Mars Lander Center of Gravity

[timster68]

I have to ask how I can measure the CG for this rocket. It’s the Semroc Mars Lander (I started building it 12 years ago). I have read the CG needs to be past the nose cone, but I don’t know how to measure it if I am supposed to balance the rocket by putting two pencils into the landing gear slots on either side of the rocket.

 

[Funkworks]

There's a plumb bob method that can work for objects with certain symmetries but it won't work for a rocket, so I just use OpenRocket.

 

[neil_w]

How can a 2D-cutout possibly be used to determine CG?

Indeed, balancing a very short stubby thing like Mars Lander can be quite challenging. OR can help, if you measure the weight of each component carefully, but not much help if it's already built.

My guess is yeah, you just need to be creative about finding a way to balance it on pencils or whatever.

 

[Funkworks]

How can a 2D-cutout possibly be used to determine CG?

My bad, a model rocket doesn't satisfy the symmetry requirements to use this method. The 2D method can work only if the 3D version has uniform mass distribution and thickness. I remember having used it but forgot that the object was quite different and did satisfy the requirements..

 

[Daddyisabar]

Oddrocs can be downright troublesome! Hard to swing test and impossible to balance on the fonger. Do your best, be generous with your nose weight and motor (Quest D16 4) for18mm mount and all will be fine. By the third or fourth flight you should have the nose weight reduced close to optimal for that motor.

 

[Steve Shannon]

Here’s one way. Connect the top and the bottom of the Mars Lander to lightweight strings which go to either end of a straight rod of equal length as the Mars Lander. The straight rod must be parallel to the centerline of the Mars Lander. Find the point on the straight rod where the suspended Mars Lander balances.

 

[neil_w]

Here’s one way. Connect the top and the bottom of the Mars Lander to lightweight strings which go to either end of a straight rod of equal length as the Mars Lander. The straight rod must be parallel to the centerline of the Mars Lander. Find the point on the straight rod where the suspended Mars Lander balances.

Beautiful!

 

[Jeff Lassahn]

tape a thread to somewhere on the rocket close to where you think the CG might be. Look at how it hangs. The CG is on the line pointed to by the thread. Do this attached to a few different places to check your guesses.

 

[BABAR]

Here’s one way. Connect the top and the bottom of the Mars Lander to lightweight strings which go to either end of a straight rod of equal length as the Mars Lander. The straight rod must be parallel to the centerline of the Mars Lander. Find the point on the straight rod where the suspended Mars Lander balances.

I was thinking put a lightweight plastic rod through the launch lug and attach at forward and aft ends with a loop of string, but this likely would break off the launch lug. Your idea is clearly better.

 

[neil_w]

Drill a small hole in the center of the body. Screw in an eyelet and tie a string to it to check balance. Depending on which way it tilts, drill another hole on the appropriate side, move the eyelet to it and try again. Repeat until you find the CG. Easy peasy.

To mostly compensate for the asymmetric mass removed by drilling the holes in the body, you can start by drilling a series of holes, spaced 1/8" apart, along the entire body before you start, and then just move the eyelet from hole to hole until you get closest to the CG.

Wait, did you say if a destructive test was OK?

 

[user 13]

Have you modded the kit?
Have you gone heavy on the paint?
Have you you used any glues other than white/yellow elmers type glue?

If you answer yes to any of those questions then you might have a C/G issue.
If you build it stock, don't go heavy on the paint, or use epoxy type glues it will be stable. As long as it's built true.

Wayne Kellner did a great job of designing the kit back in the '60s and Carl did an great job reproducing it, but it does not suffer overbuilding/over painting/ or modding well.

I don't even paint the wraps. The lower body/legs and nose cone are all that get painted.

Not exactly what you asked, but I hope helpful.
I have flown my clone many times, and in winds up to 10mph with no stability issues at all. The most I have seen is a slight weather cocking into the wind.

I have only seen problems when people go heavy on the paint/fill the spirals/mod it for a bigger engine or try and get fancy with more complicated landing gear. Don't fly it in strong/gusty winds.

EDIT:

If memory serves me the C/G of the ML should be just about the top (forward) edge of the BT-100 body tube. Also keep the weight as close to 4Oz as possible. If the C/G moves forward even a tiny bit and/or the weight edges above 4 Oz the stability margin deteriorates fast. Add in some wind or a dirty launch rod and problems will start.

I believe with mine I balanced it on my finger just at the edge of the BT-100. It will be tippy, but will give you a good idea of how close you are.