BLUE MOUNTAIN ROCKETEERS DISCUSSION FORUM

Is there anyone out there that has ground tested their MAWD altimeters? Actually what I am looking for is someone that has successfully ground tested the MAWD altimeter. I have several times unsuccessfully tested two! Here is the deal. I have set up two NEW Perfectflite MAWD altimeters in a 5 inch rocket. I tested them one at a time with the same result. In both cases upon power up the proper mach and main settings are beeped out as was the last flights altitude. (In this case it was the altitudes from the factory.) Then in both cases I got the re-cycling 3 beeps telling me that both the apogee and main charged have continuity. Ok, here is where I’m having an issue. What I am trying to do is to create a “rapid decrease in pressure” to simulate a launch condition. What I decided to do was to put the rocket on it’s side, level with the ground then I armed the altimeter. After it powered up and got to the re-cycling 3 beeps I placed a shop vac first to about 2 inches from the vent holes and turned it on. After letting it run for several seconds I turned it off, nothing. It was still doing the 3 beep thing. I moved the vac closer and closer repeating this procedure with the same result, nothing. I ended up with the vac right up to the side of the rocket with the same result. I repeated the same with the other altimeter with the exact same results. I read the owners manual (no mention of a “G switch”) and it said that this thing only needs to sense a difference of 160 feet of altitude to arm itself. So what gives? Does anyone have a better Idea on how to ground test these things?
Thanks!

Cameron

As long as you are getting 3 beeps the altimeter is reporting armed with continuity on both charges and waiting for launch. Once launch is detected, the beeping will cease so that's what you're looking for as an indication that you have created an adequte pressure decrease.

It is fairly simple to put the altimeter in a sealed container with a hose for suction (and subsequent release to allow pressure to increase). I like using little Christmas bulbs to test my electronics (need a clear container or at least a a viewport). However with the 'new and improved' bulbs that allow the rest of the string to work if/when a bulb burns out, you will get continuity even with a 'bad' bulb so be aware of that.
I also like to perform this test when connected to a computer in test mode, to confirm the outputs are functioning.

For the most accurate testing, you should use live e-matches (or whatever you plan to use for eject charge inititation (no need for actual charges)) with the electronics mounted in the avionics bay and then the bay installed in the airframe (included any/all sealing that is designed to prevent eject gasses from getting into the AV bay).

Plug up all but one of your altimeter vent holes and then attach a hose to that remaining hole.
I find that flexible rope caulk works well but you can use anythign that will provide a seal.

Now, arm your electronics, and apply suction (it doesn't take much to arm the altimeter).
Once the beeping stops (liftoff detected) you can slowly decrease suction and allow the pressure to equalize within the AV bay. The better job you do here, the more 'delay' between apogee and main fire events.
Set your main deploy altitude low to allow more flexibility in this regard.

Don't forget to set Mach delay to zero or you'll have that timer to deal with while sucking.

If you cannot get the altimeter to arm, it could be the altimeter but most likely you have a leak someplace.

In flight, these leaks are evident by 'spikes' at apogee and/or main deployment events.

Hey Bob thanks for the input.
I ended up using something similar and did successfully test both MAWD altimeters! I took the electronics board out of the rocket and placed it into a modified almond jar from Costco. Some fittings, some tubing and some negative pressure supplied by your truly and wa-la! Both altimeters "fired" the Christmas bulbs! Ye-haw!

OK, so the things work, I still wanted to see that the altimeters would fire not only the main ejection charges but also just wanted to verify that these MAWD altimeters would pop this release device that I call the “Tender Descender” (similar in theory to the Tether) which was using an Estes igniter. WOW! Guess what? Anyone that tells you that ground testing everything is a good idea, you should listen to them!

I got the altimeters to fire the main charge every time I tried. I did find though that the charge that was needed to eject the nose cone was over 10 times that which was calculated, as I am using two size 4 nylon screws as shear pins to hold the nose on. The real eye opener was the absolute fact that the MAWD will not fire an Estes igniter! I tried in and out of the rocket. I modified my vac chamber and I put the Christmas bulbs back in to verify that the MAWD’s were still working. The bottom line here is that the MAWD’s are a low current device and even with new 9 volt batteries they will not pop an Estes igniter! Back to the drawing board!

So it seems that the bad news is actually the good news. The bad news is I had to scrub the launch at Brothers this weekend. The good news I that I did not throw this thing 3500 feet into the air and have the mains not deploy!

Hmmm, looks like I will have to become a TRA member in order to fly this thing up at Mansfield in August…

Thanks again,

Cameron Tinder

Yea, those estes igniters do require a lot of current to get them going!

I'm curious as to why you chose #4 nylon screws and why just two of them?
I've found #2 nylon screws to work well on my rockets (54mm - 98mm).
I also prefer 3 (though 4 could/would work too (just be 'stronger')) as just two screws could allow the N/C to 'wobble' and potentially bind. Using 3 or 4 screws will hold the N/C straight.

As for needing 10 times the calculated force, I'm curious how you are calculating your charges.
Smaller diameter rockets will require significantly more PSI in order to generate desired lbs-force on the N/C.
Of course any/all leaks in the airframe will make a huge difference as you need to generate adequate lbs-force before leaks can bleed off PSI.
I typically look for between 150 and 200 lbs force (ground testing starts at a theoretical force of 100 lbs).
In a 1.5" airframe, 56 psi is required to generate 100 lbs-force but go up to a 4' airframe and just 8 psi will generate the same 100 lbs of force on N/C.

I picked up a grain scale (used for BP firearms (15 grains/gram) as it allows higher resolution when measuring charges.

As you stated, ground testing is the ONLY way to confirm your charges are adequate.

Hey Bob,

I am using the #4 nylon screws because that t is the smallest the local hardware store carries. If I had smaller screws I might have gone with 3 or even four of them. Since I have some time, I may go on line and see if I can find the smaller screws and use more to prevent possible binding, though I do not believe that was even a factor here.
As far as the calculated charge goes, I used a program called Rocketry Tools by Jack Anderson (WWW.rockethead.net). The rocket in question is a 5” diameter rocket. The section that comes apart is about 4 inches long but is fairly tightly packed with parachutes. It is for this reason that I called it 2 inches long. Using these numbers and saying that I want 100 pounds of force it tells me that I only need .1 grams (1.5 grains) to pop the top. Well, I started with 3 ½ grains and worked my way to 20 grains (1.3 grams) before the top was popped at all! OK, in all fairness to this test, I was using 3f powder and the charge was between parachutes (Possibly cushioning the blast?) and the nose cone, though snug, is not that tightly sealed. Anyway, I am changing the way that I am packing the chutes so I will be retesting everything as soon as I get more of these low current matches. Again, the lesson learned for me is one that I already know, and that is to just simply test everything variable even though “I know it will work”!!
Just a side note here this rocket is a somewhat modified Jay-Hawk of carbon fiber construction that is almost 8 feet tall and is fitted with an onboard video camera. Total weight with the J275 motor, video camera and 4 parachutes is 13.25 pounds! This thing is set up as to video the ground until apogee. When the nose is popped, the shroud is removed along with the mirror, so that the rocket will then be able to lookout at the view! Yes, just another reason to make sure that everything works!
Best regards,

Cameron

I think you'll be well served to hunt down some #2 nylon screws (McMaster Carr is a reliable source).

Nothing wrong with 3F powder (biggest reason to go with 4F is for motor eject when/where you need to ensure BP gets down through that little hole).

One personal note I have is to use the calculator but never use less than 1 gram (15 grains).
I checked a D12 once and found approx 10 grains, 29mm AT reloads come with 1 gram.

Another interesting item is the affect of laundry. My gut instinct is that laundry takes up space therefore less charge needed but in practice I've found that the laundry somehow 'absorbes' some of the force. It is very important to pack for ground test just like you would for flight.

Since I don't like to eject with too much force, I have adopted redundant electronics on all projects 54mm and greater. I use a comfortable primary charge and then a much larger (50% - 150%) for back-up.

I also got some feedback that an Adept staging timer with 9 VDC NiCad will push enough current to ignite an estes igniter. Definitely something to test yourself before just trusting.

One other MAWD tidbit:

On older units (I have firmware 1.0/software 1.0 (which counts as old;-)), If power is interrupted during data collection, the unit will stop overwriting the last flights data .
When you download flight data, you’ll get current data up to where the power is interrupted then ‘jump’ to whatever data was stored from the previous flight.

The latest firmware will report an even 20,000 if/when it experiences a power drop out.