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Same engine, same rocket, WAY different altitude. . . Help please!

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CrocketRocket

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Launching slightly a modified Wizard and Yankee rocket with payload bay and an Estes Altimeter. Getting bizarre altitude readings.

Wizard (B4-4):
910 ft
1,038 ft
699 ft
620 ft

Yankee (B6-6):
986 ft
670 ft
603 ft
1024 ft

Same day. Perfect weather. No wind. I would expect the altitudes to be more consistent. Is this kind of variation normal? Or is my altimeter giving me inaccurate data?

Open Rocket predicts my modifications should cause the rockets to be in the 575 ft max altitude. So I find the 603-699 fights to be plausible. Is it possible for some flights to randomly coast 300 - 400 feet higher? Does that happen?

(First time back into Model Rocketry in 30 years)

Thanks
 

kuririn

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I assume you put vent holes in your payload bays to equalize barometric pressure for the altimeter?
 

dpower

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Unless there was severe weather cocking (off-vertical Flight) in some flights, but not others, altitude won’t vary that much. Is the altimeter in the payload bay? If so, how many holes are in the bay and how large? Any obstruction between the holes and altimeter?
 

BEC

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OK - first of all, the Estes Altimeter is a really tight squeeze into BT-20, so we really need to see your installation. Because it fits so snugly, I really wonder about what your static port venting looks like.

Also, the Estes Altimeter NEVER locks in a reading after it reaches apogee. So if it ”sees” a lower pressure at any time (say when pulling the nose cone off the payload compartment while accidentally blocking the static ports with your hands) you will fool it into thinking it’s gone higher. I am guessing that your over-1000 foot reported altitudes and maybe the over-900 foot altitudes were caused by accessing the altimeter after the flights while pulling a bit of a vacuum on the payload compartment. Depending on where the static ports are you could have even caused that by just pulling the altimeter out far enough to see the display since it is such a tight fit in BT-20.

So.....bottom line, we really need to see your installation, and no, you should not see such wild swings in altitudes on the same motors unless the flight path deviated significantly from one flight to another as dpower suggested.

Oh...and welcome to TRF!
 

kuririn

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Bernard, actually the Estes altimeter doesn't fit a BT-20.
At least not the older version I have.
BT-30 minimum, and that leaves very little side clearance.
 

BEC

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That was my first reaction when I read the OP. So I went back into the hobby room, grabbed an Estes Altimeter and a BT-20 model and....it can be done. But it’s really snug. It deforms the tube a bit, but it goes in.

First pic is an Estes Altimeter part way into the payload section I created at the top of the sustainer of a Sterling Siver. Second picture is end on, with the altimeter all the way in. Third picture is the altimeter in the payload section of a Ghost Chaser, which has a PST-20 payload section. I had to use needle nose pliers to pull it back out again, but it can be literally squeezed in.

But because of that, it will be really easy to block the static ports upon installation or removal, even quite unitentionally, and assuming they are there at all.....

5E387CEC-B940-4C4B-AE2E-D360835D3C50.jpeg
87EB181A-6334-4DDF-AE81-107B4E94FDD1.jpeg
9A8DE335-CE62-4F28-ACF4-A57DF92DA19C.jpeg
 
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kuririn

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Yep, you're right Bernard.
I can see where a body tube snug against a sampling port could lead to erratic readings.
For the OP a Flight Sketch Mini will fit in a BT-20 easily.
I think they're going for around $40 nowadays.
1128202347[1].jpg

Laters.
 

CrocketRocket

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AWESOME help and info guys!

For those original flights. . . I had sanded the plastic casing of the Estes Altimeter to give easier clearance for the BT-20 body tubes. And I had 3 vent holes drilled towards the bottom of payload bay.

Flights were super straight and vertical, no weather cocking.

I think you guys are right. I'm probably sometimes creating a vacuum when the altimeter is pulled out. I was using small wire tool that allows me to easily hook and fish out the altimeter quickly. I guess I fished it out too fast, or maybe blocked the vent holes with my hand.

I will remove the altimeter more slowly. For my next flights I will remove the front plastic faceplate of the altimeter. This will allow air to more easily pass around the altimeter, and not create a vacuum when being removed.

Thanks for your help. It's exactly what I needed to know!
 

CrocketRocket

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3 holes drilled in bottom of payload bay. each hole =1/8" diameter. 120 degrees separation.
 

BEC

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So that's plenty of venting even for the Estes Altimeter, which is fussier about this than most....but I do suspect you probably just wrapped your hand around them to hold the section while pulling the altimeter out. Since the Estes Altimeter is unique in this not-so-great feature of not locking in the peak altitude data after a flight, even after it's been on the ground for awhile, it is uniquely susceptible to this issue.

The FlightSketch Mini, the PerfectFlite Firefly and the Altus Metrum MicroPeak are all really better choices for your application and they are all significantly lighter and so will reduce the performance penalty you are paying for carrying your current device. All three can give you more data, too, though only the FlightSketch Mini does it with a device you probably already have - an iOS or Android device. It is, however, a little more complicated to operate vs. simply turning it on and getting it in "ready" mode. The Firefly and the MicroPeak both are even simpler than the Estes Altimeter in that all you have to do is turn them on and wait a bit, and they are ready to go. No mode changes required. But the tradeoff there is that you have to count LED flashes to read out the result.
 

BABAR

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Anybody flown the Estes with another altimeter to compare reliability? Of course, I guess you’d need three altimeters, as if two differed you wouldn’t know which is wrong.
 

kuririn

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Anybody flown the Estes with another altimeter to compare reliability? Of course, I guess you’d need three altimeters, as if two differed you wouldn’t know which is wrong.
And if three differed you'd need four. And if .........
 

Zeus-cat

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I gave up on my Estes Altimeter; it never worked for me. I was impressed that he got any readings; even if they were bad.
 

BEC

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Anybody flown the Estes with another altimeter to compare reliability? Of course, I guess you’d need three altimeters, as if two differed you wouldn’t know which is wrong.
Yes, I've done that sort of thing with a bunch of different altimeters, including the Estes unit. If it detects a launch, and if one doesn't confuse it as we've been discussing by pulling a partial vacuum on it getting it out of a payload section, then it gives reasonable results - within a few feet, generally less, of others flown on the same flight.

It also helps to fly models for which one has lots of data when doing comparisons. For me it's the Nova Payloader. Across five examples of the model I probably have over 300 instrumented flights. So a real outlier is easy to spot.

And if three differed you'd need four. And if .........
I have some flights I did for my R&D for NARAM-60 in which I flew 9 altimeters in one model....:eek:


In general, even if the same make and model, they don't agree exactly on a given flight. There is less scatter, device to device of the same type, with PerfectFlite devices in my experience. But I figure a single-digit number of feet one way or the other on a flight to several hundred is pretty good agreement.
 

CrocketRocket

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Thanks everyone. I made the modifications, and got consistent altitude data.

621ft
626ft
679ft
677ft
611ft

So . . . much better this time.

Removing the faceplate off the altimeter made access way easier too.

Thanks for the advice.

I'm including pics
 

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BEC

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Yes, that is much better. Were the two that went higher from a different date code than the others, assuming otherwise all the same designations? Or perhaps those were longer delays allowing longer coast times?

Those values all look plausible for that model on an Estes or Q-Jet B.

Interesting to see that altimeter seems happy without the top case. Good to know.
 

CrocketRocket

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Short answer:
Slight weather cocking probably caused the shorter altitude flights in some of today's launches. All of today's launches were on new B6-6 engines bought last week.

Long answer:
Date codes? Actually, that's what created this whole mess in the first place.

My original launches were on a Wizard modified payloader using OLD Estes B4-4 engines. (OLD = got them at a yard sale 10 years ago). WAY high at over 1,000ft and 900ft. . . when Open Rocket was predicting 575ft. At first, I thought I must be an amazing rocket builder, coaxing awesome bonus altitude during the coasting phase!

Nope! The next two flights, were only in the 699ft-620ft range = VAST decrease for the same but newer B4-4 engines. The field I was launching in was very rough with scrub brush and wild grasses. The standard streamer was having a hard time dealing with the additional weight of the payload section, and the Wizard was always coming in hotter than I wanted. 25 meters per second! Maybe I damaged the rocket in some minor way, ruining its aerodynamics? I didn't know.

So I built two Yankee rockets to handle this same payload. The longer root cord of the Yankee, I felt would handle the hard landing a bit better than the Wizard's tiny fins. The Yellow and Orange Yankee Rocket's sustainer sections both weigh the same at 11g (nosecone not included). Their fins were made in the same batch. I cut new extra large streamers 1.75" x 60" from party tablecloth.

Flew those Yankee rockets a bunch of times on the new B6-6 engines. Yet again, most flights were in the 600ft range, but the Yellow and Orange rocket each had one bizarre flight that recorded an altitude in the 986ft, 1024ft.

That's when I wondered if Estes engines vary in their performance. . . and asked the forum.

But no. It was the vacuum issue ruining the data.

Today's launches were consistent. Removing the cover from the Estes Altimeter goes a long way to helping the altimeter fit in the BT-20 18mm tube, and seems to reduce a vacuum forming during removal. NOT accidentally covering the vent holes with my hand, is also VERY, VERY important.

Thanks everyone.
 

Rocks&Rockets

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I flew two Flightsketch Mini altimeters together in the nose of my 3" IRIS this weekend. Although one lost power just after apogee, they agreed within 1 foot altitude, 1188' & 1187'. Even after battery failed I was able to retrieve the data that had been saved... VERY happy with mine. Hoping FS comes out with a deployment version soon. Winter seems to be our new flying season around here. Better stock up on those little batteries.
 

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BEC

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I‘ve seen good consistency when flying multiple FS Minis together as well.

Usually when they chop off the data like that they’ve taken just enough of a shock to jar that little coin cell in the holder. He’s tweaked the design a couple of times, but it still occasionally happens to me. It’s usually at ejection. It’s hard to tell on your two screen shots if that’s the case, but I see in looking at your flights in the online log that it was after ejection some. Maybe they were rattling together inside the nose cone?
 
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Joshua Smith

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I have to +1 the Flight Sketch Mini. I lost mine on it's maiden flight but was still super impressed. I just noticed this thread and saw that they were back in stock, just ordered 2. The Bluetooth experience is great, the app is well-featured yet simple. You just turn it on, tap ready for launch in the app and ur ready to fly. Like @Rocks&Rockets I can't wait for the deployment capable version as well. Covid kinda slowed things down as you might expect
 

BEC

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I mount them down flat on a velcro pad.
Seems secure enough given the slight mass. Didn't come off. Methinks bat contact bounce at deployment.
Yeah, it’s kind of hard to imagine what caused the “contact bounce” with them mounted like that....but that’s certainly what the truncated data trace shows. The reason I was wondering is because it looked like it was past ejection, based on the data trace. Too bad we can’t see the accelerometer data in the online logs yet. Then it would be pretty apparent. I know Russ will get there eventually. And it’s why I’ve started taking screen shots of the acceleration info from the app and posting those as part of my uploads to the online log, at least sporadically.
 
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