4" Pershing 1A and 2B scratchbuild

Just a quick note, I wanted to replace my original Pershing1A nosecone after some damage to the tip after landing on some hard surface(the tip was almost all resin and gel-coat on the performance nose cone for the first two inches, so no real strength....In any case, I tried one of the proline black 5:1 4" concicals with aluminum tip. What an excellent product! For the first time in my life, I didn't need to sand the shoulder, clean up anything, apply tape to get it to fit. It fit perfectly into my 4" loc cardboard tube. I removed the aluminum tip by unscrewing, lopped about 2" off the end, and re-profiled it on my drill press and belt sander to a more angled pershing like tip and reinstalled it with a little bit of added shot/epoxy that was needed for CG. I cut and instaled a bulkhead and used three screws to hold the shoulder into the cone section so I could remove it and add more weight for larger motors if needed. I've painted it and am just waiting for some replacement vinyl from stickershock. If anyone is considering a nice cone, these are first rate products and well worth the price in my opinion. The aluminum tip will help protect against landing damage and since the cone was hollow all the way to the aluminum tip I was able to use less nose weight than before. I re-simmed using rocksim equations and can move the CG back a bit so can now fly it on G104 and G79W's and G-77R's in the hp 100 and 120 casing, as well as G75 and G80's

Frank


Frank

Beautiful Work and awesome Launch Pics!!!:clap:

This Thread has pushed me over the Top to want to build a Pershing.

Very Cool!!! :w:

I love the Pershing 1a! My friend and I had the old Estes one and it always produced "interesting" flights... especially on a Compost Dynamite E20!

I also have noticed that even though it's tracks quite straight, there is some coning of both my small version here and my 7.5" version after burnout. I did some reading and determined that the coning seems to be related to overstable designs with small fins. I've been using openrocket, so I downloaded the trial version of rocksim and used the rocksim CP calculation instead of borrowman(which matched openrocket) and that moved back the CP an inch or two. My design and fin placement is more scale than the estes version so my CP is at 15" from the rear of the rocket instead of 13" for the estes version with larger fins....my initial openrocket was saying CP at 17" from the rear for a 42" long 4" diameter rocket. This will allow me to fly on slightly larger motors and still keep a caliber or more of margin..I can use the 120ns G79W or G77R motors in addition to G80's and G75's. I could also do G64 and G76's but I'm not a huge fan of the 40-120 casing personally....

burkefj said:

I also have noticed that even though it's tracks quite straight, there is some coning of both my small version here and my 7.5" version after burnout. I did some reading and determined that the coning seems to be related to overstable designs with small fins. I've been using openrocket, so I downloaded the trial version of rocksim and used the rocksim CP calculation instead of borrowman(which matched openrocket) and that moved back the CP an inch or two. My design and fin placement is more scale than the estes version so my CP is at 15" from the rear of the rocket instead of 13" for the estes version with larger fins....my initial openrocket was saying CP at 17" from the rear for a 42" long 4" diameter rocket. This will allow me to fly on slightly larger motors and still keep a caliber or more of margin..I can use the 120ns G79W or G77R motors in addition to G80's and G75's. I could also do G64 and G76's but I'm not a huge fan of the 40-120 casing personally....

Click to expand...

Out of curiosity, which software is more accurate to you? OpenRocket or RockSim?

My 7.5 Pershing 1A also cones when launched with a K550W (mainly toward the end of the burn when the thrust drops off). I have a K1103X that I am going to try for it's next launch to get it up to speed faster which I hope will minimize the coning.

I haven't used rocksim enough to know how close it is, but from what I've read it is less pessimistic than normal borrowman assumptions.

As for the coning, it is supposed to stem from overstability, and large moment of inertia.

A cut and paste here....:

Try to reduce the "moment of inertia" of your rocket. Your fins are trying to keep the rocket straight. They apply a limited torque due to their force being applied out at the end of a lever arm with the fulcrum at the CG. Now, if your rocket has most of its mass near the CG, it will have a lower moment of inertia, and snap back into line quicker than a rocket with most of its mass out near the nose and tail. The CG might be in the same place, but imagine spinning a three foot dowel with a 1 lb lump of clay at the center, or a 3 ft dowel with 8 ounces of clay at each end to see the difference.

Third, keep the rocket stable, but not overly so. Remember: the rocket rotates (including coning) around its CG. The farther forward of the rocket's center that the CG is, the bigger the circle the tail end will describe in the sky when (not if) it cones.

Making over stable rockets by adding lots of nose weight exacerbates the coning problem two ways: it increases the moment of inertia, which means you get bigger pitching moments before the fins can counteract them, and also means that the gyroscopic effects are larger. You also move the CG forward, so the magnitude of any coning is much worse.

terryg said:

My 7.5 Pershing 1A also cones when launched with a K550W (mainly toward the end of the burn when the thrust drops off). I have a K1103X that I am going to try for it's next launch to get it up to speed faster which I hope will minimize the coning.

Click to expand...

I have removable nose weight in mine (fender washers) that I might remove for the next flight to test this. My launch pad version (4 inch) has always flown perfectly with no issues at all. Even with the coning the 7.5 inch has given an impressive flight with a K550 (lots of flame and smoke on a low and slow flight).

You can see the coning as you step thru this series of launch photos:

https://fotos.nahcpj.com/sb_2013/h5d43a6b8#h5d43aae8

I'm curious, what is the distance from the rear of the rocket to the front of the forward fins, and where is your CG? For both the large version and the launch pad version?

Frank

terryg said:

I have removable nose weight in mine (fender washers) that I might remove for the next flight to test this. My launch pad version (4 inch) has always flown perfectly with no issues at all. Even with the coning the 7.5 inch has given an impressive flight with a K550 (lots of flame and smoke on a low and slow flight).

You can see the coning as you step thru this series of launch photos:

https://fotos.nahcpj.com/sb_2013/h5d43a6b8#h5d43aae8

Click to expand...

This is the rocksim file for the 7.5 inch version. It is based on the one at Rocket Team Vatsaas's web site. I will try to dig out the launch pad version and make a sim file for it this weekend as I just built it per the instructions and it has always flown great. It is amazing just how light the launch pad version is.

View attachment big-ass-Pershing.RKT

I see yours is a little stretched, but our cp's are similar, I think I'm running mine with the CG a little behind where the cone hits the main body diameter, which is probably over-stable and part of the cause for the coning...I'm curious to try moving it back on my smaller version first.

I don't know where relative the launchpad wants their CG.

Frank

terryg said:

This is the rocksim file for the 7.5 inch version. It is based on the one at Rocket Team Vatsaas's web site. I will try to dig out the launch pad version and make a sim file for it this weekend as I just built it per the instructions and it has always flown great. It is amazing just how light the launch pad version is.

View attachment 188919

Click to expand...

This is what mine is based on:

https://vatsaas.org/rtv/arsenal/bradrocs/pershing/pershingdesign.aspx

The Launch Pad kits never give CP positions as they are midpower kits built with low power parts and designed and tested by test flights. They use paper shrouds stiffened with with CA coatings and use recessed motor mounts and light weight balsa to minimize weight. I have to repair some fins on mine to get it flight ready again and will do a sim file at the same time. I am planning to fly them again at a launch in mid December and will be flying the 7.5 on the faster burning K1103 "propellant X" and will remove nose weight and record exactly where it balances out before I launch it. I figure the extra speed should help the stability forces from the small fins and the long nose to give a better flight.

Here is the new cone finished with markings from stickershock.....I'll also throw in a picture of my 4" madcow Patriot also with stickershock markings. For my 20th work anniversary they now give you a prepaid gift card, so this is what I bought, set up for dual deploy with an easy mini.....

Frank

Quick update, with the CG moved back a bit had a flight this morning on a G-75, video shows just the liftoff...still just a bit of coning, but it should fly find on an H-128 based on the new balance point.... Just over 3 pounds ready to fly in this video.
https://www.youtube.com/watch?v=g0xmjelLZks&feature=youtu.be

Frank

OMG - cant wait to see this on the flightline! Great job Frank!