Quick question about two RMS reloads...

[DrugCoder]

I bought a F52-5T and a F52-8T. The only thing that puzzles me about them is the max liftoff weight is way different. The 5T is listed at 32 oz. while the 8T is listed at 18 oz. If the only difference in the two is the delay, why would the max liftoff weight be so different? Is it just that way to make sure it deploys as close to apogee as possible?

 

[shreadvector]

Yes, you have it correct! This is the most common beginner mistake: thinking that the bigger number after the dash means "more powerful".

As you correctly deduced, they are both identical in propulsion and the only difference is the delay time. The same motor with a long delay will only be able to safely launch a lighter rocket than a short delay. A heavy rocket will not build up as much speed, and it will decelerate in less time and turn over and head back toward the ground ("ground" means the Earth, the top of your skull, the roof of nearby fine luxury automobiles or patol cars, etc.).

As for basic weight lifting power, you will see the effect of the total-impulse (the letter such as "F" vs. "G") and the average thrust (F10 vs. F52 or G35 vs. G80) on the recommended maximum liftoff weight.

The good old Aerotech Motor Matrix helped everyone understand these weight recommendations. It needs to be updated....
https://www.coastrocketry.com/Aerotech-Motor-Matrix.pdf

 

[shreadvector]

I forgot to mention that a relatively lightweight rocket will need a shorter delay time if it is very high drag. Examples include flying saucers, large diameter rockets, boost gliders with large span wings, etc.

 

[DrugCoder]

That matrix answered my questions. However, I have the AT Arcas that I'd like to fly with the F52-8T. The matrix said it could hit 1000 ft with the 5T, how bad would it be to fly it on the 8T?

It says "recommended" for the lift off weight. How close does everybody stick to these numbers?

Is the liftoff weight INclude or EXclude the weight of the motor?

 

[shreadvector]

That matrix answered my questions. However, I have the AT Arcas that I'd like to fly with the F52-8T. The matrix said it could hit 1000 ft with the 5T, how bad would it be to fly it on the 8T?

It says "recommended" for the lift off weight. How close does everybody stick to these numbers?

Is the liftoff weight INclude or EXclude the weight of the motor?

The recommended maximum liftoff weight is the complete rocket with motor (and wadding, but you have a baffle...). simply the complete rocket as it would be sitting on the pad.

The longer delay will allow it to arc over and build up speed as it plunges back toward the ground. If you launched it perfectly straight up and it actually flew straight up, it will not build up much speed. Of course, in the real world it will not fly straight up. It will travel along an arcing trajectory which is affected by wind at launch time (weathercocking) and your launch pad (Mantis pads are notorious for the "Mantis Dance" where the short alumimum rod deflects, whips and the entier pad can lift up and tile/move/dance unless anchored with large bricks or other weights).

A simple simulation program can show you the effect of launch conditions and a non-vertical rod on the trajectory and also show you the velocity along the way - including as it arcs over and builds up speed and it plunges back toward the tops of our skulls.

Let's say it still ejects pretty high up. If it is streaming downward at 100 or 200 MPH, the parachute may never leave the main body tube and it could continue plummeting to it's destruction. Other failure modes include a violent high speed ejection with a body tube "zipper" caused by the shock cord being yanked backwards by the parachute. Also there is the possibility that the rocket will plunge into the parachute and wither get caught/tangles or the chute could whip backwards and get tangled in a fin.

I've seen all of these multiple times. It still makes me cringe every time and it's not even *my* ~$100 rocket kit.

 

[DrugCoder]

sounds good.. Looks like the AT Strongarm will fly on the 8T. Any suggestions for other kits that would be good for that motor?

What is the best launch pad to handle MPR thrusts without the "mantis dance"?

 

[shreadvector]

Small diameter and lightweight models like the Cheetah or Mustang.

As for launch pads, I'm not an expert on the commercial pads currently oout there. I know the Mantis because I have a constant need to help/warn folks who show up at our club launches with them. Virtually every one of them has gone out and bought a 6 foot long 1/4" diameter steel launch rod to replace the 5-ish foot long aluminum rod that comes with the Mantis. You lose most of a foot of the rod in the built in anchor, so you really need the extra length.

I build my own with PVC pipe and a cruciform union in the center. I drill a hole to fit the rod and then secure the rod with a shaft collar above and below with a thumbscrew replacing the allen set screw they came with. Us a big deflector and it will not tip over. Legs are 2 feet long.

Hopefully ohters will chime in with commercial pad suggestions.

 

[astrowolf67]

The Mantis pad will be fine, IF, you anchor it to the ground as Fred stated. And, I would swap the aluminum rod out for a steel rod (can be found at a Lowes, or Home Depot pretty cheap). Otherwise, there are plenty of inexpensive PVC pipe plans here at TRF, or at EMRR (www.rocketreviews.com).

A very inexpensive, effective way to anchor your Mantis, is to drive a tent stake into the ground at an angle under it, and run a bungee cord from the pad to the stake. It doesn't have to be tight, just snugg enough to keep the pad solidly grounded.

David

 

[Zeus-cat]

This is an extreme example of what can happen if you deploy your main chute too late. I don't know who this rocket belongs to and I certainly don't want to embarass anyone, so I cropped out as much of the people from the one photo as I could.

It was a large rocket, maybe about 12 feet tall and 12 inches in diameter. The altimeter charge failed to fire the apogee ejection charge and deploy the drogue parachute. When the altimeter fired the charge for the main chute the rocket was coming in intact on a ballistic trajectory and travelling very fast. The eyebolt connecting the shock cord to the booster snapped instantly as the chute came out. The booster section did not noticeably slow down at separation. The upper section of the rocket landed softly under the parachute and appeared to suffer no damage from the long distance photos I took (I did not inspect it close up).

Fortunately the rocket had arced away from the crowd and landed about half a mile away. We felt the impact when the booster hit the ground.

 

[falingtrea]

The Aerotech website also has the latest catalog, and the catalog lists the recommended motors for all their models.