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Orbital Transport - Stability problems

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RedShark92

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Howdy all,

This is my first post here, actually, I've been browsing for a few weeks. I was big into all this stuff when I was a kid - I had kinda become a BAR a few years back but then got stalled by general life events... Among the kits that I bought then was Estes' reissue of the Orbital Transport. I started building it then, never got it finished and now that I have something resembling free time again I got the kit out and finished putting it together.

It's just about ready to fly right now and I thought I'd do a swing-test with it. Right now it's failing the swing test, with either a B or C engine and with or without the glider attached.

I added a bit of weight to the hollow nosecone, by way of placing four pennies inside and this seems to have cleared it up. Without the glider, It'll now pass the swing test with the extra weight and I think it'll be OK with the glider, too, because the CG hardly moves when I mount the glider on there.

Anyway - just curious about this - I've never had a problem with stability on anything built from a kit before, though I have had to make adjustments to some customized or modded rockets to get them to work, occasionally.

Aside from that, I'd think the long and lean Orbital Transport, with those huge wing/fins at the base would be super stable. Wondering if I messed something up during the construction but I can't think of anything.

Also a little concerned about the extra weight. I think I'll probably be flying this on motors with short time-delays...
 

foose4string

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Welcome to the forum.

Unless you went real heavy on filler and paint, I don't see any need for extra nose weight on the booster. This design has been proven flight worthy time and time again without the need for extra weight. If you feel more comfortable with it, then go ahead and leave it, but I doubt it's needed(and the B motor may no longer be a worthy option). A B6-4 motor gives the OT a real nice flight to about 300-400 ft - plenty high for a boost glider and a perfect delay. A C6, and you'll have a long walk for the booster and glider(given the slightest wind and depending how well the glider is trimmed).
 
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chanstevens

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Howdy all,

This is my first post here, actually, I've been browsing for a few weeks. I was big into all this stuff when I was a kid - I had kinda become a BAR a few years back but then got stalled by general life events... Among the kits that I bought then was Estes' reissue of the Orbital Transport. I started building it then, never got it finished and now that I have something resembling free time again I got the kit out and finished putting it together.

It's just about ready to fly right now and I thought I'd do a swing-test with it. Right now it's failing the swing test, with either a B or C engine and with or without the glider attached.

I added a bit of weight to the hollow nosecone, by way of placing four pennies inside and this seems to have cleared it up. Without the glider, It'll now pass the swing test with the extra weight and I think it'll be OK with the glider, too, because the CG hardly moves when I mount the glider on there.

Anyway - just curious about this - I've never had a problem with stability on anything built from a kit before, though I have had to make adjustments to some customized or modded rockets to get them to work, occasionally.

Aside from that, I'd think the long and lean Orbital Transport, with those huge wing/fins at the base would be super stable. Wondering if I messed something up during the construction but I can't think of anything.

Also a little concerned about the extra weight. I think I'll probably be flying this on motors with short time-delays...
You're more likely goofing up the string test, either not using a long enough length of string or not getting it moving fast enough.

It's plenty stable with B and C motors, though it certainly wouldn't hurt to eyeball it versus the header card and make sure you attached all the fins ;).
 

Initiator001

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I built an Orbital Transport 35+ years ago.

I have flown it on motors from B4 to D8 and never had a stability issue.

Although it is tired and worn, my OT is still quite flyable. :)

Orbital Transport1.jpg


Orbital Transport Launch.jpg
 

Micromeister

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Concur with everyone's posts,
I've been flying Orbital Transports for about as long as they've been around LOL! in all kinds of Up and Downscales without a single gram of extra nose weight.

As Chan mentioned your likely not getting the OT moving fast enough or using a long enough string. Further Swingtesting is a very very conservative stability test. I use it all the time on ODD-Rocs and other difficult to simulate models known most of the time the model will end up a good bit overstable;)

Long and Short, I'd remove the pennies you've added as all you've actually done is lower the attained altitude your model will be able to achive. and possibly make it so overstable it'll weathercock into the wind after leaving the rod causing a crash.
Micro Maxx Minimum Diameter .281" to BT-80 4 D-12 Clustered versions of the OT have been flying just fine without any additional noseweight for a long long time.
Hope this helps.

s Pic&Flt Page_01-14-04.jpg
 
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hardinlw

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Stability is a tricky thing. The fins on a rocket will stall at around 15 degrees angle of attack (the angle between the fin and the airstream). At low angles of attack, the body tubes do not provide much force or pitching moment, but as angle of attack goes up, they do. For those two reasons, most rockets are stable over a rather limited range of angles of attack, say 0 to 12 or 15 degrees. If you start the swing test of your OT with the pointing straight up, it will not align itself with the direction of travel because the fins are stalled and the body tube is making a fair amount of lift. You need to start out with the rocket aligned in the direction of travel. If it stays there, it is stable at least over some small range. Adding tail weight until it will no longer stay straight will tell you what the CP is and then you will know how stable it is at the nominal CG.
 

mjennings

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I don't have an OT, so I can't speak specifically for it but swing testing a model like the OT is a mixed bag because you actually have to do two swing test one parallel to the wings and one perpendicular to the wings, due to the cross sectional differences.

The swing test starts to get iffy once you get models 3+ feet long, because the goal is to keep the model at a low angle of attack like the rocket would see in flight. To do this the string length and speeds need to increase significantly to be effective. I once tried to swing test a Mean Machine, it did not end well.
 

RedShark92

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Thanks for all the responses, sorry I didn't come back in sooner. I decided not to add the extra weight and flew it twice today, along with a few other models.

Both times were with a B4-2 and it flew very well, if not very high. Had a nice slow lift-off and perfect glider separation and flight both times.

I'll try a B6-2 next time, I am a little iffy on going with a longer delay because it seemed to start to arc over before the ejection charge blew on the B4-2... But we'll see, depending on the performance with a B6-2, I may try a B6-4 to get her a little higher.
 

foose4string

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Thanks for all the responses, sorry I didn't come back in sooner. I decided not to add the extra weight and flew it twice today, along with a few other models.

Both times were with a B4-2 and it flew very well, if not very high. Had a nice slow lift-off and perfect glider separation and flight both times.

I'll try a B6-2 next time, I am a little iffy on going with a longer delay because it seemed to start to arc over before the ejection charge blew on the B4-2... But we'll see, depending on the performance with a B6-2, I may try a B6-4 to get her a little higher.

Good to hear about your successes!

Unless you're dealing with a fair amount of wind or you built heavy, I think the B6-4 will be just fine.
 

RedShark92

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Good to hear about your successes!

Unless you're dealing with a fair amount of wind or you built heavy, I think the B6-4 will be just fine.
Thanks. I'll probably do at least one run on a B6-2, just to be safe... I tend to be conservative with engine strengths and delay times. I'm in Central VT and any field I can find is going to be surrounded by trees and wind is not unusual, so this is usually safer. If the B6-2 run goes OK I'll move it up to the B6-4. I don't think I'll ever be flying this on a C, though.
 

RocketsNorth

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I don't think I'll ever be flying this on a C, though.
Hey Red, I launch my OT this afternoon on the B6-4 and had some minor damage on landing. I'm wondering if a shorter delay would have prevented that :confused2:

I'm hoping for another clam day to try launching a C motor once I get it fixed.
 
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