I had the same confusion as well until it was made apparent that the
protections are against a sustainer motor starting in a horizontal or downward
facing orientation. Slightly off vertical or high dispersion flights are not
what the guidelines are concerned about. With a Raven or RRC3 you can program
the sustainer start using simulations to predict how high the sustainer is at a
particular point in time in a "reasonably vertical" flight. The Eggtimer can
sort of do this via the launch detect altitude (LDA), but then you cannot use
it for recovery because LDA may not be reached. The simulations are better if
they can be matched up with some actual flight data of the sustainer alone or
the stack flown without actually starting the sustainer. The later often
happens unintentionally as the sustainer start criteria are dialed in to match
the simulation.
On May 12, 2018, at 11:29 PM, David Erbas-White <derbas@xxxxxxxxxxxxxxxx>
wrote:
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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