TARC frustration: advice needed

[mikec]

You want to ejection charge to go off right at apogee, or a second after you've hit apogee.

Yes, but for TARC you want to hit the altitude and duration targets, and if you are hitting those targets it doesn't much matter when ejection occurs as long as it always occurs close enough to apogee that there's no damage to the rocket or the recovery system.

You were 269 feet short of the altitude target, so unless your ejection was way early you need a bigger motor, as you said. The F35 will help but it's lower-thrust so weathercocking may be more of an issue.

 

[boatgeek]

im sorry i meant 179 m which is 587 ft

179m is more plausible, but it's hard to tell in the video. It's something that's really easy to see in person if you have experience, though. Do you have a mentor helping you out?

Either way, definitely check that the altimeter is getting good air samples.

 

[firpisto34]

179m is more plausible, but it's hard to tell in the video. It's something that's really easy to see in person if you have experience, though. Do you have a mentor helping you out?

Either way, definitely check that the altimeter is getting good air samples.

my computer keys arent working I MEANT 779 FT. 80 ft from the goal

 

[afadeev]

Yes, but for TARC you want to hit the altitude and duration targets, and if you are hitting those targets it doesn't much matter when ejection occurs as long as it always occurs close enough to apogee that there's no damage to the rocket or the recovery system.

Right, unless it fires prematurely, and stops you from reaching your target altitude!
Basically, you don't want ejection charge timing to be a factor in reaching the desired altitude, but it will be a factor in the descent time.

It was straight, off the rail. Weather-cocking is the culprit, try tipping the launch rail about 5 degrees WITH the wind. That will compensate for the weather-cocking. Of course, the angle required will vary with wind speed.

Changing the rail angle will also effect the altitude.
You can adjust for that by removing some ballast, if you have it built into the rocket.
How much to remove? You will need to experiment to find out.

Higher speed off the rail, or adjusting payload (eggs) placement to produce CP (center of pressure) closer to 1.0 caliber (diameter of the airframe) off CG (center of gravity), will mitigate weather-cocking.

As for the actual length of the Delay, you really have no control over that.

Sure you do.
You can buy motors with different delay lengths, or drill existing delay grains to shorten them, if appropriate.
If you suspect that the delay charge fires before you hit apogee, you definitely need motors with longer delays.

a

 

[boatgeek]

my computer keys arent working I MEANT 779 FT. 80 ft from the goal

Ah, OK, that makes a lot more sense. If you can't lighten the rocket up, you'll want a little more power. The amount of weathercocking isn't that unusual, so you may as well plan for that if you can get the larger motor in time. You probably know this, but the F35 uses a different casing than the F39. You might also want to check your delays. Both of those motors have the same burn time so if the F39-9 is maybe popping a little early, the F35-8 will definitely be early. If you have an opportunity to do another launch, an observer as far away as possible can see better if the delay is before or after apogee.

 

[Ez2cDave]

If you have an opportunity to do another launch, an observer as far away as possible can see better if the delay is before or after apogee.

Binoculars ( I like 10 x 50mm ) really help.

 

[gshaiffer]

Thanks for the offer. Still probably won't happen, but I'll keep it in mind.

Neil
Some comments made as a TARC mentor since beginning. I personally believe the egg capsule shown in one of the post would not be allowed, would check with TARC officials before using one. Sounds like your team has made a good choice with the Apogee foam egg protectors inside a body tube. I would add that is is a very good idea to enclose the eggs foam assembly(ies) with a sealed plastic bag. Most teams twist the bag at the end of the package and use tape to seal, then cut off the excess bag. Foam is easier to clean than rocket body interiors. Don't give up, I saw a team make it to finals on pure luck, details will be omitted. Note that the rules list specific altimeters that can be used for altitude recording. additional altimeters may be flown but no
pyro allowed. Recommend for flight testing that clay equal to the weight of 3 eggs be flown until test show possibilities, they must fly some egg loaded
practice flights until egg packing is proven to return eggs with no damage. I saw one egg capsule fall from 900 ft with total 'chute failure and both eggs survived so it can be done. Adequate separation of eggs from each other required. Note that in addition to 3 eggs this year the egg capsule/altimeter bay assembly must recover on at least two parachutes of nominally the same size. Stop watches and two to 3 story stairwell can provide space to do drop test of capsule/parachutes decent speeds without burning motors. One team I was with was flying F-35 motors with foam centering rings...didn't work for their
rocket so they switched to Gator board Foam core with wood covering instead of cardboard. U-Line source makes it expensive due to minimum purchase
requirement but individual sheets may be available in your area. Only supplier in Colo Springs closed, had to buy from U-Line. Another mentor in the area had the team fly pill bottles with B-Bs as adjustable ballast. They knew the weight of a single B-B and could increase and decrease ballast buy simply counting for small adjustments. Single use motors are expensive but would stay with them for eighth graders...later might consider reloads. Same lift off weight every time important plus the more flights the more likely success. Do not depend on only flying with rocket sections..they don't launch often enough. Find a local land owner that will permit you to launch as a restricted team use (not public launch) and plan on launching every weekend and any
no class days that students are not in school (staff dev for example plus holiday closures). It asks a lot for parents to support this activity along with volunteer mentors but it has paid off in the number of students that have participated and gone on to engineering degrees resulting in aerospace employment or related jobs. We have one young man graduating this spring with Degree in Aerospace, another has been working this past four years
for model rocket company. Long post I know but a couple more comments. Open Rocket works but I believe RockSim from Apogee is better but takes time to master. There are RockSim tutorials available on Apogee website and there is a good discount for registered TARC teams to obtain RockSim. Also there is a cripple 30 day download (yes that term is not PC) demo version available. While at the Apogee web site sign up for the Peak of Flight news letter and take the time to download the past issues from the archive. There is nearly 460 issues but there is also a content file and if all issues are available then selective reading can be made. Some TARC specific, others on construction, or recovery, fin sizes etc. Mentioning fins Clipped delta seems to be one of the best choices, no fin material past the aft end TTW construction, fin can a replaceable module from body tube, teams using electrical tape to secure assemblies together.

George Shaiffer NAR 80201 L2
COSROCS
direct email [email protected]

 

[Steve Shannon]

One of the teams I mentored started out placing their raw eggs in ziplock bags, but later changed to Saran Wrap. It was perfect, fully conforming to the egg, yet fully sealing the mess when the inevitable hard landing happened during flights.
I think your statement about not allowing that egg capsule comes from this rule:
“Rockets may not be commercially-made kits designed to carry egg payloads with the only modification being the addition of an altimeter compartment.”
But that only refers to using entire rocket kits. It’s permitted to use parts from a kit or commercial parts purchased outside of a kit. That’s addressed in the FAQS:

You can fly a kit for practice and learning flights, but you must advance to an original design for your actual qualification and finals flights. For this official rocket you can use parts from several kits or any part you want from one kit, as long as you don’t use one complete kit with minimal modifications. We want to see originality and design effort go into the rocket that you compete with; it is a major learning objective for TARC.

https://rocketcontest.org/about-the-contest/faq/

 

[cwbullet]

Press and Seal might work better.

https://www.glad.com/food-storage/plastic-wrap/press-n-seal/

I have found it superior to Saran Wrap.

 

[neil_w]

Press and Seal is a great idea for this application.

 

[firpisto34]

quick question, can I pack an RMS system the night before?

 

[Steve Shannon]

quick question, can I pack an RMS system the night before?

Absolutely

 

[Ez2cDave]

Press and Seal might work better.

https://www.glad.com/food-storage/plastic-wrap/press-n-seal/

I have found it superior to Saran Wrap.

"Adult latex balloons" work well . . . Cut it to length, insert the egg, and fold it over . . . Presto !

Do not tie a knot in it, as that will create a "pressure point" that could easily crack an egg.

 

[Ez2cDave]

I think your statement about not allowing that egg capsule comes from this rule:
“Rockets may not be commercially-made kits designed to carry egg payloads with the only modification being the addition of an altimeter compartment.”
But that only refers to using entire rocket kits. It’s permitted to use parts from a kit or commercial parts purchased outside of a kit. That’s addressed in the FAQS:

Precisely correct !

 

[aerostadt]

I remember one TARC just took small sheets of foam wrapped around the egg and then just taped it up with package tape and put it in the payload section as is. Worked like a charm. I made an estimate for the TARC 2018 that the equivalent drop height without a parachute is about 7 feet.

Descent Impact Loads


Here is way to estimate or descent impact loads:

1) First, get an estimate of the descent rate. One way to do this is to go into the computer simulaton software like RockSim and choose a tube. Just use the "mass over-ride" option to assign the mass of your rocket. Then attach a parachute to the tube and look at your descent rate. This is how fast your rocket will fall with your chosen parachute diameter.

2) Now, you can estimate the equivalent drop height (H) for your rocket without a parachute by using the following formula:

H = 0.5 x ( (velocity)^2 ) / 32.2

where H is the drop height in feet, velocity has units ft/sec., and 32.2 ft/sec^2 is the acceleration of gravity.

For example, if your rocket has a descent rate of 21.25 fps, the impact will be the same as if the rocket was dropped from 7.01 feet without a parachute. Imagine dropping your rocket from 7.01 feet on a hard concrete floor or soft turf without a parachute. This will be the same impact load.

 

[firpisto34]

what should I use to attach our fins to the rocket, apogee says to use wood glue i think but I dont know if thats strong enough.

 

[dhbarr]

what should I use to attach our fins to the rocket, apogee says to use wood glue i think but I dont know if thats strong enough.

Woodglue is stronger than cardboard or wood.

 

[Ez2cDave]

what should I use to attach our fins to the rocket, apogee says to use wood glue i think but I don't know if that's strong enough.

If you use "wood glue", be sure to get the Waterproof Elmer's Wood Glue !

https://www.walmart.com/ip/Elmer-s-Carpenter-s-Wood-Glue-MAX-8-fl-oz/17011278

 

[boatgeek]

Titebond II is also a good wood glue. You want it to be yellow and not white.

 

[Ez2cDave]

Titebond II is also a good wood glue. You want it to be yellow and not white.

Agreed . . . Whichever "aliphatic resin" ( yellow "wood glue" ) you purchase, make sure it's Waterproof !

 

[firpisto34]

thanks to everyone that replied but I have another question. Our rocket flies to a good altitude but comes up a little short on time: around 39-40 seconds. We are currently using 2 12 inch parachutes right now and I think using the 2 15 inch parachutes will slow it down too much. What can we do to get the desired time without changing the altitude too much

 

[Steve Shannon]

thanks to everyone that replied but I have another question. Our rocket flies to a good altitude but comes up a little short on time: around 39-40 seconds. We are currently using 2 12 inch parachutes right now and I think using the 2 15 inch parachutes will slow it down too much. What can we do to get the desired time without changing the altitude too much

The descent speed is based on the area of the parachutes. Two 12s are too little area? Two 15s are too much area? The answer seems simple. Do something to increase the area of from that of the 12s or reduce the area of the 15s. There are several ways to do that. You can research reefing, spill holes, or making custom chutes. One of my teams had the idea to have chutes made by a family member who knew how to sew. They made a range of chutes.

 

[Ez2cDave]

thanks to everyone that replied but I have another question. Our rocket flies to a good altitude but comes up a little short on time: around 39-40 seconds. We are currently using 2 12 inch parachutes right now and I think using the 2 15 inch parachutes will slow it down too much. What can we do to get the desired time without changing the altitude too much

Basically, you need to increase the drag of your recovery system.

Here are a few ideas:

(1) Increase the size of one, or both, parachutes ( Remember that, this year, the parachutes must be within two inches of being the same diameter ).

(2) Change the "shape" of the parachutes you are using. ( This year, the "shape" of both parachutes must be the same - round, hexagonal, octagonal, x-form, etc. ).

(3) If you are using plastic parachutes, one, or both, of the parachutes could easily be increased in size or "trimmed down", as necessary. ( The rules do not say that the parachutes have to use identical materials. The only restrictions are that their diameter be within 2" of each other and that they have the same shape).

(4) If larger parachutes are used, the Descent Rate can be "fine-tuned" by "reefing" one, or both parachutes. ( Basically, "reefing" is temporarily "shortening" the shroud lines to reduce the efficiency of the parachute, by preventing it from opening to 100% diameter. Taping the shroud lines together, with a band of electrical tape is a good way to do this in a "non-permanent" and easily-adjustable way. RE-CHECK and VERIFY the location of the tape band. by measuring it, BEFORE and AFTER each flight, because it can SLIDE along the shroud lines, if not VERY TIGHT. Once you determine the optimum position for reefing the parachute, a KNOT can be tied in the shroud lines, instead of the tape - MEASURE IT, CAREFULLY ).

(5) Using a "Spill Hole" in one, or both parachutes, will decrease the efficiency of the parachute and increase the Descent Rate. This would permit larger parachutes and allow the Descent Rate to be adjusted.

These links may be helpful.

https://billsrockets.blogspot.com/2018/08/time-to-do-some-tarc-math.html

https://mostlymissiles.com/descent.htm

https://descentratecalculator.onlinetesting.net

Good luck to you & your Team !

 

[aerostadt]

Here a theoretical calculation that I used as a mentor for the TARC 2018 year to calculate parachute size. The biggest problem is knowing precisely what is the chute drag coefficient, which can depend on parachute shape, reefing (shroud length), etc. Someone else's parameters, assumptions, etc. can vary.

Descent Time = Flight Time - Burn Time
Descent Time = 43 - 6 = 36 sec.

Egg Payload Average Descent Rate = 800 feet / 36 sec = 22.22 fps

Of course, this is just one approach. Other approaches as mentioned above are available.

 

[firpisto34]

Here's a video of our most recent launch.


If you watch the first 8 seconds of the video (especially slowed down to .25 speed), you can see that the rocket kind of shakes or vibrates on the way up. Iv'e been looking on the internet and the only reason I found for this is "thrust oscillation" cased by the motor on the way up but the article I found that info on was talking about real NASA rockets, here's a link to it: https://www.technologyreview.com/s/414364/whats-the-deal-with-rocket-vibrations/

A theory I have is that the oscillation is caused by weathercocking but maybe its the opposite and the rocket isn't stable enough. Some people I have talked to also told me they've had this problem also so if anyone has any idea it would be really helpful

Also, if you keep watching the video after the parachutes deploy, you can see that the section with the eggs and the 2 parachutes twirls and spins while coming down which reduced time and actually caused one of the eggs to crack. Why is this happening and how can we avoid this in the future. Any ideas or contributions are much appreciated.

 

[aerostadt]

I am wondering how long is your rocket? I would call that fish-tailing and it might be due to wind. In the TARC 2018 competition I noticed one team made their rocket the minimum length and it looked to me that it fish-tailed a little bit and perhaps didn't go straight up. I noticed that another team made their rocket about a foot longer or more. I thought that it would not do well, but under similar windy conditions it looked to me that it went up much straighter with no fish-tailing. Afterwards, I thought to myself the longer rocket was better and that the additional weight of light cardboard is no big deal, if it produces a straighter flight. I don't recall which motors they used (I think both teams angled slightly into the wind), but I think a flight going up straight under windy conditions is better for consistency and, thus, better for reliably dialing in the target altitude.

 

[Ez2cDave]

Here's a video of our most recent launch.

If you watch the first 8 seconds of the video (especially slowed down to .25 speed), you can see that the rocket kind of shakes or vibrates on the way up. Iv'e been looking on the internet and the only reason I found for this is "thrust oscillation" cased by the motor on the way up but the article I found that info on was talking about real NASA rockets, here's a link to it: https://www.technologyreview.com/s/414364/whats-the-deal-with-rocket-vibrations/

A theory I have is that the oscillation is caused by weathercocking but maybe its the opposite and the rocket isn't stable enough. Some people I have talked to also told me they've had this problem also so if anyone has any idea it would be really helpful

Also, if you keep watching the video after the parachutes deploy, you can see that the section with the eggs and the 2 parachutes twirls and spins while coming down which reduced time and actually caused one of the eggs to crack. Why is this happening and how can we avoid this in the future. Any ideas or contributions are much appreciated.

I am wondering how long is your rocket? I would call that fish-tailing and it might be due to wind. In the TARC 2018 competition I noticed one team made their rocket the minimum length and it looked to me that it fish-tailed a little bit and perhaps didn't go straight up. I noticed that another team made their rocket about a foot longer or more. I thought that it would not do well, but under similar windy conditions it looked to me that it went up much straighter with no fish-tailing. Afterwards, I thought to myself the longer rocket was better and that the additional weight of light cardboard is no big deal, if it produces a straighter flight. I don't recall which motors they used (I think both teams angled slightly into the wind), but I think a flight going up straight under windy conditions is better for consistency and, thus, better for reliably dialing in the target altitude.

Good observations, gentlemen !

As for the spinning during recovery, I believe that one of the parachutes has unequal-length shroud lines, causing it to "spill". My ultimate "correction", after equalizing the shroud line lengths, would be to run a line up through middle of one of the parachutes and attach the second parachute ABOVE the first one. By arranging them in a "linear" manner, the descent should have far less oscillation.

As for the "fish-tailing", I believe that the rocket is "under-damped", being short, with relatively small fins . . . G.Harry Stine, in the Handbook of Model Rocketry, 7th Edition addresses this issue :

Dave F.

 

[afadeev]

A theory I have is that the oscillation is caused by weathercocking but maybe its the opposite and the rocket isn't stable enough.

If the rocket was not "stable enough", it would have veered way off vertical, or turned into a "sky writer":


But we are all guessing here based on video evidence.
The way to find out for sure is to build your rocket model in OpenRocket or RickSim, and compare the location of CG vs. CP. Have either you, or the team, modeled your rocket?

Also, did you actually fly an altimeter to measure the actual speed off the rail? In flight?
That data should give you answers to most of your questions.

Also, if you keep watching the video after the parachutes deploy, you can see that the section with the eggs and the 2 parachutes twirls and spins while coming down which reduced time and actually caused one of the eggs to crack. Why is this happening and how can we avoid this in the future. Any ideas or contributions are much appreciated.

Your rocket should not be spinning wildly under two fully opened chutes.
It looks like 1 of the 2 chutes did not fully inflate (time stamp 0.17-0.30), and the payload swung around wildly. Around 0.31 the second chute at least caught enough air to semi-inflate, but is still failing to provide equal coefficient of drag to that of the primary chute.

What was the delta between the modeled vs. observed descent speed under those 2 chutes?

As for the spinning during recovery, I believe that one of the parachutes has unequal-length shroud lines, causing it to "spill".

That is a good guess.
Could also be that the lines on one chute were tangled up.
Have both chutes been checked for construction abnormalities?
Did both land with lines relatively untangled?
Did you use a swivel on both chutes to keep the line tangling to the minimum?

My ultimate "correction", after equalizing the shroud line lengths, would be to run a line up through middle of one of the parachutes and attach the second parachute ABOVE the first one. By arranging them in a "linear" manner, the descent should have far less oscillation.

As anyone who has ever sailed will tell you, "taking" the wind away from the leeward sail / chute will reduce its effectiveness by a huge amount. This is great competitive advantage in sailing, but will also negate the effectiveness of the leeward (2nd level) chute.

You compensate for that by flying two larger chutes, but that simulating desired chute sizes unnecessarily difficult. Or just get two equal distant chutes to work properly, like NASA, SpaceX do with full size capsules (2-4 chutes in parallel), and these model Saturn V do as well:






a

 

[Ez2cDave]

As anyone who has ever sailed will tell you, "taking" the wind away from the leeward sail / chute will reduce its effectiveness by a huge amount. This is great competitive advantage in sailing, but will also negate the effectiveness of the leeward (2nd level) chute.

You compensate for that by flying two larger chutes, but that simulating desired chute sizes unnecessarily difficult. Or just get two equal distant chutes to work properly, like NASA, SpaceX do with full size capsules (2-4 chutes in parallel), and these model Saturn V do as well:

Yes, two larger chutes would offset being in the "lee" of the lower parachute. However, they don't want to increase mass significantly, if they can help it, due to altitude reduction concerns.

Actually, I should have mentioned before that the two "inline" parachutes should be separated by about 3ft, or so. The larger the parachutes, the greater the separation distance needed.

 

[boatgeek]

Here's a video of our most recent launch.


If you watch the first 8 seconds of the video (especially slowed down to .25 speed), you can see that the rocket kind of shakes or vibrates on the way up. Iv'e been looking on the internet and the only reason I found for this is "thrust oscillation" cased by the motor on the way up but the article I found that info on was talking about real NASA rockets, here's a link to it: https://www.technologyreview.com/s/414364/whats-the-deal-with-rocket-vibrations/

A theory I have is that the oscillation is caused by weathercocking but maybe its the opposite and the rocket isn't stable enough. Some people I have talked to also told me they've had this problem also so if anyone has any idea it would be really helpful

Also, if you keep watching the video after the parachutes deploy, you can see that the section with the eggs and the 2 parachutes twirls and spins while coming down which reduced time and actually caused one of the eggs to crack. Why is this happening and how can we avoid this in the future. Any ideas or contributions are much appreciated.

Does the rocket oscillate that way on every flight regardless of wind speed? If so, it's probably a minor fin alignment issue. As long as the rocket flies consistently, don't worry about it. If it is more pronounced in higher winds, I'd guess an oscillation damping issue as described above. Because this year's TARC has three eggs that tend to be near the nose, the rockets tend to have a pretty high pitch/yaw moment of inertia. That would tend to make them more likely to be underdamped.