer and using both pyro events of the PET2+ maybe 0.5 second apart, so if the first igniter burns the second one still has a chance to get the sustainer going.
The TRS is there to help find the rocket and also as a backup in case the airstart ignition fails and thus there's no sustainer motor charge to release the main chute. Per Andrew's suggestion it will be wired up to a backup ejection charge for the main chute. It's not going to be involved in the airstart at all, and has its own battery, so it's a completely separate system.
So, what do you think? Is this a flyable configuration?
Thanks!
Terry McKiernan
