Mjölnir Rocket

[The Astronaut Farmer]

I was talking in a thread about how I had an Oddroc idea. Here is my announcement on our local rocketry forum:

I have no idea why I want to build this rocket so badly, but ever since I saw the Thor scene (spoiler alert!) where Thor's hammer takes off from the ground I have wanted to build a Mjölnir rocket. So that's what I intend on doing. I've done a lot of thinking on it and its time to do some doing. The first thing I want to do is think more, by taking some advice.

I modeled it up in Catia to make sure the dimensions I found online looked okay. Here is what it looks like:







The hammer is 6.5x6.5x11". The chamfers are 45 degree angles, and 1".

The shaft is 2" diameter and 15" long.

I try to do something new every rocket, this time it is making a minimum diameter rocket. I currently plan on epoxying polycarbonate fins onto the shaft. Not sure on that exactly yet, I'll need to do more research. I don't want the fins to be very visible as that will ruin the look I'm going for. The shaft will be where the motor gets placed. Not sure on motor retention yet. I plan on using a motor ejection charge so I don't have to cram avionics in there. I plan on making the entire thing out of fiberglass, and epoxying separate sections of fiberglass together to make the hammer section. This seems like a terrible idea because of how much drag the hammer would create and therefore how strong it would have to be. Someone convince me to do something else!

I plan on epoxying a coupler tube inside the hammer. The booster tube can then be secured to it, and the ejection charge will separate the rocket into two sections. The parachute will be stored in the coupler tube that is inside the hammer.

Another major problem will be the disrupted airflow that the hammer causes. I have no idea if I can get fins big enough to mitigate that problem. Anyone have any insight on that?

Anything else I'm not thinking of in addition to that? Also I tried to find that particular video online but didn't see it, perhaps I can find it later.





Changes in design since then:

3" body tube diameter (hopefully my tiny hands can still grip it like a hammer!)
54 mm MMT
1526K160-6 by CTI is currently the motor choice. I figured a slow burning motor would allow for more airflow to get to the fins, therefore bringing the Cp down.
As of now I plan on having that coupler tube that comes out of the hammer section, and using shear pins to have the separation occur there (although would shear pins even be necessary with all the force acting on it ).
Wide, square polycarbonate fins.

Questions still: Thickness of fiberglass plates for hammer section, and thickness of polycarbonate for fins. Anyone with experience want to guide me?

[new2hpr]

I'd definitely build it first in a much smaller LPR formfactor until you get the stability locked down. Maybe the flat top surface will help, like spools and flying pizza rockets. I've never claimed to understand the aerodynamics of those, but they work, without fins.
-Ken

[rockets4kids]

Spools, saucers, and many oddrocs gain stability from base drag. I am pretty sure you are not going to get any with this design.

[rstaff3]

I'd put four large fins protruding out along the thin axis of the head. Probably not in a true 'x'.

[The Astronaut Farmer]

I'd put four large fins protruding out along the thin axis of the head. Probably not in a true 'x'.

Wouldn't that provide more weathercocking in one direction if the fins are not symmetrical?

Spools, saucers, and many oddrocs gain stability from base drag. I am pretty sure you are not going to get any with this design.

Could you describe to me how base drag works? Is it because there is so much drag at the bottom of the rocket it produces stability by counterbalancing the center of gravity?

[DavidQ]

I had been considering a similar rocket. I was planning to launch it with the handle leading the way, and have it land with the hammer end up. The hammer head could be the fins that way, and with a rear eject, the parachute would hold the hammer upright on the way down.

[jimzcatz]

Flying it upside down(hammer down) might provide better stability. Thor ROCKS!!!

[DavidQ]

Here's a rocksim that I just put together, using the squarish head as the fins. It's a BT60-sized handle, cuz that's the size range I work with.

Uses Pods, so may require rocksim v 9.0.

But, that's not reallly upside down. Thor catches the handle, so its best if the handle leads the way. He'd take a big conk if the hammerhead lead the way.

[rstaff3]

Wouldn't that provide more weathercocking in one direction if the fins are not symmetrical?

As long as there is enough fin area in both planes, it should be fine. It may have a slight tendency to weather cock but I think the fins would be less of an issue than the rectangular frontal area. I have a normal 4fNC whose fins are like a squashed 'x' and it flies great. Then there are the class of winged rockets that can be made to fly...very asymmetrical! I suggested the slightly asymmetrical arrangement to move as much fin are from below the hammer's head as possible.

[rstaff3]

Flying it upside down(hammer down) might provide better stability. Thor ROCKS!!!

I agree. On both accounts!

[Barracuda]

Just fill the hammer head full of helium and be done with it

[rstaff3]

Could you describe to me how base drag works? Is it because there is so much drag at the bottom of the rocket it produces stability by counterbalancing the center of gravity?

I can't give you a detailed technical description but...

It doesn't counter balance the CG but it does move the dynamic CP. It has to do with the air flow characteristics, velocity, turbulence, etc. An area of low pressure forms at a rocket's base, which has some restorative benefits. These benefits are only realized on short, fat rockets. For instance, the Estes Fatboy is more stable than a Barrowman analysis will tell you. When you get to saucers, spools, pyramids and the like, then this becomes a major factor. Art Applewhite did some wind tunnel tests which showed the CP of the saucers is behind their base. Many of these squat oddrocs are seen to wobble during flight. I worry that an oblong structure will tend to go unstable rather than merely wobble. Assuming no fins of course.

[Micromeister]

Flying Thor's Hammer Upside down just wouldn't be right LOL!

When designing my flying Brick. I did exactly what Dick suggested. Added a lower bodytube and 4 large, thin, Clear Polycarbonate fins.
Made for a very stable 2nd place Maryland funny meet model

[The Astronaut Farmer]

Thank you Micromeister, saved me making a subscale version! I was almost sold on having the hammer part down, but I do believe that I'll keep it in the original configuration. I'll get some nice big polycarb fins, four of them. I presume that rail buttons are usually placed near the bottom of the rocket so it stays on the rail the longest to make the rocket more likely to be stable coming off the rail? If this is the case, then I could probably get away with putting the buttons on the hammer part, correct?

[adrian]

I worry that an oblong structure will tend to go unstable rather than merely wobble. Assuming no fins of course.

Worry not.



On a D12 it flies fine. On a C6 it makes barely enough speed to generate the drag to stabilise it, and it does indeed wobble.

[The Astronaut Farmer]

thor.rktOkay, I haven't done much work with Rocksim so I thought I would model it up for experience. I seem to have the basic shapes right, but when I launch it everything reads NA and it goes to an endless loop. I attached the .rkt file (hopefully). If you need more information to diagnose my problem let me know.

[rstaff3]

Adrian: Nice! I stand corrected.