# AT-SU motor question

#### hornet driver

##### Well-Known Member
First, feel free to give me an over simplified answer. In looking at the AT motor packaging--I don't have one in front of me---I noticed max lift off weight. Does anyone know what the speed at end of rod/rail is? 4,5,6 ft.? and is it a conservative number in your estimation?--H

#### grouch

##### Well-Known Member
Fast, faster, fastest?

#### Rob702Martinez

##### Zip-Tie Oversight Committee
TRF Supporter
I could be wrong, 30mph is the speed off the rail where the flight is stabilized.

#### Tonimus

##### Well-Known Member
I suspect the liftoff weight is a function of the thrust of the motor, not the length of the rod. Most likely a 3:1 or 5:1 thrust to weight ratio.

#### MikeyDSlagle

##### Well-Known Member
Your max liftoff weight will also take into account the delay on SU motors. F42-4 will have a lower max liftoff weight than an F42-7. They can both lift the same weight, but the lighter rocket will coast longer/go higher so it can have a longer delay. Too long of a delay on a heavy rocket and it may pop the chute after it hits the ground.

I think Tripoli requires a minimum 3:1 thrust to weight but not positive. I go for around 5:1. I like slow liftoff and have put up several with an off the rail speed of 41 ft/S.

Speed at end of rod/rail is just what it sounds like. That is how fast your rocket is going when it reaches the end of the rod/rail. Generally the longer the rod/rail the faster the rocket will be going when it leaves or reaches the end. Lower thrust motors will need a longer rod/rail.

As for the thrust, you are looking at the spike right at the beginning, first .2 seconds or so. If your rocket doesn't hit the speed you need on a 4ft rail, it may possibly reach the needed speed using a 6ft rail.

One rocket I've been simming is going 35 ft/S at the end of a 6 ft rail, with a particular motor, but 42 ft/s off of an 8 ft rail, same motor.

Hope this helps

What motor are you looking at?

Mikey D

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#### Peter Olivola

##### Well-Known Member
This is a dangerous over simplification. Beginning with G class motors it's possible to build a rocket with a lighter than optimum weight. This would reverse your assumption about delay length. As motor size increases it becomes even easier. The best way to deal with all the variables involved is to use a simulation program.

Your max liftoff weight will also take into account the delay on SU motors. F42-4 will have a lower max liftoff weight than an F42-7. They can both lift the same weight, but the lighter rocket will coast longer/go higher so it can have a longer delay. Too long of a delay on a heavy rocket and it may pop the chute after it hits the ground.

I think Tripoli requires a minimum 3:1 thrust to weight but not positive. I go for around 5:1. I like slow liftoff and have put up several with an off the rail speed of 41 ft/S.

Speed at end of rod/rail is just what it sounds like. That is how fast your rocket is going when it reaches the end of the rod/rail. Generally the longer the rod/rail the faster the rocket will be going when it leaves or reaches the end. Lower thrust motors will need a longer rod/rail.

As for the thrust, you are looking at the spike right at the beginning, first .2 seconds or so. If your rocket doesn't hit the speed you need on a 4ft rail, it may possibly reach the needed speed using a 6ft rail.

One rocket I've been simming is going 35 ft/S at the end of a 6 ft rail, with a particular motor, but 42 ft/s off of an 8 ft rail, same motor.

Hope this helps

What motor are you looking at?

Mikey D

#### Dave A

##### Well-Known Member
TRF Supporter
Unless your rocket has an unusual shape or has a poor aerodynamic shape, you can bank on an easy shortcut to the 5:1 rule. (3:1 on a slow motor will layover at about 45 deg)
To eliminate all the metric conversions use:
Avg thrust ÷ 22.5= min wt w/ motor in pounds.
Example: F42
42÷22.5= 1.9 lbs is min weight for safe speed off the rail. It doesn't hurt to add a little safety factor. You launch a lot, buy a sim program.

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#### MikeyDSlagle

##### Well-Known Member
Hornet, here is a bit of info that might help: check out the section about max liftoff weight, it talks about thrust ratios and, of all things, delays.

https://www.aerotech-rocketry.com/c...uctions/HP-SU_Instructions/hp_su_in_21000.pdf

This is a dangerous over simplification. Beginning with G class motors it's possible to build a rocket with a lighter than optimum weight. This would reverse your assumption about delay length. As motor size increases it becomes even easier. The best way to deal with all the variables involved is to use a simulation program.

So you are saying it's okay to use a shorter delay on a rocket that will go higher? That would reverse my "assumption about delay length". He was asking about max liftoff weight, not delays on super light rockets or sim programs. Your larger SUs come with adjustable delays and not set times, which is why I asked the OP what motor he was talking about. In those situations the user sets the delay so it's max liftoff weight will be based on thrust. You could also give a motor a minimum liftoff weight for the delay it has if you wanted to.
But even still, lighter rockets need longer delays.
Once you get into the realm of super light rockets on powerful motors, you will have to use some other form of deployment other than motor eject.
You can compare the numbers yourself if you want to. Two identical motors with different delays, the shorter delay will have a smaller liftoff weight...because of reasons stated in previous post. I just grabbed the F42 as an example.

Mikey D

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#### scsager

##### Slightly burned-out old guy
The OP is asking about the "Max liftoff weight" PRINTED on the header card of an Aerotech Single Use motor.

The question - "Does anyone know what the speed at end of rod/rail is? 4,5,6 ft.? and is it a conservative number in your estimation?"

Here is the simple answer - Max Liftoff weight as printed on the package - does NOT translate to rail speed.

The weight printed on the package CHANGES depending on the delay of the motor. The listed weight is lower with Longer delays, and greater with Shorter delays. This takes into account the recovery phase of the launch.

>>> If you are curious about the rail speed, you will need to simulate your rocket with Rocsim or OpenRocket. <<<

#### Peter Olivola

##### Well-Known Member
No. I'm saying lighter rockets don't always go higher than heaver rockets. It is possible to build a rocket too light for maximum altitude beginning with G and on up. But don't take my word for it. Whistle up Open Rocket and build a 4" TTFN with a 38MM motor mount and play.

So you are saying it's okay to use a shorter delay on a rocket that will go higher? That would reverse my "assumption about delay length". He was asking about max liftoff weight, not delays on super light rockets or sim programs. Your larger SUs come with adjustable delays and not set times, which is why I asked the OP what motor he was talking about. In those situations the user sets the delay so it's max liftoff weight will be based on thrust. You could also give a motor a minimum liftoff weight for the delay it has if you wanted to.
But even still, lighter rockets need longer delays.
Once you get into the realm of super light rockets on powerful motors, you will have to use some other form of deployment other than motor eject.
You can compare the numbers yourself if you want to. Two identical motors with different delays, the shorter delay will have a smaller liftoff weight...because of reasons stated in previous post. I just grabbed the F42 as an example.

Mikey D

#### Peter Olivola

##### Well-Known Member
Again, a dangerous over simplification. Above an average G, a heaver rocket can go higher, requiring a longer delay, than a lighter rocket. The bigger the motor, the more pronounced the differential. Your assumption applies to black powder motors and small composites only.

The OP is asking about the "Max liftoff weight" PRINTED on the header card of an Aerotech Single Use motor.

The question - "Does anyone know what the speed at end of rod/rail is? 4,5,6 ft.? and is it a conservative number in your estimation?"

Here is the simple answer - Max Liftoff weight as printed on the package - does NOT translate to rail speed.

The weight printed on the package CHANGES depending on the delay of the motor. The listed weight is lower with Longer delays, and greater with Shorter delays. This takes into account the recovery phase of the launch.

>>> If you are curious about the rail speed, you will need to simulate your rocket with Rocsim or OpenRocket. <<<

#### Tonimus

##### Well-Known Member
Here's the exact reason for the difference in Aerotech recommended max liftoff weights quoted from their SU motor instructions:

"Maximum liftoff weight is a recommendation that is provided only as a general guideline and should not be used at the exclusion of overall considerations of rocket flight stability, recovery and safety. Use the following formula for estimating maximum liftoff weights. For short delay motors use a 6:1 average thrust-to-weight ratio (8:1 for long burn motors). For medium delay motors use a 12:1 average thrust-to-weight ratio. For long delay motors use a 15:1 average thrust-to-weight ratio."

#### MikeyDSlagle

##### Well-Known Member
Here's the exact reason for the difference in Aerotech recommended max liftoff weights quoted from their SU motor instructions:

"Maximum liftoff weight is a recommendation that is provided only as a general guideline and should not be used at the exclusion of overall considerations of rocket flight stability, recovery and safety. Use the following formula for estimating maximum liftoff weights. For short delay motors use a 6:1 average thrust-to-weight ratio (8:1 for long burn motors). For medium delay motors use a 12:1 average thrust-to-weight ratio. For long delay motors use a 15:1 average thrust-to-weight ratio."

This is what I referenced to in my second assumption, I mean post.

#### Tonimus

##### Well-Known Member
Mikey, I know. I was just spelling it out plainly for people who aren't willing to click on the instructions.

Ahh
:horse:

#### MikeyDSlagle

##### Well-Known Member
No. I'm saying lighter rockets don't always go higher than heaver rockets. It is possible to build a rocket too light for maximum altitude beginning with G and on up. But don't take my word for it. Whistle up Open Rocket and build a 4" TTFN with a 38MM motor mount and play.

Challenge accepted. Unable to find any single use 38mm G motors. So...where can I get one of those?

But just for kicks, I made a 4" TTFN (TaTa for now?) And found the optimal weight for said rocket on a 29mm AT SU G80: 19.6oz to over 1400 feet. That is using the new composite goose down material, available at the same store as the single use 38mm 'G' I would think. :smile:

Maybe the OP will chime in as to whether or not he has gotten the answer he was looking for, until then I'll just observe.

Mikey D

#### Peter Olivola

##### Well-Known Member
If you wanted to find data that supported your mistaken hypothesis, you found it, but that's not what I said. The optimum weight issue changes beginning with G class motors and becomes more pronounced as motor size increases. I suggested 38 MM because it would be easier to see the effect of weight change using commonly available materials and techniques.

Challenge accepted. Unable to find any single use 38mm G motors. So...where can I get one of those?

But just for kicks, I made a 4" TTFN (TaTa for now?) And found the optimal weight for said rocket on a 29mm AT SU G80: 19.6oz to over 1400 feet. That is using the new composite goose down material, available at the same store as the single use 38mm 'G' I would think. :smile:

Maybe the OP will chime in as to whether or not he has gotten the answer he was looking for, until then I'll just observe.

Mikey D

#### watermelonman

##### Well-Known Member
Forget the 38mm G motors, as long as you are simulating load up some big K motors. The point is that you can establish that rockets can be built below optimal weight, so the next step is to find the crossover point.

#### GregGleason

##### Well-Known Member
I have a spreadsheet do my "heavy lifting".

Greg