High Powered Rocket Car - coming along finally! 27 lbs, J+K motors possible

[bzzh8c]

Rocket car is coming along finally..... weighs 27 lbs empty.

Been working on this design for awhile - making sure it's going to do what I expect it to do and going to be safe.
38mm and 54mm motor mounts in the back, thinking I could run it on anything from a G80 with an adapter (sim says 22 mph) up to a J and K motor combined (sim says 250+ mph possible but speed is obviously limited by wheels, and fin less importantly). Will work our way up slowly in power till something tells us - that's it.

I have 1500' of heavy cable to use as a guide . Eventually will run past end of cable at speed so I designed magnetic cable guide attachments that release when the car reaches the end and allow it to carry on. When we get to higher power motors, will need lots of runon space, so planning to run on the ice of Lake St. Clair (miles and miles of empty area) this winter (if the car is still running).

Parachute canister which runs off a 2 second timer attaches next to fin in rear when motors get big enough to need it.

I think it's going to work!

 

[markkoelsch]

Are the wheels plastic?

 

[dr wogz]

Interesting..

3 wheels would have less drag, no?
Wheel pants / fairings? (For more aerodynamics)
A wing front & back to ensure it stays 'grounded'? (also the downward force to maintain wheel traction..)
A TTW vertical fin would have less drag (and a little stiffer) than the brackets & bolts you're currently using..
Do you have bearings in those wheels?
With a single rod as your axle (and unsupported), are you worried about the wheels gaining or loosing 'toe in' as it hurdles down the raceway?
Any suspension thoughts? (even just a ±1/2" to ensure full contact..)
Wheel oscillations at that speed might send it into a very bouncy run.. Are the wheels balanced?
Wheel camber thoughts?
Wheel toe-in adjustments? to ensure it runs straight & true?
Running on ice.. wouldn't rails / runners be better?
Any method of emergency braking, should you need it?

 

[rstaff3]

Cool. 27 lbs seems awfully heavy but I've never built a rocket car :confused2: I'd be interesting in all the things dr wogz brought up.

 

[dr wogz]

Mr. Bzzh8c, I don't mean to come across as arrogant or condescending. It's the designer / engineer in me coming out. These are questions I'd ask myself, and I would expect my fellow colleagues to also ask.. having done a few r/c cars in my past, I've seen a few things. 27lbs at speed has a bit of energy..

 

[Charles_McG]

27lbs at speed has a bit of energy..

Since the energy all comes from the rocket motor, given the same motor, wouldn't vehicles of different masses end up with the same energy at burnout? Just different velocities? (Ignoring base drag, air resistance and other frictions)

((Also assuming the same wheels, so the energy you dump into angular momentum is the same for two vehicles of different masses))

 

[jimzcatz]

This whole project is a bad idea in so many ways. It really has nothing to do with hobby rocketry other than propulsion. We should divorce ourselves from it totally. I can see the headlines now. "Rocket car........"

 

[rstaff3]

The lake seems long enough, but I sure hope there are no fishermen/boaters on it when this launches Unless there is a mechanism and large staff to make sure that's the case, I have to agree with Jim. For some reason when I read this my mind said it was to be launched on the same drag strip as the other car that was described recently.

 

[dhbarr]

This whole project is a bad idea in do many ways. It really has nothing to do with hobby rocketry other than propulsion. We should divorce ourselves from it totally. I can see the headlines now. "Rocket car........"

Rocket cars have come and gone for a long time, from cold power through RMS and up to the current Estes Blurrzz.

Regardless of the project, though, safety -must- come first. My primary concern here is for the bearings & wheels. How many RPM are we expecting? What safety margin before they fly apart?

 

[rstaff3]

I'd be more worried about the guide line and where this would go should that break free.

 

[Reinhard]

27lbs at speed has a bit of energy..

Since the energy all comes from the rocket motor, given the same motor, wouldn't vehicles of different masses end up with the same energy at burnout? Just different velocities? (Ignoring base drag, air resistance and other frictions)

((Also assuming the same wheels, so the energy you dump into angular momentum is the same for two vehicles of different masses))

It's the delivered impulse that stays constant. The energy is actually lower on higher mass rockets.

The energy can be expressed as a function of of impulse and mass in the following way to show this.
E = p^2 / (2 * m)

If you're wondering where the missing energy ends up, it's in the exhaust, where most of the energy of our rather slow vehicles ends up anyway. Things start to look different when rockets become fast (Oberth Effect).

Reinhard

 

[dhbarr]

I believe OP mentioned a wide range of motor choices, thus "at speed" could be a few hundred km/s.

 

[bzzh8c]

I was warned I would get "skewered" for posting about this large scale rocket car. I'll try and answer some questions from Paul while I assure everyone that this project is going to go slowly, and as soon as it seems unsafe (I've been doing HPR for 30 years, and an automotive engineer with the same in experience) I'll put a lid on it. Heck, I might not even get past an H motor which will only push this thing as fast as you can run. Here's the car that inspired me by Ky Michaelson:

https://www.the-rocketman.com/whatchamacallit/rm_yellowRktcar.jpg

which worked very well, till it got to probably 300 mph....

From above questions;

Yes, 3 wheels would have less drag (pinewood derby method), but speed is not the goal here. I want horizontal version of "low and slow" and lots of noise, pictures, video. Speeds should never get too high so that drag is an issue. 100 mph would be success in my mind, and that would be done on a frozen lake with miles of room.
Downforce using wings is a possibility. If first trials of running on bumpy ice show I need it, I'll add em.
The tail fin needs to be removable for transport. It does go through the body tube and the bottom edge is supported inside. It's rigid.
I'm not worried about path. The 1/8 wire cable will maintain it's straight path no matter what. There is no possibility for the car to go sideways and either snap the cable or break the mounts. If or when I get to high power, managing the release of it at the end so it can coast off is the thing I'm working on now. Magnetic mounts seem to be the best bet.
It's got rigid single axles for simplicity and strength.
The 14" wheels have bearings, balancing will be attempted using my bench grinder.
Wheels add A LOT of gyroscopic stabilizing at speed. This will probably help the car maintain a straight path during costing much more than the fin, and rails or skis could let it deviate quite a bit.
Parachute will be used off 3 second timer if or when we get to higher powered runs.

Again, I built this to make a real rocket car (my definition for that is the wheel RPM corresponds to the vehicle speed).
I've seen too many horizontal rockets going 200, 400, and 600 mph in a couple of seconds with 3 inch wheels attached.
These wheels are skidding the entire run. It's just a horizontal rocket and I think very unsafe.

So, skewer away. I understand the comment that this kind of unique project doesn't really belong on a model rocketry website, but that's why I put it in the "Watering Hole" which suggests rocketry related topics. I will certainly exercise caution, safety, and energy management once I get it running.
If folks would prefer I not update this thread with info/pics/data, then let me know.

But I certainly will share what ends up being accomplished with my hero - Ky Michaelson.

 

[dhbarr]

Cool beans! Get great photos

 

[rstaff3]

I don't think you've been that skewered. You will be IF YOU DON'T POST PICTURES!

 

[ericoberg1]

Its Crap. No Seat.

 

[MikeyDSlagle]

Doesn't need a seat, just handlebars.

 

[manixFan]

Doesn't need a seat, just handlebars.

The heck with handlebars, needs a tow rope for a skater to ride behind - it's running on a frozen lake.

Oh, wait....


Tony

 

[tmacklin]

I see absolutely nothing remotely political with this project and thus urge you to pursue it until the bitter end. Unfortunately, it seldom gets cold enough in north Texas to freeze my pond.

[video=youtube;mUtiGxgX4g0]https://www.youtube.com/watch?v=mUtiGxgX4g0&list=FL7e6x-xp9lSosd4JrqNviDg[/video]

 

[markkoelsch]

I think ever releasing it for the guide wire is a mistake. At that point you have no control over its direction. That worries me. A longer guide wire, an active method to stop it while on e the wire- I think you need to focus on that.

 

[jimzcatz]

This entire thread needs to go away. Yea, I said it.

 

[Incongruent]

Does it have to be High Power? Can't you do a rocket car that runs on low or mid power engines?

 

[bzzh8c]

Does it have to be High Power? Can't you do a rocket car that runs on low or mid power engines?

Been there done those already, several times. Fun but more power would be cool. I'll take steps to control it and be safe.

 

[bzzh8c]

Rocket exhaust - yum yum

 

[Incongruent]

This is just my opinion, but I think maximizing efficiency on a lower powered rocket car would be much more purposeful and safe than going high power in a big rocket car for the sake of it.

Pinewood Derby would be a fun low power project.

Pinewood Derby is hiding in a corner.

 

[tmacklin]

If you're gonna be a bear, you might as well make it a grizzly!

[video=youtube;_JENG0Z8AMk]https://www.youtube.com/watch?v=_JENG0Z8AMk[/video]

 

[bzzh8c]

Well, that's where we differ. My purpose isn't to do something that's been done 1000 times before. But I will start low power, and go slow, and work my way up gradually. I am confident the setup will tell me when it's reached it's limit of safe operation and I'll stop. Guaranteed.
But I hope to have some videos and pics of a very cool looking and loud car moving down the wire by then!

 

[mrwalsh85]

I don't know... I think you need to rethink your guide wire. Your opinion is that the wire will not allow the rocket car to go anywhere. I digress. Ever see a clothesline, tightened to it's maximum tension (that you can apply)? Once you hang clothes on it, either it reaches the maximum tension of the actual clothesline, or the supports start to sag inwards, allowing vertical movement.

Same should apply for rocket cars. You would need two very very very heavy anchors that would allow you to tighten your guide wire to it's maximum. Did I mention heavy? I can see the guide wire giving at least 5-10ft over 1500'...

As far as the topic goes, Ky Michaelson had this subject published in High Power Rocketry a long time ago. No reason it can't be documented/discussed here... Within reason.

 

[bzzh8c]

Agreed and thanks for the input.
The "anywhere" word came from my thought that the 5-10 feet of deviation over 1000'+ wire is far less angle than that experienced in rod whip we see today on for high powered launches. And how many times have we had 20+ lb rockets come whistling back down towards us? I think relatively speaking this project is at worst medium risk. I myself feel it is actually a low risk endeavor since I'm going to slowly ramp up in power. Unlike many who jam the biggest motor they can in a virgin rocket because they're afraid it will be a one timer and they just want a good launch picture.

I'm going to be careful, believe me.

 

[bzzh8c]

And again, I expect if I get to high powered runs, they will take place on Lake Erie, miles from anybody, in the middle of winter with the wire secured by threaded rods through the thick ice.