On Wed, 20 Nov 2002, Andrew Case wrote:
> On Wed, 20 Nov 2002, Randall Clague wrote:
>
> > Last night at the KISS III redux BBQ, we rejected the idea of testing
> > devices made for HPR use, since they're black boxes.
>
> I see. I thought the baro gave an output something like a voltage
> proportional to sensed pressure, which is then used to determine
> altitude. If it's a black box that pops the chute at some preset
> altitude without giving some measurable signal you can monitor to
> see if it's affected by vibration then the story's completely different.
>
> If I understand correctly what the baro is doing, it measures pressure
> at some sample rate, and detects apogee when the measurements stop
> going down and start going back up again. If this is the way it works,
> then it might get spoofed at MECO if the location where the baro samples
> pressure has high speed air flowing past it[1]. This creates a low
Actually, barometric altimeters have been used in "high power" (hobby)
rocketry for some time and they are fairly well characterized. A single
hole in the airframe works pretty well as a static port, as long as it is
placed away from obstructions on the vehicle exterior and is several
calibers (body diameters) away from any changes in diameter, such as the
nose or a fairing.
All of above is true until the vehicle nears Mach 1. At this point, air
pressure on the surface of the vehicle will suddenly increase and trick
the barometric altimeter into thinking it is decending. Hence, the reason
for using accelerometer-based apogee detection.
I realize most of the readers of this list are well aware of all of this,
but I think it bears repeating because some have expressed concern about
the reliability, testability and usefulness of these devices. In addition
to being an amateur rocketeer, I am a high-power guy, and I have seen
these devices work reliably in rocket flights at every monthly launch!
I have also tested both the accelerometer and barometer in my units
several times. If I wanted to vibration test under high altitude
conditions, I would probably go to Radio Shack and buy a woofer speaker
small enough to put in my bell jar, and rig a fixture out of epoxy and
fiberglass to mount the unit to the cone. Voila! Vibration under vacuum!
I guess my point is that these units are actually pretty damn good, but
you just need to spend a little time with one to learn its capabilities.
I have flown the G-Wiz, AltAcc and RDAS. The RDAS has an excellent flight
recording system, but the algorithm for detecting launch may not detect
launch on a hybrid propelled rocket because of thrust oscillation. The
G-Wiz and AltAcc will do better, but the AltAcc is a bit sloppy on apogee
detection. The G-Wiz used be consistantly late in detection, but recent
units have better firmware.
The makers of RDAS designed their equipment to be updated by the user, so
one can download and update the firmware at will. They also included
breakwire launch detect in the unit so that it can be flown with hybrids
or other blowdown motor systems which experience combustion instability.
Some folks find this to be more trouble that it's worth, but I like the
high data rates and extra data collection channels available on the RDAS.
Heck, the Gummit uses the RDAS in missile test programs! But, for plain
ol' rocket flyin' I fly the AltAcc.
I gently suggest that those engaged in experimental work not add to their
workload through mistrust of unfamiliar equipment.
-Dave McCue
Fun fact: a Mityvac vacuum pump from an auto parts store or Harbor
Freight Tools and an old jelly jar with a hose barb in the lid make for a
great barometric test system! Use the brake fluid collection jar included
in the kit as a vacuum reservoir, and valve the air out of the test cell
(jelly jar) at the rate desired. A Tee connector and a second valve allows
the controlled reintroduction of air.
Cheesy ASCII art below:
Mityvac fluid jar valve Tee & jelly jar valve
|------| ____
| |----------| |------------X------------t------------------X---
|------| |__| __|__
| |
| |
| alt.|
| in |
| jar |
-----
Proportional font people will be lost, I'm afraid...
_______________________________________________
ERPS-list mailing list
[EMAIL PROTECTED]
http://lists.erps.org/mailman/listinfo/erps-list
