Max weight at lift-off

[Scotty Dog]

Looking for a list that gives the max weight a motor will lift . I found one BUT I cant remember were I found it. Thanks Scotty Dog

[cjl]

You can figure it out yourself easily enough. Look at the liftoff thrust of the motor, and divide it by 5. If it's an Estes or Quest motor, the liftoff thrust spike is short enough duration that you want to be a bit more conservative than this, so divide the liftoff thrust by 8 or so.

[ScrapDaddy]

I know this will be much more helpful ;)

http://www.commonwealth.net/rocketstore/estes/EngineData.pdf

[cjl]

Unless the question wasn't estes specific...

That is a good resource for Estes though. It accounts for a couple more factors than I did, such as whether the delay will be short enough for a heavy rocket.

[ScrapDaddy]

Well quest motors have the same general characteristics as estes motors, just A wee bit weaker, like and estes C6 is a 9Ns impulse with a average thrust of 4.76 Ns while a quest C6 has a 8.76Ns total impulse with a 3.45 Ns average thrust. What I would do is if you used a quest C6-3 I would go with the max liftoff weight of the estes C6-5...

[cjl]

1) Newtons (N) are a unit of thrust. Newton-second (Ns) are a unit of total impulse. A quest C6 has 8.76Ns of total impulse, and 3.45N of average thrust. Sorry - pet peeve of mine (and this isn't specific to you - a lot of people here do it).

2) In most cases, if an Estes C6-5 works, then so will a Quest C6-5. I would also tend to use Estes only for rockets in the upper end of the lifting capacity of a C, due to the higher thrust.

I really wasn't referring to Estes vs Quest though in my brand comment above. I was thinking Estes vs AT, Apogee, CTI, etc.

[rocketsmith]

Scrappy, the Quest motor has 30% less avarage thrust, so if one followed your advice and the rocket was marginal at takeoff it would probably arc over and deploy a few feet from the ground. The correct way is to take the avarge thrust in newtons, divide by 4.45 to get thrust in pounds and then divide by 5 again to find the may liftoff weight in pounds. Then divide by 16 to find weight in ounces.
On another point, the best advice I ever took was this; "I never learned anything while I was talking." Maybe you could get some value from this age old wisdom. In other words, shut up and listen, and then, when you actually have something useful and correct to say, people will take it for the value it has. Or just keep running your ignorant 12 year old mouth and prove know everything already.

[cjl]

The correct way is to take the avarge thrust in newtons, divide by 4.45 to get thrust in pounds and then divide by 5 again to find the may liftoff weight in pounds. Then divide by 16 to find weight in ounces.


This method normally works quite well for high power motors (oh, and I'm assuming you meant multiply by 16 for the lb-oz conversion). With Estes motors, it's not as good though, because of the thrust spike at ignition. Using the average thrust alone, using this method, an Estes D12 could only lift 8 ounces, when in reality, it can safely lift 12 or 13. The same is true of high power motors which do not have a roughly constant thrust throughout the burn. A more accurate method would be to use the average liftoff thrust (in other words, the average over the first half second or less of the burn) rather than average thrust when running this calculation. What the motor does after liftoff is less critical to the stability of the rocket.

[ScrapDaddy]

1) Newtons (N) are a unit of thrust. Newton-second (Ns) are a unit of total impulse. A quest C6 has 8.76Ns of total impulse, and 3.45N of average thrust. Sorry - pet peeve of mine (and this isn't specific to you - a lot of people here do it).

2) In most cases, if an Estes C6-5 works, then so will a Quest C6-5. I would also tend to use Estes only for rockets in the upper end of the lifting capacity of a C, due to the higher thrust.

I really wasn't referring to Estes vs Quest though in my brand comment above. I was thinking Estes vs AT, Apogee, CTI, etc.


Woops.... I'll get into a good habit :) I see very few rockets that are built about the C6's max liftoff weight and need a D10 or a D21.... Hence why I'm struggling to find a rocket for my D24....

[ScrapDaddy]

This method normally works quite well for high power motors (oh, and I'm assuming you meant multiply by 16 for the lb-oz conversion). With Estes motors, it's not as good though, because of the thrust spike at ignition. Using the average thrust alone, using this method, an Estes D12 could only lift 8 ounces, when in reality, it can safely lift 12 or 13. The same is true of high power motors which do not have a roughly constant thrust throughout the burn. A more accurate method would be to use the average liftoff thrust (in other words, the average over the first half second or less of the burn) rather than average thrust when running this calculation. What the motor does after liftoff is less critical to the stability of the rocket.


For BP motors I believe that the formula is Max thrust/10 for this very reason the thrust profile of a BP motor looks like a mountain curving into a flatland and a compostite Curve resembles a Mesa

[uwish]

This method normally works quite well for high power motors (oh, and I'm assuming you meant multiply by 16 for the lb-oz conversion). With Estes motors, it's not as good though, because of the thrust spike at ignition. Using the average thrust alone, using this method, an Estes D12 could only lift 8 ounces, when in reality, it can safely lift 12 or 13. The same is true of high power motors which do not have a roughly constant thrust throughout the burn. A more accurate method would be to use the average liftoff thrust (in other words, the average over the first half second or less of the burn) rather than average thrust when running this calculation. What the motor does after liftoff is less critical to the stability of the rocket.

I think you are describing the difference between progressive and regressive motors.

[ScrapDaddy]

Scrappy, the Quest motor has 30% less avarage thrust, so if one followed your advice and the rocket was marginal at takeoff it would probably arc over and deploy a few feet from the ground.....

shut up and listen......

#1 that's why i said use a shorter delay then of you were using a estes motor.
And you should do your homework the peak thrust of a quest C6 is a
15.46N while the estes motor only has a peak thrust of 14.06N meaning that a quest motor would get the rocket off te pad faster..

#2 Fair enough, only if you return the favor :neener:

[cjl]

I think you are describing the difference between progressive and regressive motors.

Progressive and regressive would be one example of a situation where the average thrust would not be representative of liftoff thrust. Dual thrust motors would be another example.

[rocketsmith]

It's a rule of thumb to do the calculation I suggested. Yes, multiply for ounces, not divide. If you use the liftoff thrust number for the calculation and, say you are using Blue thunder or a fast white propellant you can dramatically overestimate the max liftoff weight. If the rocket is right at the max liftoff weight and it is a relatively high drag design it will tend to get off the pad, founder in the air and then nose over. If using electronics it's okay, usually it will kick out the laundry before any badness happens. But with motor eject, you are asking for a lawn dart or a zipper at the best ( G104t are famous for this). I know becoause I like slow and low flight and have had one too many pound the ground even when it sim'ed out fine with the delay suggested. I now err on the side of extra altitude rather than a month of repairs. I guess for BP motors it's not such a big concern. I don't fly anything smaller than an F anymore so I guess I forgot.

[ScrapDaddy]

That's one "F" motor makes up 52% of my Ns useage the rest of them are "a" and b motors and unlike you, I have yet to fly anything later than a "F" :D talk about the other end of te spectrum :) Long live Estes!

[cjl]

It's a rule of thumb to do the calculation I suggested. Yes, multiply for ounces, not divide. If you use the liftoff thrust number for the calculation and, say you are using Blue thunder or a fast white propellant you can dramatically overestimate the max liftoff weight.

Not at all. In most cases, Blue Thunder and fast white propellants have fairly flat thrust curves, and the liftoff thrust is nearly identical to the average thrust. Liftoff thrust is definitely what matters, although in some cases, you will need a fairly short delay.



If the rocket is right at the max liftoff weight and it is a relatively high drag design it will tend to get off the pad, founder in the air and then nose over. If using electronics it's okay, usually it will kick out the laundry before any badness happens. But with motor eject, you are asking for a lawn dart or a zipper at the best ( G104t are famous for this). I know becoause I like slow and low flight and have had one too many pound the ground even when it sim'ed out fine with the delay suggested. I now err on the side of extra altitude rather than a month of repairs. I guess for BP motors it's not such a big concern. I don't fly anything smaller than an F anymore so I guess I forgot.

Again, liftoff thrust is all that matters for it to be able to get off the pad safely. It is true that in some regressive motors, on a maximum weight rocket, you will need a very short delay (and this should be factored in to the decision to fly that motor).

[rocketsmith]

Off the pad safely is not the full flight profile. That's my point. Using average thrust to do the calc. is a more conservative method. It also leads to an even greater margin of safety for the flier as well as the spectators. But hey, use whatever calcs. you like. All flights are heads up to me anyway.

[cjl]

Off the pad safely is not the full flight profile. That's my point. Using average thrust to do the calc. is a more conservative method. It also leads to an even greater margin of safety for the flier as well as the spectators. But hey, use whatever calcs. you like. All flights are heads up to me anyway.

What about progressive motors though? If your motor has a progressive burn, average thrust is too optimistic, and you will be leaving the pad more slowly than anticipated.

I agree that all flights should have people paying attention though. There's always a chance for something to go wrong.

[ScrapDaddy]

One quick question for CJL if the minimum impulse for an "F" motor is 40.01 Ns then how is it that it only 52% of your Ns usage?

[cjl]

The parenthetical value in my signature (52% F) is simply stating the equivalent motor class if you were to put my total impulse usage this year into a single motor. So, it's saying that my cumulative total motor usage this year is the equivalent of a 52% F motor. The reason it's 52% is because the percentage values are based on each individual motor bracket. A A 40.0001 Ns motor would be a 0% F motor for example. The formula you can use to find it is pretty simple - percentage of motor class = (total impulse - min impulse for motor class)*100/(min impulse for motor class). For the case in my sig, you would do this:

(60.9-40)/40 = 0.52 = 52%.


Oh, and that value will change significantly as of tomorrow :D
(assuming good weather of course)

[ScrapDaddy]

The parenthetical value in my signature (52% F) is simply stating the equivalent motor class if you were to put my total impulse usage this year into a single motor. So, it's saying that my cumulative total motor usage this year is the equivalent of a 52% F motor. The reason it's 52% is because the percentage values are based on each individual motor bracket. A A 40.0001 Ns motor would be a 0% F motor for example. The formula you can use to find it is pretty simple - percentage of motor class = (total impulse - min impulse for motor class)*100/(min impulse for motor class). For the case in my sig, you would do this:

(60.9-40)/40 = 0.52 = 52%.


Oh, and that value will change significantly as of tomorrow :D
(assuming good weather of course)

ohh so you are stating that's your total Ns usage is 52% of a F motor.... Good luck I am about 68 Ns ahead of you!:D

[rocketsmith]

What I am saying is that, in the case of say a J570, max thrust is over double the average thrust. With a progressive burn like a Redline, using the average thrust will give a fairly accurate assesment of lifting capacity in a real world senario. The difference between the average thrust of a progressive motor and the thrust at take off is generally negligable relative to the huge spike at lift off of a hot regressive burn like the J570 compared to it's average thrust. The big spike in thrust will get you off the pad but it is the duration of the burn that does the work. I like the idea of using the largest component of a data field to make assesments rather than the statistically anomalous portion. However, as an engineer, I can see the value of paying attention to maximum and minimum values, especially when designing materials and structural components. I think that with the dynamics involved in flight, averaging the data gives a better assessment of final outcomes.

[ScrapDaddy]

What I am saying is that, in the case of say a J570, max thrust is over double the average thrust. With a progressive burn like a Redline, using the average thrust will give a fairly accurate assesment of lifting capacity in a real world senario. The difference between the average thrust of a progressive motor and the thrust at take off is generally negligable relative to the huge spike at lift off of a hot regressive burn like the J570 compared to it's average thrust. The big spike in thrust will get you off the pad but it is the duration of the burn that does the work. I like the idea of using the largest component of a data field to make assesments rather than the statistically anomalous portion. However, as an engineer, I can see the value of paying attention to maximum and minimum values, especially when designing materials and structural components. I think that with the dynamics involved in flight, averaging the data gives a better assessment of final outcomes.


Hence the formula Peak thrust/10

[cjl]

Hence the formula Peak thrust/10

Which does not really accurately reflect the capability of a constant thrust motor.

[ScrapDaddy]

Which does not really accurately reflect the capability of a constant thrust motor.

Well why don't you give me a formula that worlds for all motors:rolleyes:
apperently you have to look at the thrust curve so you know which formula to use...

[cjl]

ohh so you are stating that's your total Ns usage is 52% of a F motor.... Good luck I am about 68 Ns ahead of you!:D

Exactly.

Oh, and that lead will probably vanish as of tomorrow :D

[cjl]

Well why don't you give me a formula that worlds for all motors:rolleyes:
apperently you have to look at the thrust curve so you know which formula to use...

Honestly, if you take the average thrust over the first quarter to half second of the burn, and then divide it by 5, that should give you a pretty good estimate. That's ignoring the issue brought up by Rocketsmith though - in some cases, the motor is physically capable of lifting a rocket safely, but the shortest delay available is still too long. In that case, the rocket is too heavy for the motor, but not because of liftoff capability.

[ScrapDaddy]

Exactly.

Oh, and that lead will probably vanish as of tomorrow :D

I have 9 "a" motors I want to fire along with a couple more "b" motors. But if you put 2 "f" motor in you would be a good 80ns ahead of me.

[ScrapDaddy]

Honestly, if you take the average thrust over the first quarter to half second of the burn, and then divide it by 5, that should give you a pretty good estimate. That's ignoring the issue brought up by Rocketsmith though - in some cases, the motor is physically capable of lifting a rocket safely, but the shortest delay available is still too short. In that case, the rocket is too heavy for the motor, but not because of liftoff capability.

I have a good example, a F24-4 will lift a rocket to a safe alltitude but a A24-4 will not even though the A24 has the power to get the rocket off the pad safely.

[cjl]

I have a good example, a F24-4 will lift a rocket to a safe alltitude but a A24-4 will not even though the A24 has the power to get the rocket off the pad safely.

Exactly.

Oh, and in that quote, I mistyped something. I said "sometimes the shortest delay is still too short" - I meant "too long", but you understood it correctly despite the typo.

[ScrapDaddy]

Exactly.

Oh, and in that quote, I mistyped something. I said "sometimes the shortest delay is still too short" - I meant "too long", but you understood it correctly despite the typo.

I know all too well about this, let's flash back to the "will the D24 lift a 3FNC safely, it had the inital spike to render the rocket stable, yet it didn't have a short enough delay.

[ben_ullman]

Scrappy, the Quest motor has 30% less avarage thrust, so if one followed your advice and the rocket was marginal at takeoff it would probably arc over and deploy a few feet from the ground. The correct way is to take the avarge thrust in newtons, divide by 4.45 to get thrust in pounds and then divide by 5 again to find the may liftoff weight in pounds. Then divide by 16 to find weight in ounces.
On another point, the best advice I ever took was this; "I never learned anything while I was talking." Maybe you could get some value from this age old wisdom. In other words, shut up and listen, and then, when you actually have something useful and correct to say, people will take it for the value it has. Or just keep running your ignorant 12 year old mouth and prove know everything already.




#2 Fair enough, only if you return the favor :neener:

And as his post count continues to rise, he has missed the point yet again! :rolleyes:

[ScrapDaddy]

And as his post count continues to rise, he has missed the point yet again! :rolleyes:


You know you aren't showing the entire post, which would justify my *politicly incorrect responce* I stated that the quest motor would actually have more lifting power and I had the data to prove it. His post, in which he stated that the quest motor had less average thrust, which then he followed up with that ment that the quest motor had less lifting power, which is not correct because of the inital spike of BP motors, the rocket would be well off the pad and on it's way to the moon.

[ben_ullman]

You know you aren't showing the entire post, which would justify my *politicly incorrect responce* I stated that the quest motor would actually have more lifting power and I had the data to prove it. His post, in which he stated that the quest motor had less average thrust, which then he followed up with that ment that the quest motor had less lifting power, which is not correct because of the inital spike of BP motors, the rocket would be well off the pad and on it's way to the moon.

Ever heard the phrase nickel and diming someone? You havent made any large mistakes, but all these little comments and statements add up to an overall crappy experience.

Ben

[ScrapDaddy]

That's a 2 way street there.

[ben_ullman]

That's a 2 way street there.

your paving your future man. This is not a Q&A board, its a forum, with equal participation from all parties. I think everyone here has been more than gracious enough to you. But its not that you CANT get experience its like you dont WANT to go get experience? I mean, is it to hard for you to just go build a kit and fly it then say "so when I flew this, it did this, and I was wondering about this_____?" You would be amazed how much things would improve for you.

I mean, if you sat in word and typed 50 questions that you had (im sure you could do that) and make them reasonably answerable. Then start tackling each question. Decide what you need to do/fly to maybe get these questions answered. Believe me, checkilists are almost required in HPR. I was at almost 2 pages for my2 stage O to N Terrier Sandhawk.

Ben

[ScrapDaddy]

Ok, I have your point, after over 20 attempts, you are the first to break through to me. No more "what if" question and more I tried thi and why did____ happen. Gotcha

[ben_ullman]

Ok, I have your point, after over 20 attempts, you are the first to break through to me. No more "what if" question and more I tried thi and why did____ happen. Gotcha

Seriously, the first time you post a pic/video of a rocket you flown and ask a question. You will be amazed at how rewarding the forum can be!

Ben

[ScrapDaddy]

Fair enough... But there is nothing intesting in my fleet....all just "3FNC" ohhh I know I have a bunch of parts iv been meaning to do somthing with... Stay tuned for a build/ disaster thread!

[cjl]

Seriously. Even if it's just 3fnc, we'd be happy to see pictures of it being built, sitting on a table looking nice, or flying.

[ben_ullman]

Seriously. Even if it's just 3fnc, we'd be happy to see pictures of it being built, sitting on a table looking nice, or flying.

yeah, you can do alot with 3FNC. They make great cluster rockets and something thats you can just toss in the car and go with. Not as fragile as some nicer models.

Ben

[ScrapDaddy]

I had plans 2day to build a RamJet. Only to be interupted by....

[ben_ullman]

I had plans 2day to build a RamJet. Only to be interupted by....

rockets can wait man, remember this as long as you live, they call it a hobby for a reason. (unless your Jim flis, then its a job)

Ben

[ScrapDaddy]

rockets can wait man, remember this as long as you live, they call it a hobby for a reason. (unless your Jim flis, then its a job)

Ben

I stay in a car for a 4 hour road trip, only to tell me that you have to be 18 to see him, pure Bull****