TARC Altimeter Anomalies

[Faraday's Cage]

I'm currently participating in the Team America Rocketry Competition, and I've made it into the finals which take place on Saturday. The official (required) altimeter for TARC is the Perfectflite Alt15k. On the last few test launches I've done, there have been some odd anomalies near apogee. The anomalies could be a problem with the altimeter itself, or they could be a result of the ejection charge firing into the electronics bay. Since my team could potentially be disqualified if there is anomalous altitude data, I need to make sure that the anomalies aren't a result of my rocket design. I've attached a zip archive of the Rocksim file for my rocket, as well as the relevant Perfectflite altimeter data files (readable with this program) and images of the altitude graph. If anyone could offer advice as to why I am having anomalous results, I would be thankful.

-FC

View attachment Flights.zip

 

[n5wd]

I'm ...in the Team America Rocketry Competition,....there have been some odd anomalies near apogee. The anomalies could be a problem with the altimeter itself, or they could be a result of the ejection charge firing into the electronics bay.

Well, on the plus side, you've still got five days until you have fly. That said, I can't tell from your Rocksim file how you've got your egg capsule/altimeter situated to come down on the 15" chute. Is it separated from the booster?
If so, describe, please, from tip of nosecone to bottom of the assembly, how it's put together.

General question: if you have a bulkhead on the bottom of the nosecone assembly, did you seal the bulkhead well so that the charge can't get into the bay where the altimeter is? If so, how did you do that? And, is the altimeter on the nose cone side of the assembly or on the bulkhead side of the assembly? If lower in the assembly, where's the ports for the altimeter (to get a look at the outside air?).

 

[bobkrech]

It's a bit late to be worring about this now, and you may be worrying about something that may not be an issue.

You made the finals so I have to ask why you are looking at the plotted data. The plots are interesting, but TARC uses the beeped altitudes, so the plots aren't relevant.

To answer any questions we need to know.

• Have you changed anything after the qualifications?
• What were the beeped altitudes for the flights you supplied the data plots of so we can make a comparison?
• For those of us who don't use RS can you provide a drawing of your rocket and indicate the positions of the altimeter vents, and their number and diameters?
• How is the altimeter compartment sealed/isolated from the ejection charges?

Bob

 

[hardinlw]

Upward spikes in altitude indicate a rapid decrease in sensed pressure which would not be the case if your ejection charge was venting into the altimeter chamber. That would be a rapid increase in pressure or a negative altitude spike. Those spikes are so sharp I don't believe they are real. The altimeters have a filtering algorithm built in to eliminate sharp spikes, so I reiterate Bob's question: What altitude was beeped out? Does it match the peak of the spike or the "real" altitude (ignore the spike)? At the finals, the beeped-out altitude is all that will be used. During test flying, you need to download these traces to make sure the beeped out altitude is correct and that you don't have a design flaw that causes problems. Some things I'd look at are altimeter mounting (you don't want it bouncing around) and the location and condition of the altimeter vent holes. Ideal is 3 or 4 holes equally spaced around the circumference at least one body diameter behind any discontinuity (like the joint where the nose cone attaches). Holes should be perpendicular to the surface and clean, no ridges or globs of paint to disturb the airflow.

 

[Larry Curcio]

You made the finals so I have to ask why you are looking at the plotted data. The plots are interesting, but TARC uses the beeped altitudes, so the plots aren't relevant.

[/LIST]Bob

Hi, Bob.

Like... technology, technology, technology!

This rule really really has to be changed. (And I'm holding you responsible! ) The NAR already disallows the use of non-recording altimeters in record attempts. It should ignore altitude beep-outs (as, say, a *sane* person would) in TARC attempts.

The NAR should also require altimeter temperature correction for record attempts, and it should teach TARC contestants to do it as well - by requiring it be done.

FWIW, I'd argue that the plots are relevant, because they show what really happened. They could show you why your simulation was way off (or *that* it was way off). They could show you why your performance varied so much between the downward spike (which would have resulted in a valid altitude beep-out) and the upward spikes.

It ain't the contest, after all, it's the engineering.

Regards,
-Larry(Now if I had only listened in English class...)C.

 

[n5wd]

This rule really really has to be changed. (And I'm holding you responsible! ) The NAR already disallows the use of non-recording altimeters in record attempts. It should ignore altitude beep-outs (as, say, a *sane* person would) in TARC attempts.
The NAR should also require altimeter temperature correction for record attempts, and it should teach TARC contestants to do it as well - by requiring it be done....

It ain't the contest, after all, it's the engineering.

Hi, Larry. I'm obviously NOT Bob (I understand he's a lot better looking than I), but if you'll allow me MHO....

what you're asking for would be nice if this were a college engineering challenge that was designed around an engineering project course. But, it's not. It's middle schools and high schools, where you're competing with band, football, drama club, choir, AP classes, dance, basketball, debate, homework, YMCA after-school activities, Imagination Destination, cross-country, FFA, and kids who are discovering the opposite gender.

I'd say my TARC teams over the last four years have been relatively average - and those are just SOME of the other activities our team members have been involved in at the same time they're doing TARC. We're lucky to get one afternoon a week from the kids, and maybe, towards the end of the qualification time, a couple of weekend days. Maybe, if we're lucky, a couple of Spring Break days.

That means, out of the 30 weeks between getting started (we started meeting the second week of school, right after we had our parents meeting on the Thursday of the first week, and school starting on Tuesday of the first week), taking away a week for Thanksgiving, three weeks for Christmas/Winter Holdiay, a week at Spring break, and our week of snow days... that means we had a total of 24 weekly one and-a-half hour meetings to introduce the kids into rocketry, build two little 'uns and an egglifter (Quest courier) to get them SOME building experience and to go out and fly them a few times so they can see how rockets really work, introduce them to RockSim, talk about how to derive engineering changes from observed data, rocket design, parts ordering, construction, destruction, reconstruction, and general mayhem. And, you want to add another few hours of theory and application to the challenge?

Sorry, but I really think that measuring the altitude from the perspective of the standard altimeter is just fine for this application... even if it is something as non-technical as counting the beeps.

 

[Larry Curcio]

Hi, Larry. I'm obviously NOT Bob (I understand he's a lot better looking than I), but if you'll allow me MHO....

what you're asking for would be nice if this were a college engineering challenge that was designed around an engineering project course. But, it's not. It's middle schools and high schools, where you're competing with band, football, drama club, choir, AP classes, dance, basketball, debate, homework, YMCA after-school activities, Imagination Destination, cross-country, FFA, and kids who are discovering the opposite gender.

I'd say my TARC teams over the last four years have been relatively average - and those are just SOME of the other activities our team members have been involved in at the same time they're doing TARC. We're lucky to get one afternoon a week from the kids, and maybe, towards the end of the qualification time, a couple of weekend days. Maybe, if we're lucky, a couple of Spring Break days.

That means, out of the 30 weeks between getting started (we started meeting the second week of school, right after we had our parents meeting on the Thursday of the first week, and school starting on Tuesday of the first week), taking away a week for Thanksgiving, three weeks for Christmas/Winter Holdiay, a week at Spring break, and our week of snow days... that means we had a total of 24 weekly one and-a-half hour meetings to introduce the kids into rocketry, build two little 'uns and an egglifter (Quest courier) to get them SOME building experience and to go out and fly them a few times so they can see how rockets really work, introduce them to RockSim, talk about how to derive engineering changes from observed data, rocket design, parts ordering, construction, destruction, reconstruction, and general mayhem. And, you want to add another few hours of theory and application to the challenge?

Sorry, but I really think that measuring the altitude from the perspective of the standard altimeter is just fine for this application... even if it is something as non-technical as counting the beeps.

Suffice it to say that we ask the kids to do sims; we can certainly ask them to download the data and judge flights by what happened, rather than by ejection spikes.

If American kids are too dumb and preoccupied to do that, then we can have the contest in China.

 

[bobkrech]

Larry

I do not disagree with your basic premise, however for TARC 2011, the only metric is the beeped out altitude, nothing else. An investigation of pressure discontinuities for a team that is in the finals hopefully with the rocket in question is not going to accomplish anything useful at this late date.

The beeped altitudes have filtering to eliminate the effect of ejection charge spikes in a properly configured altimeter bay, but the rocket designer is responsible for locating the sampling holes where turbulence doesn't effect the pressure readings.

Years ago, NAR did look at the accuracy and repeatability of commercial altimeter. IIRC the results were published in SR. Of all altimeters in the price range acceptable to TARC age competitiors, Perfectflite was the only mass produced in sufficient quantity (over 1000 on the shelf) altimeters that were accurate and precise enough for competition.

Provided that all altimters are precise, they should be repeatable from launchto launch, so that a rocket tuned to a reading of 750' should always go back to that reading, independent of whether it's actually 745' or 755'. On the day of the finals, every competitor has nominally the same weather conditions (not always true) but if their rocket is tuned into 750', and they can compensate for temperature and wind, they should be able to tune the rocket to hit 750' within the repeatability of their motors.

Unlike TARC, an altitude record which occurs at ??? ASL location, on any day, with a variable temperature and barometric pressure. Temperature compensation to correct the barometric altitude should be required, but to do this you need to measure the outside air temperature, not the payload temperature.

Bob

 

[Larry Curcio]

Larry

I do not disagree with your basic premise, however for TARC 2011, the only metric is the beeped out altitude, nothing else. An investigation of pressure discontinuities for a team that is in the finals hopefully with the rocket in question is not going to accomplish anything useful at this late date.

Bob

Yup. I agree. Never contested that.

The beeped altitudes have filtering to eliminate the effect of ejection charge spikes in a properly configured altimeter bay, but the rocket designer is responsible for locating the sampling holes where turbulence doesn't effect the pressure readings.

Bob

Sometimes the filtering works, too.

<< Good stuff deleted>>

Temperature compensation to correct the barometric altitude should be required, but to do this you need to measure the outside air temperature, not the payload temperature.

Bob

Still agree, but don't see the problem with measuring outside air temperature at the launch site. That is, you have to have a high altitude flight before nonstandard temperature lapse with altitude makes a difference, so launch site temperature is sufficient.

BTW, not to be too hard on American kids. The team that posted the data certainly downloaded it and interpreted it intelligently. They belie the notion that I'm asking too much of TARC teams.

Regards

 

[kjohnson]

Not every team has the resources available to do this. We are talking about a very broad cross section of the country here from suburban to rural, affluent to cash strapped, and everywhere in between. Requiring this would limit the participation in the event. It's hard enough for some teams to make it financially the way it is.

kj

 

[Larry Curcio]

Not every team has the resources available to do this. We are talking about a very broad cross section of the country here from suburban to rural, affluent to cash strapped, and everywhere in between. Requiring this would limit the participation in the event. It's hard enough for some teams to make it financially the way it is.

kj

I'm sorry. I must disagree again. The kids are required to do sims. They can download data - if not on the field, then in a library.

My real beef is with rocket education programs in general, and not TARC, and I can see I'm getting nowhere in this thread.

I stand down.

Regards,
-LarryC

 

[n5wd]

I'm sorry. I must disagree again. The kids are required to do sims. They can download data - if not on the field, then in a library.

Larry, you're wrong - the kids are NOT required to do sims. No where in the rules, nor no where in the competition (unless you're going for one of the presentation prizes) do you REQUIRE sims... it's merely a real good thing to do. But not all teams choose to do it. Do I, as a sponsor, tell a team that's not using Rocksim that they can't play? Nope. It's not required, but merely a real good (with all the emphasis in the world that I can muster) idea.

Whether or not it's a good thing, or bad thing, this year even though we (I a NAR/TRA L2 and our mentor team, a NAR/TRA L2 and an NAR L1) demonstrated it, taught it, suggested it, and did everything but plead with the kids to do it, not one of our teams used Rocksim the way we wished they would. I'm sure the kids had a lot of reasons why they chose not to do it -what matters to them is that one of the teams is flying this weekend, competing for the $60,000 in prizes. Yeah, now that we've done the after-action briefings with the teams that didn't qualify, they're beginning to see what they missed. But, whether it's a real good idea or not, using any computer software is not required for TARC. If you want to change that, send Trip a letter, and good luck with that.

My real beef is with rocket education programs in general, and not TARC,

Yeah, if I was teaching a course in engineering using rocketry, there'd probably be a lot of simulations cross-referenced with real world data. That's not what TARC is, though, even though we wish it were more like it than less.

and I can see I'm getting nowhere in this thread.

What? Because you couldn't convince us that it was necessary? Again, it's the rules of TARC that you're fighting, not us.

 

[Larry Curcio]

Larry, you're wrong - the kids are NOT required to do sims. No where in the rules, nor no where in the competition (unless you're going for one of the presentation prizes) do you REQUIRE sims... it's merely a real good thing to do. But not all teams choose to do it. Do I, as a sponsor, tell a team that's not using Rocksim that they can't play? Nope. It's not required, but merely a real good (with all the emphasis in the world that I can muster) idea.

Whether or not it's a good thing, or bad thing, this year even though we (I a NAR/TRA L2 and our mentor team, a NAR/TRA L2 and an NAR L1) demonstrated it, taught it, suggested it, and did everything but plead with the kids to do it, not one of our teams used Rocksim the way we wished they would. I'm sure the kids had a lot of reasons why they chose not to do it -what matters to them is that one of the teams is flying this weekend, competing for the $60,000 in prizes. Yeah, now that we've done the after-action briefings with the teams that didn't qualify, they're beginning to see what they missed. But, whether it's a real good idea or not, using any computer software is not required for TARC. If you want to change that, send Trip a letter, and good luck with that.



Yeah, if I was teaching a course in engineering using rocketry, there'd probably be a lot of simulations cross-referenced with real world data. That's not what TARC is, though, even though we wish it were more like it than less.



What? Because you couldn't convince us that it was necessary? Again, it's the rules of TARC that you're fighting, not us.

You're right. Which is to say, I'm sorry I wrote in a way that must have been offensive to you, and you are right to write this way in return.

You have my sincere apolgies.

Regards,
-LarryC

 

[Zeus-cat]

I have seen these spikes in my Alt15K data too. They are caused by the ejection charge going off. Here is a small chart of the spikes that I have seen. I set my altimeter bays up with 3 or 4 holes evenly spaced around the bay.

As you can see it is slightly more common for me to get spikes (57%) than to have clean data (43%). Most of these flights were on the same rocket with the same altimeter. So sometimes they were up, sometimes down and sometimes smooth.

Spikes
7 Up
12 Down
1 Both up and down
15 No spikes