Ah, interesting. I was wondering how this worked. In the
rocketryforum.com post someone said "a PET2+ can't do it, but an RRC3
can". So I read the manual for the RRC3 and it said nothing about
detecting if the rocket is off vertical when it fires the sustainer.
But, it does have the ability to use up to 3 logic gates to trigger the
airstart, including ranges for velocity and altitude. So I see what you
mean -- you sim the flight, figure out how high you should be if things
go well, and set the triggers appropriately.
The Raven has even more features, and 4 pyro channels, but it's similar
in that the logic gating is based on velocity and altitude, and also
optionally barometric pressure, but not tilt. So for both altimeters
the "safe to airstart" is inferred from other data, not directly
determined by pitch/roll/yaw.
So ... suppose I replaced the PET2+ with an RRC3 (yes, I'm being cheap)
or a 2nd EggTimer (if I'm being really cheap and I like soldering) and
used the velocity & altitude gating to determine if it's OK to
airstart. Would that satisfy the safety requirements?
Thanks
Terry
On 5/13/2018 7:03 AM, Dave Peterson wrote:
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:You're not remotely being a pain - I think it's a great item to discuss.
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.
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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