Most model rockets blast off at a much higher acceleration rate than full size rockets. Is it possible to have model rockets take off slowly to more closely mimic the acceleration speed of full scale rockets?



Most model rockets blast off at a much higher acceleration rate than full size rockets. Is it possible to have model rockets take off slowly to more closely mimic the acceleration speed of full scale rockets?

In: 15

Yes, by weighing them down. But why would you want to?

I honestly don’t think model rockets accelerate faster than large ones. The space shuttle is going 100MPH by the time the tail passes the tip of the crane. Its all perspective. I mean, the initial acceleration of a model is faster, since its power to weight ratio is probably much higher, but the overall acceleration is no where near the same.

A long time ago, there was a multistage Apollo rocket model made by Estes. It took either 4 b motors or one bigger one. I think it was a d motor. With the 4 smaller motors, it took off very realistically. The bigger motor took off like a bottle rocket. I only saw a completed one once and it was really cool.

No, not really. Some things just don’t scale very well, and model rockets don’t for a number of reasons.

If you balance a broom on your palm (vertically), it’s pretty easy to do and it’ll stay upright even if your movements are pretty slow. If you try the same with a pencil, it’s way harder and you need to be very quick.

It’s the same story with rockets. The smaller you make it, the more quickly it wants to tip over. Compare the way a pen falls to the way a cut tree falls.

Getting a small model rocket to almost hover in the air without tipping over would be very challenging and would probably require a very fast active stabilization system of some kind. Or you may need gyroscopic stabilizers. If the rocket flies fast enough, the fins at the bottom will stabilize it, but that doesn’t work at low speeds.

There would be other problems as well, such as dialing in the thrust just right, and getting a long enough burn time.

The thrust-to-weight ratio for most model rockets is *waaaay* higher than the TWR of orbital rockets; mostly because mass scales exponentially with size but thrust doesn’t. You can get a similar visual effect by lowering the TWR on your model rocket to something close to that of an orbital rocket – by either reducing the thrust or increasing the weight (or probably both).

The primary reason model rockets take off so slowly is because they are passively stabilized, whereas full-sized rockets are often actively stabilized. Full-sized rockets that do rely on passive stabilization do need to accelerate more quickly than actively-stabilized rockets do.

Passive stability is achieved using the fins on the bottom of the rocket, similar to the way the feathers on an arrow or a dart help it fly straight. In order to do this, the rocket needs a decent amount of velocity. That’s why model replicas of real rockets still have fins on them.

Active stability relies on being able to move the engine slightly so that the thrust can steer the rocket instead. This is certainly possible to do with model rockets, but it is a lot more difficult than just putting fins on the bottom of the rocket.

As such, model rocket motors are designed to produce relatively high levels of thrust with a short burn time. This gives them high acceleration.

Model rockets accelerate quickly because they have a higher thrust-to-weight ratio. So you need to put a less powerful rocket engine in them or increase the weight.

Increasing weight will rescue the max attitude they fly to.

If you reduce the trust and amount of fuel you burn the same the altitude will decrease a bit because of an increase in gravity lost. More work is needed to be done if to counteract gravity if the flight time is longer.

After a look at model rocket engines, it looks like their burn time them is less than 3 seconds for smaller variant. I am not sure why but I suspect it is simpler to manufacture them that way but it might be regulated.

If you look at rocket artillery that is in size a lot more comparable to a model rocket they have an engine burn time of similar short duration and will accelerate quickly, so the conclusion is for small scale rockets short burn time and high acceleration is more efficient.

As a comparison, a the space shuttle solid booster had a burn time of 2 minutes

There is a couple of reasons to keep large rocket acceleration down.

One is so even when most fuel is used do like to keep max acceleration at around 3g to protect the crew. Even for satellites they and the rocket would need to be stronger and heavier for higher acceleration.

Another reason is to keep speed and therefore drag down in the lower part of the atmosphere so use the fuel you have more efficiently. The drag also results in a lot of forces on the rocker you like to reduce.

small rockets are stronger than large ones because of the cube-square law and do not have weight-sensitive payload. So one of the major reasons larger rockets accelerate slower is not relevant.

That said there is a larger rocket that accelerates very quickly the it was an anti-ballistic missile accelerated at 100g and no model rocket is close. There was even an experimental HIBEX low level interceptor that accelerated at 400g

It’s a size thing, too. Make a huge model rocket and it will appear a lot slower-moving, even at the same rates.

An orbital rocket is a difficult balance of thrust vs total weight, so on launch it is very close to 1, while over the very long burn time the weight drops massively so the thrust to weight increases.

Model rockets have very short burn times, they want a T/W of about at least 3 to reach target altitude efficiently, the longer the burn time, the lower your launch T/W can be