[roc-chat] Re: PET2+ OK for sustainer airstart?

  • From: David Erbas-White <derbas@xxxxxxxxxxxxxxxx>
  • To: roc-chat@xxxxxxxxxxxxx
  • Date: Sat, 12 May 2018 23:29:09 -0700

On 5/12/2018 10:52 PM, Terry McKiernan wrote:


Allen, Eric, David,

Thanks for all your responses.  I'm still pretty new to rocketry and so I have a lot of questions and learn new things each launch.

I do want to stress that I'm in favor of safety and following the rules!  I'm an aerospace engineer by trade and work on government contracts, and trust me, they have LOTS of rules ... I'm used to it :).  I'm just trying to find out what they are.

As you can imagine, my situation is this: I did some research, selected what I thought were the right components, assembled the rocket, and now with only a few weeks remaining before the June launch, noticed something in a forum that said maybe it would not be allowed.  So sure, I could go buy a Raven or similar altimeter and probably get it before the June launch but hey, it's another $150 so if I don't have to, I'd rather not.  I just bought & assembled my EggTimer TRS which was quite a project, plus 2 PET2+ units, and was hoping this combination would give me the flexibility for lots of different flights.

Allen, the Tripoli document you provided is very helpful.  I read through it and as you said, there are specific guidelines for multi-staged flights.  In particular on page 20:

"The upper stage(s) of HPR rockets must be using electronic recovery, and not rely on motor ejection."

That's covered; the EggTimer TRS will be in charge of the chute deployment.

And then:

"If required, make sure that the staging electronics have a feature to inhibit staging events if the rockets flight profile does not follow expected behavior."

That's where I get stuck -- the "if required" phrase.  I can't tell if it's required or not.

If it matters, this staged rocket is made of 2 L1 rockets that we've flown with H and I class 38mm motors, and that what I want to use for the staged flight.  I have two I-180W motors on hand so I could use those, or perhaps get an H class for the sustainer if that would improve safety.

Once again I appreciate everyone's input, and I'm not trying to be a pain.  I'm trying to avoid 2 things: either buying something (like another altimeter) that I don't really need, and/or showing up in June and finding out I can't launch.


You're not remotely being a pain - I think it's a great item to discuss.

I was struck by the "if required" portion of the guidelines as well, and I don't think it's very far off from what I had stated previously.

If you'd asked me about 3 years ago, I probably would not have personally 'required' it for launches that I would have been associated with (as either RSO or overseeing CP3).  However, with failures that I've witnessed since that time, along with associated advances in electronics that help mitigate staging failures, I probably would 'require' it if I were asked to be associated with a project.

There are areas of the sport where we're not given leeway, and areas where we DO have some leeway -- this appears to be one of them.  I'm just suggesting how I might lean in this situation.

One other item that would perhaps come into play were I involved in this - is it a proven two-stage design?  In other words, I've you've purchased a two-stage kit that has been proven to work by others, that would tend to remove that factor from the equation in terms of it being a safe design.  The fact that you're experimenting with your own new design, with your associated 3D printed coupler, would tend to make me want to ensure that other safety factors were followed more cautiously.  I would say this of ANY new, unproven design - not just yours, so don't take it personally.

Thanks again for bringing this up as a topic, it's appreciated!

David Erbas-White


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