How big should my sugar rocket to provide thrust (100m distance) on my skateboard with me sitting down

[Zwalter]

Okay, I know what you're thinking, its a stupid idea and it is. I was just wondering how much potassium nitrate and sugar will I be needing for me to thrust on my skateboard. And what pvc diameter would you guys recommend and the nozzle size? I am planning 2 rockets on the side of the skateboard.

Distance I would like to cover is around 100 meters. I weigh 70kg. I will be sitting down on my skateboard throughout the entire process. Would appreciate some answers, thanks.

[wumpus]

When working with rocketry/fireworks, the goal is to be as far away as possible while the rocket is burning.  This violates that basic principle.

[sevenperforce]

This is a very bad idea. Please do not do this idea.

That being said, if you do this very bad idea, it is to your advantage to know a little bit about what you’re doing and how to go about it.

Do not use two rockets; it will be impossible to light them simultaneously and they will burn at different rates. Use one.

The safest way to do this would be to mount the rocket motor on a pair of wheels fixed to a very long dowel, which in turn would be mounted to the front of your skateboard. You would be facing into the exhaust, of course, but if the dowel is long enough it should be fine. That way you can just let the rocket motor tow you and you could release the dowel to abort. Experiment with a motor in advance (perhaps using sheets of paper hung at intervals behind the rocket exhaust) to figure out a safe distance.

If you insist on mounting a rocket motor to the skateboard itself, then please protect yourself by mounting a metal plate of some kind to the underside of your skateboard and putting the motor under that.

You can rig a crude torsion system to determine the thrust level of your rocket motor. Attach a wooden broom handle firmly to a stable structure like a tree, attach a test motor to it, and video the test-fire to identify how much the thrust causes the broom handle to bend before springing back. Then you can attach weights to the same point on the broom handle and figure out the thrust level from that.

Again, this is a very bad idea and you should not do it. 

[StrandedonEarth]

This idea is only good for winning Darwin Awards. Because even if you survive an engine RUD, it’s still likely to destroy your reproductive capability if you’re practically sitting on it. 

That said, a pusher motor arrangement will not want to go in a straight line

[RCgothic]

4 minutes ago, StrandedonEarth said:

 

That said, a pusher motor arrangement will not want to go in a straight line

I'm not touching the main subject of this thread, but this is pendulum rocket fallacy. A pusher rocket is no less stable than a puller.

[wumpus]

6 minutes ago, RCgothic said:

I'm not touching the main subject of this thread, but this is pendulum rocket fallacy. A pusher rocket is no less stable than a puller.

The pendulum fallacy relies on that if the rocket is free to rotate, then it doesn't matter where the exhaust is as long as it is inline with the center of mass.

Once you add wheels (which provide a fulcrum for the exhaust to torque around), you are in a different case and rocket placement matters.

 

But where you don't want to be is on the skateboard.

[Vanamonde]

Neither Squad, Take Two, Private Division, the forum, the moderators, or really anyone else endorses the proposal presented here. 

[Kerbart]

Has it been mentioned what an exceptional bad idea this is? You can use one of those pocket pull spring scales to have someone drag you along at a constant speed to see what kind of force is needed to propel you forwards. The rest is then just math that, as a KSP player, you should have no issues with.

[sevenperforce]

58 minutes ago, wumpus said:

1 hour ago, RCgothic said:

1 hour ago, StrandedonEarth said:

That said, a pusher motor arrangement will not want to go in a straight line

I'm not touching the main subject of this thread, but this is pendulum rocket fallacy. A pusher rocket is no less stable than a puller.

The pendulum fallacy relies on that if the rocket is free to rotate, then it doesn't matter where the exhaust is as long as it is inline with the center of mass.

Once you add wheels (which provide a fulcrum for the exhaust to torque around), you are in a different case and rocket placement matters.

In reality it will be almost impossible to get a homemade rocket's thrust vector lined up properly with the skateboard's center of mass, let alone the shifting center of mass of a whole person sitting on top. A "puller" arrangement (with a sufficiently long dowel) would be more stable if you could hold onto the dowel, because it would allow you to essentially gimbal the engine thrust. 

But the shifting center of mass would actually be useful in the sense that a pilot with sufficiently good reaction time would be able to steer by shifting their weight to the left or to the right and thus changing the center of mass relative to the thrust vector.

I would not want to be the pilot attempting to steer a skateboard by shifting my body mass while screaming as a homemade rocket motor is continuously (we hope) exploding under me.

51 minutes ago, Vanamonde said:

Neither Squad, Take Two, Private Division, the forum, the moderators, or really anyone else endorses the proposal presented here. 

Even this non-endorsement seems insufficiently discouraging of the very bad idea that this proposal represents.

6 minutes ago, Kerbart said:

You can use one of those pocket pull spring scales to have someone drag you along at a constant speed to see what kind of force is needed to propel you forwards. The rest is then just math that, as a KSP player, you should have no issues with.

It's a little more complicated than that. The rocket motor's burn isn't going to last long enough to do a constant-speed arrangement. You would need to determine what starting speed is required to get a 100-meter roll-to-a-stop and then figure out how much thrust is required to get you up to that speed within the burn time of the motor.

Have I mentioned that this is an exceptionally bad idea?

[Kerbart]

6 minutes ago, sevenperforce said:

It's a little more complicated than that. The rocket motor's burn isn't going to last long enough to do a constant-speed arrangement. You would need to determine what starting speed is required to get a 100-meter roll-to-a-stop and then figure out how much thrust is required to get you up to that speed within the burn time of the motor

I apologize for choosing the wrong words. What my intention was, is measuring the rolling friction. From there, as said, it's a matter of math to figure out the rest. Probably something along the lines of friction × distance = energy required, and then some voodoo math with burn time, thrust and an extra factor for air resistance. Again, nothing too hard to figure out.

[sevenperforce]

9 minutes ago, Kerbart said:

I apologize for choosing the wrong words. What my intention was, is measuring the rolling friction. From there, as said, it's a matter of math to figure out the rest.

Ah, yes, you’re right. Rolling friction will be nearly constant so you can multiply the force of rolling friction by 100 meters to get the kinetic energy that will be lost to friction, then use the mass of the skateboard and rider to solve for the necessary starting velocity.

All so you can light yourself on fire and probably blow limbs off. Best-case scenario you’d get flung off the skateboard at high speed by a steering failure and end up with road rash.

It’s a great idea as long as you aren’t riding it, though. Duct tape a potato and a GoPro to the skateboard and have fun with it. Take a video and post it on YouTube and call it the world’s fastest rocket-propelled potato. Instant sensation. 

One of the things that makes homemade rockets with PVC casings very dangerous is that PVC turns into shrapnel if it bursts. Ordinary amateur rocket motors have cardboard casings that will rupture much less harmfully if you have an overpressure event.

If you experiment at all with PVC casings, make sure you use an endcap and nozzle made out of packed clay or something that similarly will fail before the PVC does. 

[StrandedonEarth]

22 minutes ago, sevenperforce said:

Ah, yes, you’re right. Rolling friction will be nearly constant

But you also need to find the static friction, by which I mean how much force it needs to get it rolling, which will be more than it takes to keep it rolling.

The higher thrust to get going requires a hole (depth TBD) in the grain for an extra initial spike in thrust.

Of course, this should be tested from a safe distance, not from atop the skateboard the motor is mounted to!

 

 

[Nuke]

the alaskan version of this is to stick a bunch of rocket motors on your kayak. that way if you ever find yourself on fire, you can jump in the water and die of hypothermia instead. 

[sevenperforce]

51 minutes ago, StrandedonEarth said:

But you also need to find the static friction, by which I mean how much force it needs to get it rolling, which will be more than it takes to keep it rolling.

Nah, you only need to find the static friction in order to figure out how much total impulse you need at the beginning. The rocket motor burn would be very short; it doesn't last for the entire duration.

[Dman979]

Edited July 19, 2021 by Dman979
Updated warning.

[Spacescifi]

6 hours ago, Zwalter said:

Okay, I know what you're thinking, its a stupid idea and it is. I was just wondering how much potassium nitrate and sugar will I be needing for me to thrust on my skateboard. And what pvc diameter would you guys recommend and the nozzle size? I am planning 2 rockets on the side of the skateboard.

Distance I would like to cover is around 100 meters. I weigh 70kg. I will be sitting down on my skateboard throughout the entire process. Would appreciate some answers, thanks.

 

Why on Earth? Who or what are you trying to learn or prove?

Like in video games we push ourselves to fill our egos or take out aggression on inn-game enemies or simply to prove that we can beat the challenge and therefore can beat a similar or just as challenging if not more so one IRL also... after the game though.

 

By the way.... no one here is going to tell you because if you or someone is hurt or worse we may get an opportunity to visit a courtroom.

IP addresses human. They can find you and litigate you. Not even hackers are safe.... unless you have a bunch or 'burner' hardware that is not registered to you and fake addresses... but no legal, lawabiding citizen does that so...

Edited July 19, 2021 by Spacescifi

[StrandedonEarth]

I just re-read the actual question in the topic: “How big…?”  Answer: Big enough to kill you

[kerbiloid]

You need to have F5/F9.
Do you?

Edited July 19, 2021 by kerbiloid

[sjwt]

13 hours ago, Zwalter said:

Okay, I know what you're thinking, its a stupid idea and it is. I was just wondering how much potassium nitrate and sugar will I be needing for me to thrust on my skateboard. And what pvc diameter would you guys recommend and the nozzle size? I am planning 2 rockets on the side of the skateboard.

Distance I would like to cover is around 100 meters. I weigh 70kg. I will be sitting down on my skateboard throughout the entire process. Would appreciate some answers, thanks.

No, No, NO.. when you kill or injure your self or someone else its bad news for all of us 

 

 

[Deddly]

@Zwalter, when the entire forum is begging you to reconsider what you are planning on doing, please stop for a moment and think carefully about how valuable your life is. Think about how useful it is to have all of your limbs and your current state of health. Think about how seriously wrong this plan could go. Burns. You could flay yourself alive by scraping yourself along the ground, you could veer off course and crash. You could end up a quadriplegic.

These are all serious and realistic dangers. Your chance of success, however, is much less likely. Now, please ask yourself: is it worth the risk for a couple of seconds of fun?

Get into rocketry by all means, but don't do it in any way that involves you or any other living thing being close to the rockets when they are firing.

[Zwalter]

Yeah I think everyone convinced me pretty well. This was way worse than my initial thought was. Anyway, thanks for the response everyone!!

[Terwin]

If you are interested in using stored energy to propel perhaps a couple hundred pounds tens or even hundreds of meters horizontally, you might want a lower density medium.

Elastic cords come to mind, probably something between the foot-long 'bungee' tie-down cords with hooks on each end you might find in Walmart and the  kinds that people use to jump off of bridges.

If you want a more rocket-like high acceleration/short distance acceleration approach, use a 'horizontal sling-shot' type of set-up, or for a more manageable acceleration, you can tie one end to something fixed(like a tree or lamp-post) and walk it back to your launch spot.

In either case, using elastically stored muscle power tends to have fewer accidental catastrophic effects over using high-pressure gasses generated by rapidly converted chemical energy.

Should also be less expensive and more re-useable too...

[StrandedonEarth]

1 hour ago, Terwin said:

n either case, using elastically stored muscle power tends to have fewer accidental catastrophic effects over using high-pressure gasses generated by rapidly converted chemical energy.

Fewer, not zero. Potentially less catastrophic, but still dangerous. If a stretched bungee cord snaps, that’s a lot of energy whipping around. It could easily take out an eye or worse.  Steel cables that  snap under load have been known to cut people in two

Granted, modern bungee cords have many filaments, so a sudden, catastrophic failure is unlikely (if you see it starting to unravel), but still something to watch out for.