Stability on a very weird high power rocket.

[ljwilley]

I'm thinking about trying to do an upscale of an estes Lunar Scout. It has no nose cone, HUGE fins, and is very short and stubby. Even though the diameter is 3", The rocket requires an I motor to get above 700 ft. The main issue is stability... Rocksim shows that the rocket has only a stability of .3 calibers, which most of the time is really bad stability. (this was calculated with base drag added) When 200 grams of nose weight are added, it brings it up .064. This also makes the rocket weigh 2500 grams, and I don't really want to add more.

So, would it be OK with just .64 cal. of stability? And even if it is, I have doubts that the RSO's would be too happy about letting me fly something with a stability of less than 1 cal. ...

Help?

 

[ljwilley]

Oh, and almost forgot, here's a picture of what the rocket would look like:

 

[Azamiryou]

If the scaling is accurate for all dimensions, it will most likely be stable as long as you keep the CG in the same relative position.

Here's another thread on this topic.

Up/down scaling shouldn't change the aerodynamics very much unless it's really extreme.

 

[ljwilley]

So the RSOs should approve it as long as the stability is the same or higher than the original?

 

[Azamiryou]

Obviously that's up to the RSO, but that's the argument I'd make. Except I wouldn't say

the stability is the same or higher than the original

I'd say the CG is in the same relative place or farther forward. (Since we don't know the actual CP or a stability value for the original.)

Also, I'd make sure the original is actually a good flyer. Some odd rocs are kind of marginal, and I don't think I would have a lot of faith in an upscale of one of those.

 

[Art Upton]

It's a sort of Spool like short very fat fined rocket.

I'm not sure what your planned flying field is, but if I was RSO and saw CG the same or better then original, I would just put it out 'at the far pads' for the First flight.

You didn't say what type of I motor your going to test it with first, that matters for an RSO as well.

Is it short and sweet for the test flight ?
Or is it a super scarry long burn of panic ?

 

[Art Upton]

Also, I'd make sure the original is actually a good flyer. Some odd rocs are kind of marginal, and I don't think I would have a lot of faith in an upscale of one of those.

Yea, like an upscale of a Mar Leaper

 

[ljwilley]

Yea, like an upscale of a Mar Leaper

Those things are horrible! They are Styrofoam and they are NOT STABLE AT ALL!

 

[ljwilley]

It's a sort of Spool like short very fat fined rocket.

I'm not sure what your planned flying field is, but if I was RSO and saw CG the same or better then original, I would just put it out 'at the far pads' for the First flight.

You didn't say what type of I motor your going to test it with first, that matters for an RSO as well.

Is it short and sweet for the test flight ?
Or is it a super scarry long burn of panic ?

No super scary long burn of panic. Probably an I316 from Loki, I really want to try one of those.

 

[Azamiryou]

Those things are horrible! They are Styrofoam and they are NOT STABLE AT ALL!

If you upscale it, make it out of Styrofoam so it won't hurt too bad if it hits someone.

 

[ljwilley]

If you upscale it, make it out of Styrofoam so it won't hurt too bad if it hits someone.

Don't worry, I don't think I will make one any time soon...

 

[BABAR]

Very cool. Extra points if you stick the landing.

I would think it would be pretty easy to increase your stability if you don’t go perfectly to scale (not sure how much of a purist you are in that regard.)

Extending the ring fin a bit longer tailward, and or slightly increasing the angle of the pylons should pick up a whole bunch of CP without much cosmetic change.

I always thought this was more of a saucer than a standard finned rocket.

 

[ljwilley]

Very cool. Extra points if you stick the landing.

That would be awesome wouldn't it!

don’t go perfectly to scale

no

I always thought this was more of a saucer than a standard finned rocket.

Yes, but It will still have a parachute and everything.


Another thing- I did not calculate the drag of the rings/small side fins that are on the bottom of the main fins. Couldn't figure out how to do it in rocksim. So that would also bring the CP backwards.

 

[BABAR]

Yes, but It will still have a parachute and everything.

packing space kinda tight.

I guess with electronics everything is possible.

 

[ljwilley]

packing space kinda tight.

I guess with electronics everything is possible.

Already went through that. If I configurate the motor mount a certain way, I will be able to fit a shock cord, my 45" topflight chute, and a chute protector. Just barely.

 

[BABAR]

Already went through that. If I configurate the motor mount a certain way, I will be able to fit a shock cord, my 45" topflight chute, and a chute protector. Just barely.

Again, if you can compromise scale a bit….

 

[ljwilley]

I loaded the original with another A10-3t. If the CP location in the Rocsim was actually correct, the original would have a stability caliber of -.25. So, with that in mind, the upscale would have to be a lot more stable than the original even is.

 

[ljwilley]

And still, I don't want to compromise scale. I want it to be 1:1 with the original, otherwise it's not really a scale model. With the CG information on the original, I think it should fly fine.

 

[Back_at_it]

The Lunar Scout falls into the saucer / spool category and doesn't SIM worth a darn. If you upscale accurately you should have no issues with stability.

I've seen one of these upscaled. It used 24" concrete tube as the outer ring. I believe the main body tube was a LOC 7.5 tube. The whole thing was about 4ft wide. It flew on an G80 motor with no issues.

 

[RocketRev]

You can fake RockSim into adding the "landing pads" of the kit and RockSim will think they are very oddly shaped fins.

All you do is create the fins as "tube fins" on ends of the struts/fins with an "I.D." as small as you can make it. The length of the tube fin is the actual thickness. And just locate the center of each additional "Tube Fin" at the ends of each fin/strut.

As far as not having a nose cone.....I've never actually looked at one of these rockets. So I need to ask, does the parachute just sit inside the tube or is there a short coupler tube with a bulkhead on it? I ask because RockSim used to have a flat perpendicular to the body tube nose cone shape in its options. But its been a long time since I've done anything with RockSim, so this is all from memory.

Brad, the "curious" "Rocket Rev.," Wilson

 

[PhlAsh]

As far as not having a nose cone.....I've never actually looked at one of these rockets. So I need to ask, does the parachute just sit inside the tube or is there a short coupler tube with a bulkhead on it?

It's tumble-recovery

 

[ljwilley]

The coupler will have a bulkhead and will act as the nose cone to deploy the chute. The low power on it table recovery, though.