How Fast Do Model Rockets Fly? – The Model Rocket
If you’ve ever watched a model rocket zipthrough the air at top speed and disappear against the sky, then you know thatmodel rockets are fast. Watching a launch of a highpowered model rocket got mecontemplating this subject, so I began some in depth research.
So, how fast do model rockets fly? Model rockets generally fly at top speeds less than 250 mph, but there are some that fly faster. Larger high powered rockets can reach speeds greater than Mach 1.
The speed that a model rocket can reach depends on the power of the motor used to launch it and the speed characteristics of the rocket itself. This article will explore all the factors that influence model rocket speed.
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You don’t hear as much talk about model rocketspeed because it can be difficult to measure with the use of specializedequipment or a rocket simulator, and the speed for a rocket depends on numerousfactors, some inside our control and some outside of our control.
The trouble with pinpointing a model rocket’sspeed is that it is constantly changing as it goes through each stage offlight.
The model rocket launches with a burst ofpower. Its speed accelerates until the point that it burns through all itsfuel. The rocket will reach its top speed right before its motor runs out offuel.
After the rocket runs out of fuel, it entersthe coasting phase. Propelled forward by its own momentum, it will continue togo higher, but it will also gain that altitude at slower and slower rates.
The model rocket begins to slow because of theeffects of weight and drag, which I will talk about more below. Eventually therocket will stop gaining altitude. This is when the rocket reaches apogee.
The recovery system will launch shortly afterthe rocket reaches apogee as it begins its fall back to the ground under thepower of gravity.
It is difficult to pinpoint exactly how fast amodel rocket will go in the seconds that it burns up its fuel, and the topspeed a model rocket reaches doesn’t always mean that it will gain greateraltitude.
Estimating Model Rocket Speed
Even though model rocket speed changes rapidlyafter you launch the rocket, there are some ways you can go about getting anidea of the model rocket’s average speed. None of these methods are accurate,but if you’re just looking for a vague ballpark of your model rocket’s average speed,they will help you.
Using Information from the KitIf youare buying a model rocket kit, you can use the information on the kit’spackaging and motor to get a general idea of what the rocket it capable of. This is definitely not the most accurate way to determine the speed ofyour model rocket, but it might give you a general idea of the speed potentialof the model rocket in your kit.
Here is the information you will need:
Estimated maximum altitudeMost powerful motor recommendedburn out timeMost powerful motor recommendeddelay timeTypically, you can find the estimated maximumaltitude of a model rocket on its packaging or on its product description page online,and the delay time is the number in the motor’s code.
Finding the burn out time might take a little more digging. Estes’ details a lot of useful information on their Engine Chart, including the thrust duration, or how long it takes before the motor burns out. It has many commonly used motors, but it is does not include every motor you might come across.
In order to determine what kind of averagespeed you could get out of your rocket divide the estimated maximum possibleheight by the time delay plus the thrust duration listed for the strongest motoron the list of recommended motors.
An example:
Let’s take the Estes Silver Arrow Launch Set (link to read reviews and see pricing on Amazon) and see what kind of average speed estimate we can get for it.
Estimated Maximum Height: 1125 feet
Most Powerful Motor: C67
Burnout/Thrust Duration: 1.9 seconds
Delay Time: 7 seconds
Burnout (1.9) + Delay (7) = 8.9 seconds
Max Height (1125) / 8.9 = 126.4 feet per second or 86 mph
Now what does this number really tell you? Itonly tells you the average speed including its very slowest speeds as it comesto a stop. A rocket is at its fastest right before its motor reaches burnout.It’s top speed could easily double its average speed.
Using Measurements You TakeYourselfFor a more handson experience, you could estimate the average speed ofyour model rocket by measuring the rocket’s altitude at apogee and how long ittook to get there. This method will only work if you can actually see therocket when it reaches apogee.
Here is what you’ll need:
An altitude tracker like the Estes Altitrack (link to read reviews and see pricing on Amazon)A stopwatchSomeone to helpTo take these measurements, one person willneed to be using the Altitrack to measure the model rocket’s altitude atapogee, and the other will need to use a stopwatch to measure how long it takesfor the rocket to reach apogee.
There are ways to measure the altitude of therocket at apogee other than the Altitrack, but they involve rigged up anglecalculators and calculus. Using the Altitrack works on the same principles, butit is a lot more convenient.
To calculate the average speed of the rocket, you will take divide the altitude at apogee by how long it took the rocket to reach apogee.
An example:
Altitude: 900 feet
Time to reach Apogee: 5 seconds
900 / 5 = 180
If it takes 5 seconds for the rocket to reachits peak and it traveled 900 feet, it would be traveling 180 feet per second or122.7 mph.
Remember, this is not the speed that it startswith at takeoff or ends with after it reaches its peak because the rocket isconstantly accelerating until it runs out of fuel and then it is constantlydecelerating. It is an average of the speed it traveled.
Accurately Measuring Model RocketSpeed
So far, I’ve shown you how to make some basicestimates of your model rocket’s speed, but as I’ve said these are inaccurate.If you’re working on science project or want to get into model rocketcompetitions, then you’re going to want accuracy.
The only way to measure a model rocket’s truespeed is to attach a device called an accelerometer to the rocket before youlaunch it.
Products like the Jolly Logic AltimeterTwo (link to read reviews and check pricing on Amazon) will measure your rocket’s top speed and more. The AltimeterTwo includes 10 different pieces of data about each flight including the top speed, the highest altitude, motor thrust duration, peak acceleration, ejection timing, and total flight time. It can be attached to the outside of your rocket or it’s also small enough to fit inside most rockets.
If you’re looking for ever more data, you could try the Jolly Logic AltimeterThree (link to read reviews and check pricing on Amazon) which connects to your Bluetooth enabled smartphone. The launch data is sent to your phone where it creates a graph with all the pertinent information.
Alternatively, you could create a DIYaccelerometer, but this requires a deeper understanding of electronics.
How Thrust Affects Speed
Thrust is the force which moves a rocketthrough the air. It is generated by the reaction that takes place in the motorwhen it is ignited.
Thrust is what causes the rocket to overcomeweight and drag and move from the ground. If a rocket does not have enoughthrust, it will not overcome these forces and it won’t lift off.
The amount of force with which a model rocketwill launch depends up on the total impulse and the average thrust, which Iwill outline for you below.
The more thrust a motor puts out in aninstant, the faster the model rocket will fly. In the sections that follow, Iwill discuss how the classification code on a model rocket motor describes theamount of thrust it can put out.
Total Impulse
The total impulse is the amount of energyavailable in a model rocket’s motor. To think of it in layman’s terms, it isthe amount of fuel the motor has. If we compared it to a car, it would be theamount of gas in the tank.
To determine the total impulse of a modelrocket’s motor, you must refer to the code printed on the motor. It will looksomething like this: B44. The first letter in this code, in this case a B,refers to the total impulse class of the motor.
Here is a chart that shows the total impulsefor different model rocket motors.
Rocket MotorTotal Impulse
Rocket Motor ClassTotal Impulse (inNewtonseconds)1/4A0.6251/2A1.25A2.5B5C10D20E40F80G160As the letters ascend, the total impulse ofthe model rocket motor typically doubles.
The total impulse will determine how long the motorcan produce the thrust needed to continue to accelerate.
Thinking of the car example, the amount offuel in our gas tank does not determine how fast we can go, but how long we cango at a certain speed.
Average Thrust
The number that follows the letter in the coderefers to the average thrust of the motor. In the example of the B44 motor,this is the number 4.
Theaverage thrust is the rate at which the motor will use the fuel available toit.
All motors rated with a B will haveapproximately the same energy available for them to use to send the rocketthrough the air, but those with a higher number will burn up that fuel morequickly and those with a lower number will do it more slowly.
If we compare this to a car, the averagethrust would be the amount of pressure you put on the gas pedal. The average thrust relates directly to howmuch fuel is used and how fast the model rocket will go.
In case you’re curious, the last number is howmany seconds after the motor runs out of fuel that it will take before itactivates the recovery system. At this point, all thrust would be stopped andthe rocket would begin a safe descent.
How Weight, Drag, and LiftAffects Model Rocket Speed
Weight, drag, and lift are all natural forcesthat impact the top speed a model rocket can reach. In order to reach its topspeed, a model rocket must overcome the effects of weight and drag, and utilizelift forces to its advantage.
Weight
We don’t always think about it, but our weightis all about gravity. It is a measurement of the downward force that theEarth’s gravity has on us. The weight of a model rocket will tell you how muchforce will be pulling it down as the motor struggles to launch it upwards.
Theheavier a rocket is, the bigger the motor will have to be to counteract theforce of gravity.
Drag
Drag is all about the aerodynamics of therocket. Drag is created when a solid body, like a rocket, comes in contact withliquid or gas, like air.
Themore aerodynamic a rocket is, the less drag there will be, the faster it can goand further it can fly.
Lift
Lift is another aerodynamic force, but unlikedrag, lift will help your rocket travel faster. In airplanes, lift is the forcethat helps the aircraft overcome its weight.
In the case of rockets, lift is used tostabilize the rocket and keep it flying straight up. If a rocket doesn’t not fly straight up, it may encounter more drag,and it would slow down.
How Does Motor Size Affect ModelRocket Speed
The size of the motor does not affect thespeed of the rocket directly. The speed of the rocket is most stronglyinfluenced by the average thrust, or the speed with which the motor burnsthrough its fuel to create thrust.
Estes Rockets come in four sizes mini (13mm),standard (18mm), 24mm, and 29mm. As the motor increases in size, so does thetotal impulse because there is room for more fuel in the motor, but a modelrocket will also have to be larger and heavier to hold the larger sized motor.
The extra fuel available in a larger motor canbe used for different purposes, and those purposes are not always speed. Hereis what a motor can do with extra fuel.
A motor could use that extra fuel to lift a heavier rocketA motor could use the extra fuel to increase the thrust duration, or the length of time the motor provides thrust to the rocketA motor could use the extra fuel to increase the model rocket’s speed by increasing the average thrust. The faster it burns through fuel, the faster it will flySo, a larger motor will not always mean afaster model rocket.
A Rocket that Breaks the SoundBarrier
That’s right. Some model rockets made out ofpaper and plastic can break the sound barrier and create a sonic boom. Ofcourse, that sonic boom is hard to hear as the rock is already likely to beover 100 feet in the air and the boom is relatively small. It is still anamazing feat.
In order to break the sound barrier and createa sonic boom, a rocket must be traveling at over 767 miles per hour.
Now, I’ve already said that most model rocketsdon’t go faster than 250 miles per hour, and this is certainly true, but inorder for a rocket to be capable of breaking the sound barrier, it must bespecially made with that purpose in mind. The motors used to do this are alsorestricted to those 18 years of age and older.
The Apogee Aspire
But how do they manage it? Let’s look at onemodel rocket kit that claims to be able to break the sound barrier – the ApogeeAspire.
The Apogee Aspire (link to read reviews and check pricing on Amazon) can fly over a mile high using and F motor, and it can break the sound barrier when using a G motor.
It accomplishes this feat by being incredibly lightweight. The rocket is 29 inches long and it only weighs 1.85 oz. All the components of the rocket are extremely lightweight. The body it made out of thin paper tubes, the nose is thin plastic and the fins are made out of super light balsa wood.
According to altitude predictions generated using a simulator called RockSim, when paired with the Estes E128, the Apogee Aspire can reach an altitude of 2,116 feet in somewhere around 10 seconds (estimated from the burn time plus delay time). This is about 212 feet per second or 144 miles per hour on average.
With a stronger motor like the Apogee F108,the Apogee Aspire reached a height of 5,479 feet in about 16 seconds in thesimulator. This is an approximate average of 342 feet per second or 233 mph.
With an even stronger motor like the Aerotech29mm G78G10, the Apogee Aspire reached a height of 4,171 feet in about 11seconds. This is approximately 379 feet per second or 258 miles per hour.
Onething this shows us is that speed isn’t everything.The slower motor actually resulted in a higher altitude, while the faster motorhad a faster burn time and so could not go as high.
Now none of these numbers show that the rocket broke the sound barrier, but these are a) just estimates of the speed based on the information I have and b) only reflect the average speed, not the top speed. However Apogee does report that with a G motor the rocket will in fact cross the 767 miles per hour mark to break the sound barrier (albeit momentarily).
In addition, because the delay usuallyactivates the recovery after the rocket has stopped gaining altitude, thesespeeds are probably a little slower than they should be.
The speed of the Apogee Aspire can be witnessed in this video:
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