F35 max liftoff weight differentials?

[Kevlar150]

What I'm looking to know is if the max lift off weight changes as the delay gets longer with the F35 24/60 motor?

 

[rharshberger]

The delay has nothing to do with max lift off weight, the delay will affect max altitude, too short and a light rocket might not reach max altitude, to long and a heavy rocket might core sample before the delay initiates the ejection charge or the delay may initiate ejection past apogee causing damage to the rocket and/or recovery system( too short a delay can result in the same damage).

Sent from my SM-N920V using Rocketry Forum mobile app

 

[dhbarr]

A longer delay weighs ever so slightly more. It is not enough to matter unless you're pushing for an altitude or speed record.

 

[MikeyDSlagle]

Rich what you are saying is exactly the opposite of what you are saying. Lol I am not arguing or attempting to stir anything up. I am agreeing with ya.
First you say the delay has nothing to do with liftoff weight but then you explain how the wrong delay with the wrong weight will cause damage. So in effect, the delay helps determine the max liftoff weight...sorta. Alot of motors have a max weight written on the package and that max weight is affected by the delay. Heavier the rocket, the shorter the delay. It's not how much the motor can actually lift but how high it will go, which determines delay.

 

[Bat-mite]

Here is a simple answer: no. Delay is calculated to the expected time between motor burnout and apogee. Burnout time is fixed for any given motor. Apogee is determined by the total impulse of the motor and the weight/shape/finish of the rocket. You need a motor with enough thrust to get you a good speed off the pad, usually summarized as a 5:1 ration between average thrust and liftoff weight. But delay is set to try to get the apogee event to actually occur at apogee.

As Rich wrote, too early or too late, and you risk serious rocket damage due to deployment at high speeds.

You can really only get the correct delay time by running sims that accurately reflect your rocket, field, wind speed and direction, rail length, etc. Or, follow the kit manufacturer's specifications, since they have done sims and testing for you.

 

[dhbarr]

Rich what you are saying is exactly the opposite of what you are saying. Lol I am not arguing or attempting to stir anything up. I am agreeing with ya.
First you say the delay has nothing to do with liftoff weight but then you explain how the wrong delay with the wrong weight will cause damage. So in effect, the delay helps determine the max liftoff weight...sorta. Alot of motors have a max weight written on the package and that max weight is affected by the delay. Heavier the rocket, the shorter the delay. It's not how much the motor can actually lift but how high it will go, which determines delay.

heavier=shorter is only true on one side of optimum mass

Very light and fluffy rockets require short delays
Altitude optimized rockets typically require long delays, often exceeding what's available
Struggle-off-the-rod birds require short delays again

The mass of the delay train itself is negligible in almost every plausible scenario.

EDIT: re-reading the Original Post I think I initially answered a question OP wasn't asking. Fortunately I think my second response was more in line with the intended direction.

tl;dr: sims are good

 

[markkoelsch]

The max liftoff weight does not change. The delay times need to be proper for the rocket and motor you are flying as a system. This is where you need to run simulations to determine the proper delay time for a given motor and rocket.

 

[rharshberger]

The delay has zero effect on how much the motor will safely lift (meaning stable off the rail), but it will affect what happens after motor burnout.
I agree with all the statements including my own so far. Yes "fluffy" rockets can perform similar to heavy rockets.
Sims are definitely necessary whenever possible.

Sent from my SM-N920V using Rocketry Forum mobile app

 

[MikeyDSlagle]

This discussion came up some time back. Right..the delay doesn't change the "power" of the motor but if you dig through Aerotech's motors you will come across two motors (f27-4 and f27-8 for example) where the recommended max liftoff weight changes as the delay changes. Its right there in black and white for everyone to see. It doesn't take into account drag and weight optimization, but it is there, and the OP was pretty much asking the reasoning behind it. And I am fairly certain it says something about delay in one of the safety codes as well when talking about max liftoff weights. No I can't reference it, I'm at work.

 

[dhbarr]

This discussion came up some time back. Right..the delay doesn't change the "power" of the motor but if you dig through Aerotech's motors you will come across two motors (f27-4 and f27-8 for example) where the recommended max liftoff weight changes as the delay changes. Its right there in black and white for everyone to see. It doesn't take into account drag and weight optimization, but it is there, and the OP was pretty much asking the reasoning behind it. And I am fairly certain it says something about delay in one of the safety codes as well when talking about max liftoff weights. No I can't reference it, I'm at work.

That is... utterly bizarre that they differ. [initially I had thought the AT website had updated them both to 22oz MTOW]

Has anyone told Aerotech?

 

[clreynolds]

Guys, not to be argumentative, but delay times most definitely DO effect max lift off weight. Remember that Thrust-to-weight or "speed off the rail" only determine a minimum speed for aerodynamic stability. All of the responses agree that if you change the lift off weight, you must also adjust the delay for proper (safe) deployment. But the reverse is also true, if you just change the delay, you must adjust the weight of the rocket, or choose a rocket of an appropriate weight. This sounds confusing now because we have all gotten used to HP motors with adjustable delays. We choose a motor with a good TTW ratio, then choose an appropriate delay. But if the delay is fixed, you must choose a rocket of appropriate (that is to say, maximum safe) weight.

 

[MikeyDSlagle]

But...I have only seen it on single use and not on reloads so with the F35 being a reload...I don't see the weight per delay changes.
Run sims and when you get tired of fooling with delays, get an altimeter

 

[clreynolds]

I thought that this reload predated the approved modification of delays. (Boy, I am SO! Old. )

 

[dhbarr]

The idea of changing the MTOW as a function of delay timing boggles me, as it would treat a spool and an MD 3FNC identically.

 

[clreynolds]

Yes, it was never a very accurate measure, only the "Max".

 

[markkoelsch]

Guys, not to be argumentative, but delay times most definitely DO effect max lift off weight. Remember that Thrust-to-weight or "speed off the rail" only determine a minimum speed for aerodynamic stability. All of the responses agree that if you change the lift off weight, you must also adjust the delay for proper (safe) deployment. But the reverse is also true, if you just change the delay, you must adjust the weight of the rocket, or choose a rocket of an appropriate weight. This sounds confusing now because we have all gotten used to HP motors with adjustable delays. We choose a motor with a good TTW ratio, then choose an appropriate delay. But if the delay is fixed, you must choose a rocket of appropriate (that is to say, maximum safe) weight.

You are wrong. Max liftoff weight is independent of delay time. One does not actually effect the other.

You need to sim things to determine the correct delay for a given motor and rocket.

 

[Bat-mite]

You are wrong. Max liftoff weight is independent of delay time. One does not actually effect the other.

You need to sim things to determine the correct delay for a given motor and rocket.

Exactly! The keyword is "max". The "max" liftoff weight does not change; however, the "ideal" liftoff weight changes if you are stuck with a fixed delay time.

 

[MikeyDSlagle]

I got these from Aerotech's website, it is the instructions for an F20, just for an example. Maybe the OP saw something like this and wondered if the same applied to the F35. This is what I have been referring to. I clear has recommended max liftoff weight. Not ideal, max.



I am not trying to be stubborn or argumentative either. The recommended max liftoff weight clearly changes as the delay changes. I agree the delay has nothing to do with how much the motor can lift, and I for one am not arguing that point. I am simply pointing out what Aerotech has on the instructions, and what likely led to the original question. I will tap out now.

 

[clreynolds]

You are wrong. Max liftoff weight is independent of delay time. One does not actually effect the other.

You need to sim things to determine the correct delay for a given motor and rocket.

Actually, I am not. We are just talking about different things. The calculation of speed is only dependent on weight, drag and thrust. But that is not what the mfg. is talking about. They are referring to the entire flight profile, both up and down.

If you calculate a delay for a given weight and motor, you are doing exactly what the mfg is telling you. Example: Fly any 3FNC rocket on the F27-4 (OP's example) at the MTOW given by the mfg. Thrust to weight is already accounted for, and ejection happens somewhere before, at, or after apogee, but most likely late since we are already at max weight. Now fly that same rocket on the F27-8. The same liftoff speed, same max altitude, but deployment doesn't happen until a 4 seconds later. The rocket is now at high rate of speed (1/2at^2), or already in the ground. That is what the mfg said, you can't fly a rocket that heavy with that delay, and recover it safely. Note: MTOW will always be lower for a longer delay.

If you say you must adjust the delay, you again agree with the mfg, either choose a shorter delay, or a lighter rocket. That is why they offer the motor is various delays. The maximum takeoff weight is a measure that has been around since well before PC sims, or adjustable delays. It was a way to provide on more check.

 

[rcktnut]

Look at an Estes catalog same thing with the motors. Like mentioned a little something to go by, before sims were available. Made a little sense* I guess before we had sims.

* A 16 oz 4 inch dia. rocket most likely will not go as high as a 16oz. 2 inch dia. rocket.

 

[Sabrina]

When I was very new to rocketry, I flew my new rocket on a 4 second delay - great flight, beautiful, perfect! - I was really happy
Then I flew it again, but this time the motor had a 7 second delay. Great flight, beautiful, but why didn't the parachute come out? OH NO ! :y: SMASH!

"I'm never using one of those 7- second delay motors again. Those things are awful. They shouldn't even be able to sell those things!!!"

Then somebody explained it to me. - it's simple really. Those motors with the longer delays, you fly them in a LIGHTER rocket than the one I used. :smile:

Before you tell me how wrong I am, please review the motor chart in any Estes catalog, or carefully read and compare the packaging on any Aerotech "hobby" motor - the ones that don't require certification. I think these "Max Liftoff" guidelines are intended to help the average person who doesn't even know what a "sim" is.

 

[AlfaBrewer]

Let's try this another way...

NO. The max lift off weight is the same regardless of delay - IF what you're looking for is the max weight that can reach a safe velocity for a given length of rod/rail.

YES. The delay affects the flight profile, so for a given motor/delay combo a specific max weight will give the optimum delay/apogee event (assuming a typical 3/4FNC rocket).

Ultimately, the answer depends on what you're really asking.

 

[markkoelsch]

I believe any estimation of max liftoff weight based delay time is just wrong. It is an over simplification of a complex system. Furthermore, I think we have evolved enough as a hobby that technology in the form of simulators can predict this far better than some generic line in instructions.

 

[blackjack2564]

What I'm looking to know is if the max lift off weight changes as the delay gets longer with the F35 24/60 motor?

The uncomplicated answer is yes. The weight & size of rocket launched is a "function" of the interaction of Max liftoff weight and delay time. { and drag is a factor also}

Therefore a lighter, skinner rocket needs a longer delay or chute will be premature.
Heavier, fatter rocket needs a shorter delay or chute may be very late, if at all before contacting ground.

Even though technically speaking the actual weight the motor can lift is the "same". It can lift both rockets, but they will reach very different altitudes.

So another way to look at it:

a skinny rocket weighs same as fat one, but has less drag, will go higher, therefore needs longer delay.

THAT's why for simplicity sake, instructions show heavier weights & shorter delays vs lighter weights & longer delays for the same motor.

The manufacturer must make it SIMPLE for the first time [newbie] buyers to figure stuff out.
Read the chart, match motor to rocket, go fly safely.
Once they got you hooked, then you start finding about simulators etc.

Hey at least the OP had enough sense to ask.....hope we didn't scare you away from your first post........come back...LOL

 

[Bat-mite]

Semantics. What is meant by "max?" There's the maximum weight that the thrust can safely lift, which does not change. Then there's the maximum weight for a successful flight given a fixed delay.