I have read about all the different statistics of rocket motors. I understand that thrust is the amount of force the motor applies to the rocket. Total Impulse is the amount of thrust over the duration of the motor burn. My question is how does all that translate into motor performance. If I want a motor that burns low and slow, what statistics should I be looking at as opposed to wanting a motor that makes my rocket explode off the pad and put into orbit. Any help putting it into Layman terms would be greatly appreciated.
Can someone explain Rocket Science to me?
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I'll get you started.
Thrust is indeed the lifting force of the motor. It applies force to the rocket body in opposition to gravity, which is also a force. Obviously, in order to leave the ground, the motor needs to apply more upward force than gravity is applying downward force.
If you were throwing a rocket into the air, then think of thrust as "how strong is your arm?"
A rocket motor's thrust is determined primarily by the chemicals used to create the propellant, the size and location of the propellant grain core, and the size and shape of the nozzle.
Thrust is a key factor in knowing how much weight your motor can lift. If you use the arm example, then obviously a full grown man could apply more thrust than a toddler. Thrust likewise is a key factor in how fast your rocket will be going when the force is applied.
Motor burn length is not a determiner of thrust. If you have an underthrusted motor, it doesn't matter how long it burns -- it's not going to lift the rocket.
But provided you have enough thrust to lift the rocket, then burn time determines how high it will go. That is total impulse. Back to the arms, a grown man could get a good, long wind-up, applying his arm strength over a nice arc. So, two men with the same arm strength, but one with long arms and one with short arms, will throw the rocket different distances. The long-armed one will be able to apply his arm strength for a longer period of time.
However, thrust is often not uniform throughout the burn. Some motors will give you a lot of thrust at ignition, then quickly peter out. Others will burn slowly on the pad, then gain thrust as the rocket continues upward.
All that being said, your best friend for understanding this is ThrustCurve.org. Put in your rocket's dimensions and weight, then run sims to determine which motors are effective for your rocket, and how high you might expect them to go. Give it a look and come back with questions.
Thrust is indeed the lifting force of the motor. It applies force to the rocket body in opposition to gravity, which is also a force. Obviously, in order to leave the ground, the motor needs to apply more upward force than gravity is applying downward force.
If you were throwing a rocket into the air, then think of thrust as "how strong is your arm?"
A rocket motor's thrust is determined primarily by the chemicals used to create the propellant, the size and location of the propellant grain core, and the size and shape of the nozzle.
Thrust is a key factor in knowing how much weight your motor can lift. If you use the arm example, then obviously a full grown man could apply more thrust than a toddler. Thrust likewise is a key factor in how fast your rocket will be going when the force is applied.
Motor burn length is not a determiner of thrust. If you have an underthrusted motor, it doesn't matter how long it burns -- it's not going to lift the rocket.
But provided you have enough thrust to lift the rocket, then burn time determines how high it will go. That is total impulse. Back to the arms, a grown man could get a good, long wind-up, applying his arm strength over a nice arc. So, two men with the same arm strength, but one with long arms and one with short arms, will throw the rocket different distances. The long-armed one will be able to apply his arm strength for a longer period of time.
However, thrust is often not uniform throughout the burn. Some motors will give you a lot of thrust at ignition, then quickly peter out. Others will burn slowly on the pad, then gain thrust as the rocket continues upward.
All that being said, your best friend for understanding this is ThrustCurve.org. Put in your rocket's dimensions and weight, then run sims to determine which motors are effective for your rocket, and how high you might expect them to go. Give it a look and come back with questions.
Funkworks
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You would want a low average thrust, but there's a limit to how low you can go. The rocket needs to have a minimum speed off the rail to be stable and fly straight.
A higher thrust can provide a greater acceleration and ensure that minimum speed off the rail is achieved.
A lower thrust will provide less acceleration, and the "low and slow" visual, but too low and the rocket may not have enough speed once off the rail and veer off the vertical.
I have an estes V2 and I flew it on these two motors:
D21, which has an average thrust of 21 N
D10, which has an average thrust of 13 N
The first one made it pop faster than I could follow. The second was sufficient, so I'd stick with it or try something weaker if the simulation showed it was also safe.
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Nytrunner
Pop lugs, not drugs
To focus on this question, you want to pay attention to the average thrust of a motor, and its total impulse. Batmite's suggestion of thrustcurve.org is a very good one, it has all this data for commercially available hobby motors.
Looking at Funk's answer above, we have a good example. The D motors he lists both have the same total impulse of ~20 N-s, but the D21 (higher avg thrust) expends that impulse in almost half the time as the D10 (lower average thrust)
I'll use a different example: The H152, and the J150 (both are high-power motors that require certification). Both have an average thrust that's similar, but the H has only 275 N-s, and the J has ~950 N-s. Naturally, this means the H burns out much faster than the J.
Both extreme's have their own challenges.
-Rockets with high-thrust motors must be strong enough to survive that force and the resulting aerodynamic stresses.
-Rockets with low-thrust motors must be light enough to safely get up to speed on rail exit (30mph+)
Boy, with all you guys out there and them there fancy numbers, nobody told him pointy end goes up 
Wait...What? lolBoy, with all you guys out there and them there fancy numbers, nobody told him pointy end goes up
So, I think what you are saying in general terms for a given total impulse:
Low & Slow = Lower average Thrust, but make sure there is enough to get it off the pad and fly stable.
Jumping off the pad to altitude quickly= Higher thrust average. but make sure the rocket can structurally handle the acceleration forces.
Low & Slow = Lower average Thrust, but make sure there is enough to get it off the pad and fly stable.
Jumping off the pad to altitude quickly= Higher thrust average. but make sure the rocket can structurally handle the acceleration forces.
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This might be helpful:
https://www.thrustcurve.org/motorstats.shtml
https://www.thrustcurve.org/motorstats.shtml
Off Grid Gecko
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DV = (Isp) x G x log(Mf/Me)
Everything you need to know one equation
Everything you need to know one equation
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Only if you are one of the 1 in 100 that actually can do math beyond +-x and divide. Sorry sounds a bit snarky that way but it's probably worse than that.DV = (Isp) x G x log(Mf/Me)
Everything you need to know one equation
Off Grid Gecko
Well-Known Member
Had to drop it in there. When someone asks about rocket science my brain immediately turns to "rocket equation." I would have thought with KSP becoming so popular, lot's of hobby rocketeers would become more familiar with terms like Delta-V, Hohmann Transfer, etc. Very few of us here probably worry about orbital mechanics, but it's fun to dream.
Motor and fins at the back of the rocket. Rocket not too short or too long (3-4FNC). Use the latest electronics. Wear a shirt that says "Yes I am a rocket scientist. " Memorize and follow rules to the letter. Rocsim everything. Always use a rail, even for low power. But most importantly - no stinking mindsimed oddrocs.Boy, with all you guys out there and them there fancy numbers, nobody told him pointy end goes up
DV = (Isp) x G x log(Mf/Me)
Everything you need to know one equation
Except you did not identify what the variables represent . .
With regard to speed and trajectory the detailed equations for F=ma show everything when written out in full. If you are within a gravitational field say like near the earth's surface, the force of gravity will subtract velocity from the final burn-out velocity over the burn of the motor. On that basis it would be better to have a fast burn rather than a slow burn. However, the force of air drag goes up by the square of velocity, so there is a benefit in having a slower burn. Obviously, there is a compromise between balancing the velocity losses from gravity and air drag.
There is lots of theory and equations here:
https://www.rocketmime.com/rockets/rckt_eqn.html
There is lots of theory and equations here:
https://www.rocketmime.com/rockets/rckt_eqn.html
Man, you are no fun at all!!!Motor and fins at the back of the rocket. Rocket not too short or too long (3-4FNC). Use the latest electronics. Wear a shirt that says "Yes I am a rocket scientist. " Memorize and follow rules to the letter. Rocsim everything. Always use a rail, even for low power. But most importantly - no stinking mindsimed oddrocs.
Yep, rule number one about rocket club is safety through science. Rule number two is saftey through science!Man, you are no fun at all!!!
Rule 1 is push anything hard enough and it will fly
Okay (Serious face on) 2 of my favorite motors are the Aerotech H999 and the Aerotech I59. The H999 is a full H motor at 319 newton seconds. This motor is a great selection for several uses. For lighter, strongly built rockets it is a screamer. It only burns for .3 seconds. That is 3 tenths of a second. For light rockets there is this CRACK!!! and then silence as it coasts to over 5000 feet. It is also great for first flights on bigger rockets. It kicks it off the pad very quickly and then lets it coast. For rockets that are on their first flight where you may have concerns about its stability it is only under power for .3 seconds so if it is unstable its only exciting for a short time.....
The I 59 is a long burn motor at 8 seconds and a thrust average of around 14 lbs. Total impulse is 486 newton seconds. You hit the button on this motor and it just keeps burning and burning..... I have 1 rocket that works well with both motors, in fact the altitude I get from both motors is about the same. (around 5000 feet) Why the same altitude when the I59 has 167 more newton seconds? Gravity. The I59 loses 1 G of acceleration to the earths gravity for the full 8 seconds of its burn. The H999 only loses it for .3 seconds.
So which is better? Neither really. It all depends on what you want. If you want speed off the rail and a fast flight the the shorter burn, higher thrust motors shine. Slower launch? The long burn motors rule here. So, what do you want to do?
The I 59 is a long burn motor at 8 seconds and a thrust average of around 14 lbs. Total impulse is 486 newton seconds. You hit the button on this motor and it just keeps burning and burning..... I have 1 rocket that works well with both motors, in fact the altitude I get from both motors is about the same. (around 5000 feet) Why the same altitude when the I59 has 167 more newton seconds? Gravity. The I59 loses 1 G of acceleration to the earths gravity for the full 8 seconds of its burn. The H999 only loses it for .3 seconds.
So which is better? Neither really. It all depends on what you want. If you want speed off the rail and a fast flight the the shorter burn, higher thrust motors shine. Slower launch? The long burn motors rule here. So, what do you want to do?
Fun rockets go on the far pad.
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Think of it like holding a big garden hose, the more water (mass) you can pump out in a given amount of time (velocity) the more reaction (thrust) you're going to get. Now if you replaced the water with something heaver (like steel shot, for example) and you could push it out at the same rate (velocity) as the water, it would push back even more because there's more stuff (mass) behind it. So, your thrust is dependendent on the mass of the stuff coming out and the velocity that it comes out at. The longer you can keep this up, the more total thrust over time (impulse) that you're going to get. All of the math is a variation on this theme.
Note that the classic equation is only true for no gravity and no air drag.
where the delta V (DV) or increase in velocity is equal to the specific impulse (sec.) times the acceleration of gravity (32 ft/sec^2) times the natural logarithm of the total or full rocket mass(Mf) divided by the empty rocket mass(Me). (Actually, the rigorous units for Isp and Gc are different, but the final numerical result is the same.)
A slightly modified analytical equation for no air drag, but including "constant" gravity and vertical flight can be written:
DV = Isp x G x ln(Mf/Me) - G x tb
where tb = motor burn-time
where the delta V (DV) or increase in velocity is equal to the specific impulse (sec.) times the acceleration of gravity (32 ft/sec^2) times the natural logarithm of the total or full rocket mass(Mf) divided by the empty rocket mass(Me). (Actually, the rigorous units for Isp and Gc are different, but the final numerical result is the same.)
A slightly modified analytical equation for no air drag, but including "constant" gravity and vertical flight can be written:
DV = Isp x G x ln(Mf/Me) - G x tb
where tb = motor burn-time
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heada
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Simple version... A rocket goes up because the rocket motor applies enough force on it that it overcomes gravity and drag by the air. If given a fixed amount of force in the rocket motor, how that force is applied will normally change how the rocket reacts. If you apply it as a high amount of force over a short amount of time, the rocket will go very fast but maybe not as high. If you apply the force over a longer period of time, it wont go as fast but it may go higher.
A large, gas hungry muscle car may be able to use a gallon of gas to do a 1/4 mile in 3 seconds but it burned all that gas so quickly that it'll coast to a stop within 1 mile. A small, efficient car may only go 40 mph but it'll go 30 miles on that same single gallon of gas.
A large, gas hungry muscle car may be able to use a gallon of gas to do a 1/4 mile in 3 seconds but it burned all that gas so quickly that it'll coast to a stop within 1 mile. A small, efficient car may only go 40 mph but it'll go 30 miles on that same single gallon of gas.
If you are interested in the actual science a good first step is to throw out the letters and numbers we use on hobby rocket motors. They are rooted in little black powder motors where none of the burn times vary by all that much, and they all fit in roughly the same shoebox of total power. you need the whole thrust curve, or at the very least the burn time and total/average thrust to make some degree of sense of it.
We stick to designations like K550 and K480 because it was an easy extension of the Estes A/B/C days, but there is no way to tell from name alone that the latter is a far, far powerful motor other than being directly familiar with the two.
If all this spins your head a bit, I learned a lot from just reviewing Estes Engine Time/Thrust Curves charts which is in every recent Estes Catalog. It's also helpful to view the "Model Rocket Engine Performance Chart" to view Max liftoff wts for each motor. Plus you can then drool over all the rockets you wanna get to sate your Id. 
https://estesrockets.com/wp-content/uploads/Educator/Estes_Time-Thrust_Curves.pdf
https://estesrockets.com/wp-content/uploads/Educator/Estes_Engine_Chart.pdf
https://estesrockets.com/wp-content/uploads/Educator/Estes_Time-Thrust_Curves.pdf
https://estesrockets.com/wp-content/uploads/Educator/Estes_Engine_Chart.pdf
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depending on the weight of the rocket. its possible to make a light weight rocket scoot off the pad with a low thrust motor.
its possible to make a heavy rocket just make a stable speed at the end of launch rail/rod with a heavy rocket.
i can take, say, a G80 and gain 2 different altitudes and speeds depending on the weight of the rocket. put a G80( with long delay) in an apogee aspire( minimum diameter very light weight) and i might not see it again. i might not even see it come off the pad if i blink.
put that G80(with short delay) in a mini magg(i did it once. max liftoff weight for motor and delay) and it wont get off the pad very quick and not much altitude
You got it!
After reading all these great answers, it makes we wonder why Physics is not a mandatory Science in all US high schools. Usually it is Chemistry, Biology and maybe an elective science. As an engineering high school graduate student....we took Physics. There was no biology offered.. Concur review, review and review the Estes Motor charts. Understand the model rocket motors first......to get a good basic understanding.
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I only got rid of my slide rule about 6 years ago.
Nytrunner
Pop lugs, not drugs
Once you know the math, the new tools sure help save time
