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RE the 2011 FAA waiver in Amesbury
- SnowLion
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11 years 1 month ago #6534
by SnowLion
Replied by SnowLion on topic Re: RE the 2011 FAA waiver in Amesbury
Yup, all motor. You saw the beast @ the Textile Museum. Keeping it simple.
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- bobkrech
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11 years 1 month ago #6535
by bobkrech
I simmed the rocket at ~1250' with rasaero. The time to apogee is about 10 seconds so a 9 second delay is correct.
What's the diameter of the chute? IMO the 72" stock chute is too small for a 28 pound rocket giving at 24 fps descent. I think you need something between an 8' chute that will give a descent rate of ~18 fps to a 10' chute giving 14.5 fps.
If you located the CG 7.5" forward of the CP as per the instruction sheet, the rocket will maintain a vertical attutude even in wind, however you will be under chute for more than a minute with a safe recovery descent rate, so wind can be an issue even when the apogee is only 1250'.
Bob
Replied by bobkrech on topic Re: RE the 2011 FAA waiver in Amesbury
What did you do to the rocket to double the published kit weight?I am expecting to fly the Purple Pterodactyl for my L2 cert flight on April 27. I need the 15/15 rail since the beast comes in @ around 28 pounds. I plan on tossing it up on a CTI 1266J760 which should get it 800 to 1,200 feet up. The sims say a 9 second delay (the shortest for that motor) should be good but I think it'll be a little long. With the low altitude I don't think drift should be that much of an issue.
Just a heads up for the rail & any L2 peeps for witness(es).
See you all then.
I simmed the rocket at ~1250' with rasaero. The time to apogee is about 10 seconds so a 9 second delay is correct.
What's the diameter of the chute? IMO the 72" stock chute is too small for a 28 pound rocket giving at 24 fps descent. I think you need something between an 8' chute that will give a descent rate of ~18 fps to a 10' chute giving 14.5 fps.
If you located the CG 7.5" forward of the CP as per the instruction sheet, the rocket will maintain a vertical attutude even in wind, however you will be under chute for more than a minute with a safe recovery descent rate, so wind can be an issue even when the apogee is only 1250'.
Bob
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11 years 1 month ago #6536
by SnowLion
Replied by SnowLion on topic Re: RE the 2011 FAA waiver in Amesbury
Since the kit was pricey to begin with I decided to up the investment & go for the thicker fins & fiberglass air frame. That increased the weight & shifted the CG way aft so the thing became unstable. So I kept on adding nose weight until I was comfortable with the CG. I also called PML & they said the published 51" CP from the nose was wrong, it should be 56" However, RockSim said 51" so I added more weight. I think the true CP is somewhere between 51 & 56. So I'm now up to 28 pounds plus a bit. Now the 'chute that came with the beast is way too small so I ordered the 10' one. That should give me a decent rate of 15-17fps.
Does anyone want to but my 84" PML 'chute? Brand new!
I figure I'm now in it for about 7 big ones not counting the PRO54 stuff. It do get expensive as the toys get bigger.
I using 1,000lb test hardware for all the connections. The strap the PML supplied looked pretty robust so I kept that but I put a kevlar sleeve from Giant Leap over it to protect it from the charge.
Thanks for verifying my sim, it gives me more confidence I picked the right motor/delay.
Does anyone want to but my 84" PML 'chute? Brand new!
I figure I'm now in it for about 7 big ones not counting the PRO54 stuff. It do get expensive as the toys get bigger.
I using 1,000lb test hardware for all the connections. The strap the PML supplied looked pretty robust so I kept that but I put a kevlar sleeve from Giant Leap over it to protect it from the charge.
Thanks for verifying my sim, it gives me more confidence I picked the right motor/delay.
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11 years 1 month ago - 11 years 4 weeks ago #6537
by bobkrech
You really should read www.nar.org/pdf/launchsafe.pdf about launching in high winds. While almost everyone is aware of the 5:1 minimum thrust to weight ratio rule for high power rockets, many are not aware of the recommendation that the rocket thrust to weight ratio in wind should be the wind speed in mph. This prevents severe weathercocking by allowing the rocket to attain sufficient airspeed so the fins are not stalled when the rocket leaves the rod.
A J285 is a calm to light wind motor due to it's relatively low thrust. In winds in the 15 mph and up range if I wanted to launch I'd be using a H400 or I566.
Bob
Replied by bobkrech on topic Re: RE the 2011 FAA waiver in Amesbury
It all depends on the wind. What you are not accounting for in your simulations is the winds alot. If the ground level winds are blowing 20 mph, the winds aloft are likely to be blowing 40 mph or higher. If the winds gusting between 15-20 mph, the RSO is most likely going to limit any dual deployment flight to not more than 2500' and possibly less as from experience your rocket will not recover in the field.Are we still good to go to 4900' AGL on April 27? My J285 wants to know. Expecting 4100' altitude and 830' downrange at 20 MPH wind for recovery by dual deploy, closer if calmer. Seen a lot of rockets up the hill or in the swamp, so just askin'.
You really should read www.nar.org/pdf/launchsafe.pdf about launching in high winds. While almost everyone is aware of the 5:1 minimum thrust to weight ratio rule for high power rockets, many are not aware of the recommendation that the rocket thrust to weight ratio in wind should be the wind speed in mph. This prevents severe weathercocking by allowing the rocket to attain sufficient airspeed so the fins are not stalled when the rocket leaves the rod.
A J285 is a calm to light wind motor due to it's relatively low thrust. In winds in the 15 mph and up range if I wanted to launch I'd be using a H400 or I566.
Bob
Last edit: 11 years 4 weeks ago by bobkrech.
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- HPRnut
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11 years 4 weeks ago #6538
by HPRnut
Replied by HPRnut on topic Re: RE the 2011 FAA waiver in Amesbury
I will witness the L2 cert. Cone find me. I will have my Big Nuke and Hellfire with me. I am assuming you took the test. If not, I think Bill may have them.
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11 years 4 weeks ago #6541
by SnowLion
Replied by SnowLion on topic Re: RE the 2011 FAA waiver in Amesbury
TY. I took the test last year in July. See you in a few weeks.
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