I think the reality is that no one truly knows and we are struggling to find a
reasonable set of rules.
Personally if I was RSO and you were adhering to minimum distance etc, I would
let you fly without giving it a second thought.
- iPhone mail
On May 12, 2018, at 6:31 PM, Terry McKiernan <terry@xxxxxxxxxxxxx> wrote:
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
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