Margins on longer HPR rockets with long motors

[jahall4]

I have never built a rocket around really long cases, but it appears as the motor burns the CG could move forward enough to make it overstable. So how much overstable is generally considered acceptable. Some claim 2 calibers is overstable, RockSim I think uses 2.5. And certainly the rocket more likely to weather cock, but I think I have also seen other issues that may be the result of over stabilization.

What say everyone?

I’m sure this has been hashed out before, but the search function is less than perfect on this site so please be kind and consider my inquiry as sort of a latest pole.

Thanks

 

[dhbarr]

Does overstable still matter when you're going a few hundred miles per hour?

 

[tfish]

Funny you should bring this up. I was just looking around for some stuff for some projects I'm working on for this year. I don't believe, but could be wrong, that's it's not necessarily, just a long motor issue. It is for "high" mach flights. What's high mach? I'm not sure. I've posted the following a few times. Over the years I've seen a bunch of high altitude record attempts. Not all of them go as planned. Usuallly it's a rocket where the fins spans are one caliber and the rockets have around one caliber of stability. Sometimes these rockets work. Most of the times they don't. I call these types of rockets glass slippers. It does not take much for then to "break". For a few years now, I've flying min dia rockets I refer to as AeroPac Sport Flyers. They're not going to break many records. The design is pretty simple, Fin spans of 1.1 calibers and staic margins of 2.5 or more. 3(?) years ago I had a flight mishap where I lost one side of the exit cone on my nozzle. Most rockets would have been toast once this happens. Mine was able to "recover" and still flew to 24K (?). Sorry for all the ?, but I'm on a new laptop and don't have all my notes.

[video=youtube;y5wpiyQfMO0]https://www.youtube.com/watch?v=y5wpiyQfMO0[/video]

One of the threads I was rereading tonight was on this subject starts here https://www.rocketryforum.com/showt...ion-98mm-Minimum-Diameter&p=574080#post574080

In my mind I sort of think it of it like.....searching for words...two force (that work against each other) that get a running start and take some time to stop the momentum. (I'm not sure if it's the right way to think of) and your rocket needs to have enough stability built in to over come these two (additional) forces.

As for the TRF search: try typing in your search in front of the following site:https://www.rocketryforum.com

such as glass slipper site:https://www.rocketryforum.com works pretty good

Hopefully this was helpful.

Tony

 

[Voyager1]

In my own experience over-stable rockets (5 - 7 cal) can fly perfectly straight without significant weather cocking. It does depend on the wind at the site, the motor used, the velocity off the rail, etc. One option is to use a longer rod or rail so that you maximise the launch velocity and counter any wind gusts near the ground.

For example, I use an extended 2.1" PML Gamma with a Contrail 38mm/914mm hybrid motor with an I210 grain. It is over-stable at about 7.5 cal and leaves a 2m rail at about 28 m/s, but 33 m/s for a 3 m rail. In light winds it does not weather cock very much at all. Keep the velocity off the rail as high as possible and the flight is relatively straight up.

This can be an issue when using hybrid motors that are generally very long. There is a net forward-ward shift in the CoG as the N2O leaves the tank in the forward part of the motor and the grain burns.

Check out a previous thread on this subject at https://www.rocketryforum.com/showthread.php?125718-What-to-do-if-rocket-is-overstable

 

[jahall4]

Check out a previous thread on this subject at https://www.rocketryforum.com/showthread.php?125718-What-to-do-if-rocket-is-overstable

Thanks for the link John great thread!

 

[mpitfield]

I have never built a rocket around really long cases, but it appears as the motor burns the CG could move forward enough to make it overstable. So how much overstable is generally considered acceptable. Some claim 2 calibers is overstable, RockSim I think uses 2.5. And certainly the rocket more likely to weather cock, but I think I have also seen other issues that may be the result of over stabilization.

What say everyone?

I’m sure this has been hashed out before, but the search function is less than perfect on this site so please be kind and consider my inquiry as sort of a latest pole.

Thanks

I think it might help if you listed the case, motor/s, and even the rocket design you are planning on using?

I regularly fly a 54mm MD rocket with a Loki 54-2800 case, which is the second largest case next to the Loki 54/4000. This rocket is hardly optimized performance but the design is much more practical for the rocket pastures in Upstate NY. Using the last motor I flew, the Loki L-1040, on the pad she has a stability margin of 2.73 however as you can see, below, 2 seconds into the flight things have shifted around considerably. Also CG is not the only thing acting on your CP. As you move through Mach your CP will move around. This is why knowing the rocket design and motor is helpful in getting good advice.

Here is a OR "Stability vs. Time" graph of my last flight. This rocket has 11 flights on it and it flies very straight and true...at least it appears to from the ground looking up, if you can track it.

Funny you should bring this up. I was just looking around for some stuff for some projects I'm working on for this year. I don't believe, but could be wrong, that's it's not necessarily, just a long motor issue. It is for "high" mach flights. What's high mach? I'm not sure. I've posted the following a few times. Over the years I've seen a bunch of high altitude record attempts. Not all of them go as planned. Usuallly it's a rocket where the fins spans are one caliber and the rockets have around one caliber of stability. Sometimes these rockets work. Most of the times they don't. I call these types of rockets glass slippers. It does not take much for then to "break". For a few years now, I've flying min dia rockets I refer to as AeroPac Sport Flyers. They're not going to break many records. The design is pretty simple, Fin spans of 1.1 calibers and staic margins of 2.5 or more. 3(?) years ago I had a flight mishap where I lost one side of the exit cone on my nozzle. Most rockets would have been toast once this happens. Mine was able to "recover" and still flew to 24K (?). Sorry for all the ?, but I'm on a new laptop and don't have all my notes.

[video=youtube;y5wpiyQfMO0]https://www.youtube.com/watch?v=y5wpiyQfMO0[/video]

One of the threads I was rereading tonight was on this subject starts here https://www.rocketryforum.com/showt...ion-98mm-Minimum-Diameter&p=574080#post574080

In my mind I sort of think it of it like.....searching for words...two force (that work against each other) that get a running start and take some time to stop the momentum. (I'm not sure if it's the right way to think of) and your rocket needs to have enough stability built in to over come these two (additional) forces.

As for the TRF search: try typing in your search in front of the following site:https://www.rocketryforum.com

such as glass slipper site:https://www.rocketryforum.com works pretty good

Hopefully this was helpful.

Tony

Tony I recall seeing that video when it was posted and I find it remarkable that the rocket, first did not part with a fin, and secondly recovered and just kept going on it's merry way.

 

[jahall4]

Tony I recall seeing that video when it was posted and I find it remarkable that the rocket, first did not part with a fin, and secondly recovered and just kept going on it's merry way.

So what was thought to have caused the disturbance?

 

[jahall4]

Here is a OR "Stability vs. Time" graph of my last flight. This rocket has 11 flights on it and it flies very straight and true...at least it appears to from the ground looking up, if you can track it.
View attachment 336326

Thanks for posting the graph. Confirms what I suspected, but am I seeing that the center of pressure moves aft 4"?

 

[mpitfield]

Thanks for posting the graph. Confirms what I suspected, but am I seeing that the center of pressure moves aft 4"?

It depends on where you measure your CG from. I have seen this discussed before and don't recall if there was a consensus or even a standard, however in my case CG is being measured from the nosecone tip, so it is 0. With that in mind, based on the posted graph it moves from 51"ish to 45"ish, so forward roughly 6".

 

[mpitfield]

So what was thought to have caused the disturbance?

Look at the second pic :y:

 

[markjos]

So what was thought to have caused the disturbance?

See Post #3 above:

...3(?) years ago I had a flight mishap where I lost one side of the exit cone on my nozzle. Most rockets would have been toast once this happens....

Pretty remarkable recovery, indeed.

Mark

 

[jahall4]

It depends on where you measure your CG from. I have seen this discussed before and don't recall if there was a consensus or even a standard, however in my case CG is being measured from the nosecone tip, so it is 0. With that in mind, based on the posted graph it moves from 51"ish to 45"ish, so forward roughly 6".

huh? Think you read "Center of Pressure" as CG? Whatever we do lets not get those confused.:wink:

 

[jahall4]

See Post #3 above:

Pretty remarkable recovery, indeed.

Mark

Didn't make the connection, thx.

 

[Steve Shannon]

It depends on where you measure your CG from. I have seen this discussed before and don't recall if there was a consensus or even a standard, however in my case CG is being measured from the nosecone tip, so it is 0. With that in mind, based on the posted graph it moves from 51"ish to 45"ish, so forward roughly 6".

I don’t know if there’s a standard either, but all of Peter Allway’s drawing do stationing from the nosecone tip as well. That’s all I’ve ever seen in manufacturer’s documents also. If it’s not the standard point of reference it ought to be.


Steve Shannon

 

[mpitfield]

huh? Think you read "Center of Pressure" as CG? Whatever we do lets not get those confused.:wink:

I think you need to reread the part of your post I quoted and my response to it. No confusion here.

 

[mpitfield]

Ahh sorry totally my bad I even reread the post, twice. I think I saw what I wanted to. I will check the sim later when I get home



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[AeroAggie]

..<snip>...The design is pretty simple, Fin spans of 1.1 calibers and staic margins of 2.5 or more. 3(?) years ago I had a flight mishap where I lost one side of the exit cone on my nozzle. Most rockets would have been toast once this happens. Mine was able to "recover" and still flew to 24K (?). Sorry for all the ?, but I'm on a new laptop and don't have all my notes.

Tony, just out of curiousity, is that a 3 fin or a 4 fin design? I know there's quite a bit of discussion regarding roll coupling and coning of 3 fin rockets. Looks like yours settled right back into a nearly nominal flight after the wiggle, so just wondering how your data point would play into that kind of discussion.

 

[mpitfield]

Thanks for posting the graph. Confirms what I suspected, but am I seeing that the center of pressure moves aft 4"?

First off, I am starting to wade into waters that are above my snack bracket, however I will attempt to regurgitate what I recall on the topic. Just don't count on the technical accuracy of my explanation.

My simulation (OR) uses the Barrowman formulas for calculating CP. These calculations take into account velocity. This is because the CP moves around as a function of the Mach number. Specifically as you move beyond Mach .5ish the CP shifts aft, however somewhere about Mach .6ish the CP starts to shift forward. This is why the general rule I was taught and generally follow is 2 calibers for high velocity rockets. Of course if you are seriously going for a record then you are likely going to push this rule to the edge.

I re-ran my simulation and this time I used CP vs. velocity. Hopefully this screenshot illustrates the phenomenon clearly.



BTW if you are getting into Mach flights I recommend that you sim your flights using both OR and RASAero. I would not use RS for these flights.

 

[tfish]

Tony, just out of curiousity, is that a 3 fin or a 4 fin design? I know there's quite a bit of discussion regarding roll coupling and coning of 3 fin rockets. Looks like yours settled right back into a nearly nominal flight after the wiggle, so just wondering how your data point would play into that kind of discussion.

That is a three fin rocket. I've not done any 4 fin rockets yet. The one I'm playing around with in my mind might be my first 4 fin. The only reason is that it may still have a good head of steam as it's entering the thinner air.

Tony

 

[BDB]

Do any of you know if OR or RSAero factor altitude into their calculations? It seems that as a rocket gets into thinner air, the movement of CP might become irrelevant. (Someone, please correct me if I’m wrong.)


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[LithosphereRocketry]

Do any of you know if OR or RSAero factor altitude into their calculations? It seems that as a rocket gets into thinner air, the movement of CP might become irrelevant. (Someone, please correct me if I’m wrong.)


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Not sure about stability but both take it into account on altitude calcs.

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[Steve Shannon]

Do any of you know if OR or RSAero factor altitude into their calculations? It seems that as a rocket gets into thinner air, the movement of CP might become irrelevant. (Someone, please correct me if I’m wrong.)


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As air gets thinner the movement of CP forward matters even more. The restorative force is less for two reasons: smaller normal force due to reduced air density and shorter lever arm due to forward movement of CP.

 

[Buckeye]

View attachment 336347

CP Mach shift is cool and all, but this is a non-issue for most HPR fliers. As the plot shows, the most critical point is usually getting off the pad.