Need help on launch rod size

[jp_austin]

My son built a nice looking rocket. Has lots of custom pieces he designed in CAD and had 3D printed at school. ANyway he thinks it will fly and I don't doubt that it can. Just how well is another thing. He purchsed 1/4" launch lugs and I found him a 1/4" stainless rod to use as the launch rod. He was told to use 1/8" lugs for the 1/8" rods, 3/8 for the 3/8" rods etc... It fits on the rod but there is not much if any wiggle room. He got them aligned correctly and all but just seem like he is asking for trouble if he tries to launch and the rocket will not freely slide off the launch rod. Did he get incorrect info for the launch lugs? Is one sellers 1/4" lugs another's 1/8" size?

Before I give him the OK to launch this beast I want to at least ensure this aspect of his design. Think he is planning on an "E" sized motor and he has done a number of smaller rockets in the past but this one scares me and especially with it's close tolerance fit on the launch rod. Advice and assistance is greatly appreciated.

[o1d_dude]

As long as the rocket slide easily on the launch rod, I don't think there should be any problem. If there is binding, scraping, and such, you might need a smaller diameter launch rod.

I'd check the diameter of your stainless steel rod with the calipers to make sure the rod is in fact 1/4".

If all else fails, you can buy a length of 1/4" cold rolled steel very inexpensively at Home Depot/Lowe's/Menard's.

[Aksrockets]

Make sure that there is nothing in the launch lug that might restrict the rod. Check for any epoxy or other adhesive, you might run some sand paper through it to get it to a good fit.
Ive had my fair share of problems with launch lugs. A friend and I were flying a Castillo when we noticed it was hardly fitting on the launch rod. With further inspection I found that there was a small dot of epoxy on the inside of the lug. A few passes of sand paper and the rod was fitting smoother, but still tight. So, we chose a motor that was sure to kick it off the pad without getting stuck.

http://www.youtube.com/watch?v=Lm-6475ZXtQ

Good luck!

Alex

[Disaster_Guy]

Previous response is right on the money. I have had no issue with rods from Home Depot matching size of commonly available lugs. I will throw out a few other things to consider here though if you have concerns to begin with...

Just as important as the size of the rod is the base the rod will be secured within. A standard Estes style launch pad should no be used as-is with a rocket that requires a 1/4" rod (in face it will not fit a 1/4" rod without modification). If you don't have a heavy steel pad base to launch it from, it likely needs to be staked down. The length of the rod is also important. It needs to be long enough for the rocket to attain sufficient velocity for stability prior to leaving the rod.

If you have questions about the stability of the rocket, download the free program "Open Rocket" and have him model it in there. Make sure that the program shows that it is stable and also use that to ensure that the appropriate motor is used. I can not picture a rocket that really requires a 1/4" rod (due to weight) that would fly on an E motor.

[blackbrandt]

Make sure that there is nothing in the launch lug that might restrict the rod. Check for any epoxy or other adhesive, you might run some sand paper through it to get it to a good fit.
Ive had my fair share of problems with launch lugs. A friend and I were flying a Castillo when we noticed it was hardly fitting on the launch rod. With further inspection I found that there was a small dot of epoxy on the inside of the lug. A few passes of sand paper and the rod was fitting smoother, but still tight. So, we chose a motor that was sure to kick it off the pad without getting stuck.

http://www.youtube.com/watch?v=Lm-6475ZXtQ

Good luck!

Alex

Couple of questions. First, what the heck was that motor? Second of all, have you ever seen a rocket take the pad into the air because that motor was too big (oh, wait, that is impossible!) or the lugs or buttons were misaligned?

[Rocketbuilder]

It depends on how tight it is; one of my rockets is pretty snug on the rod, but an E9 still gets it off the pad fine. Like old_dude said, just make sure there is no binding or scraping. All 1/4" lugs and rods should be the same size, unless there is a manufacturing defect.
Disaster_guy is right about OpenRocket (http://openrocket.sourceforge.net/). Unless you've already calculated the stability, a simulation is a good way to be sure that it won't go skywriting.
The part about 3D printed parts caught my attention; I'd love to hear more about the details of the rocket. Maybe a picture?
If you and your son are interested in rockets, this is a wonderful resource; there's tons of information on the forum, and the people here are friendly and happy to help.

[jp_austin]

The rod is secured to a steel plate that is 3" X 4" and that is bolted to a 3/4" piece of plywood that is about 2.5' X 4'. Plenty of stability here. The rod is in fact 1/4" stainless as measured with the calipers. I will double check with him on the glue. He used some type of rubbery glue as he put one mount on the base where his 3D printer fins attach. He didn't want to use epoxy for some reason. I asked him now to go inspect the lugs closely and run a piece of sandpaper down them. He said it slides a little easier but still not terrific. If you do push from the bottom as a motor will then the rocket slides fairly easily but not like earlier loose fit type launch lugs with C type motors and and an old Estes launch rod.

Here is his rocket below. He built this with some funky lattice fins. He was unsure if they would work at all since the mesh size is fairly small. It's a bit of a guess and "lets see if it works" type of thing. See a pictures here:


All the red pieces sans the small nuts and bolts to hold the fins in, are built on the 3D printer in plastic. The pieces are not super light weight but not terribly heavy. Without the motor the thing weighs in at 485gm (17 1/8 oz). Yes he used the design tools but the lattice fins are where there is a huge unknown. Unfortunately he did not have access to some high end flow modeling tools for the fins. It may end up being a bit of a gamble.

[Rocketbuilder]

What size E motor was he planning on using? An Estes E will not safely lift a 17 1/8 oz rocket... you are going to need a composite motor. Aerotech has some single use composite E motors that aren't terribly expensive. Was that what he was going to use?
As for the fins... I'm not sure if they'll work. There's probably someone around here who could tell you. It seems it it almost more like it's relying more on base-drag than anything else.

[blackbrandt]

Maybe an E39 or something like that.

[Rex R]

estes launch rods and lugs are a smidge under size(in particular the 1/4" lugs). dusting the inside surface of the lugs with powdered graphite(lock lube) will help.
rex

[Rocketbuilder]

estes launch rods and lugs are a smidge under size(in particular the 1/4" lugs). dusting the inside surface of the lugs with powdered graphite(lock lube) will help.
rex

Huh. Weird, but smart to make them smaller. Good to know; I stand corrected.

[kruland]

Looks really cool. Those are grid fins and found on some military hardware. I honestly don't know much about them but there is certainly lots of top secret papers out there describing them.

My advice is to try to swing test it first. Then launch in a very secluded location with as few people as possible around. It could end up ugly.

Please take a movie for us.

The rod is secured to a steel plate that is 3" X 4" and that is bolted to a 3/4" piece of plywood that is about 2.5' X 4'. Plenty of stability here. The rod is in fact 1/4" stainless as measured with the calipers. I will double check with him on the glue. He used some type of rubbery glue as he put one mount on the base where his 3D printer fins attach. He didn't want to use epoxy for some reason. I asked him now to go inspect the lugs closely and run a piece of sandpaper down them. He said it slides a little easier but still not terrific. If you do push from the bottom as a motor will then the rocket slides fairly easily but not like earlier loose fit type launch lugs with C type motors and and an old Estes launch rod.

Here is his rocket below. He built this with some funky lattice fins. He was unsure if they would work at all since the mesh size is fairly small. It's a bit of a guess and "lets see if it works" type of thing. See a pictures here:


All the red pieces sans the small nuts and bolts to hold the fins in, are built on the 3D printer in plastic. The pieces are not super light weight but not terribly heavy. Without the motor the thing weighs in at 485gm (17 1/8 oz). Yes he used the design tools but the lattice fins are where there is a huge unknown. Unfortunately he did not have access to some high end flow modeling tools for the fins. It may end up being a bit of a gamble.

[Rex R]

I found out about the lugs when doing my ventris, fortunatly my arrow shaft alignment tool allowed me to get them lined up . the 'fins' should work...provided they can handle the stress(google 'MK82 snake eye bomb')

[hornet driver]

I found out about the lugs when doing my ventris, fortunatly my arrow shaft alignment tool allowed me to get them lined up . the 'fins' should work...provided they can handle the stress(google 'MK82 snake eye bomb')

Rex R hit it on the head . The fin design is draggy and hence puts a lot of stress on the attachment points. In your case they look substantial. And draggy may not be a bad thing! They do however tend to have a lot of directional authory so make sure they are true. + 1 on the graghite powder !!

[cwbullet]

Lots a drag, but the air force has used these for years.

[o1d_dude]

Rex R hit it on the head . The fin design is draggy and hence puts a lot of stress on the attachment points. In your case they look substantial. And draggy may not be a bad thing! They do however tend to have a lot of directional authory so make sure they are true. + 1 on the graghite powder !!

Yep. Mini-tube fins. Very draggy but arrow straight flight trajectory when aligned properly.

[jp_austin]

Yes he is using a large Aerotech E39 (I think). I know they were single use type and not terribly expensive. Will try this morning when few or no people are out. Going to a large area outside of town to try it out. Will takes pics and video!

Update - He got it out this morning early. Was an F32 engine and not an E after all. It managed to get off the launch rod very easily. First time for the design and it flew very well. Yes the fins were big ole drag generators but they did their job and the thing flew very straight. I have a video showing it initially looked like it cocked to the side slightly right off the launch rod but it was very straight a few feet into the flight. Will never set any impressive heights but for small fins on a self designed rocket I would say he did very well. Home built chute worked well and with such low winds only had to go about 100 yards to retrieve it. He was psyched as he worked many months in his Aerospace class in high school designing the thing. He wants to build an altimeter before he flys it again.

[MarkII]

Nice design! The grid fins are a great touch.

[cwbullet]

1/4" should be plenty big.

[Rocketbuilder]

Update - He got it out this morning early. Was an F32 engine and not an E after all. It managed to get off the launch rod very easily. First time for the design and it flew very well. Yes the fins were big ole drag generators but they did their job and the thing flew very straight. I have a video showing it initially looked like it cocked to the side slightly right off the launch rod but it was very straight a few feet into the flight. Will never set any impressive heights but for small fins on a self designed rocket I would say he did very well. Home built chute worked well and with such low winds only had to go about 100 yards to retrieve it. He was psyched as he worked many months in his Aerospace class in high school designing the thing. He wants to build an altimeter before he flys it again.

I'm glad to hear that! It's definitely very rewarding to see your own designs fly. With an F32 in that rocket, it must have been an impressive flight.
Is he hooked yet? I'll warn you, rocketry can become an addiction!

[jp_austin]

I'm glad to hear that! It's definitely very rewarding to see your own designs fly. With an F32 in that rocket, it must have been an impressive flight.
Is he hooked yet? I'll warn you, rocketry can become an addiction!

Yes he was grinning from ear to ear. He likes to do this type of thing and I have encouraged it over the years!

His previous projects were not rockets but were aerospace related. Started with a simple jet engine build and he went overboard by turning it into a twin shaft with dyno.

Junior Year:
http://www.youtube.com/watch?v=YOeQMVDiSMY

Senior Year:
http://www.youtube.com/watch?v=C2ojygEWAGs

[Rocketbuilder]

A jet engine? OK, your son is awesome.

[luke strawwalker]

The rod is secured to a steel plate that is 3" X 4" and that is bolted to a 3/4" piece of plywood that is about 2.5' X 4'. Plenty of stability here. The rod is in fact 1/4" stainless as measured with the calipers. I will double check with him on the glue. He used some type of rubbery glue as he put one mount on the base where his 3D printer fins attach. He didn't want to use epoxy for some reason. I asked him now to go inspect the lugs closely and run a piece of sandpaper down them. He said it slides a little easier but still not terrific. If you do push from the bottom as a motor will then the rocket slides fairly easily but not like earlier loose fit type launch lugs with C type motors and and an old Estes launch rod.

Here is his rocket below. He built this with some funky lattice fins. He was unsure if they would work at all since the mesh size is fairly small. It's a bit of a guess and "lets see if it works" type of thing. See a pictures here:


All the red pieces sans the small nuts and bolts to hold the fins in, are built on the 3D printer in plastic. The pieces are not super light weight but not terribly heavy. Without the motor the thing weighs in at 485gm (17 1/8 oz). Yes he used the design tools but the lattice fins are where there is a huge unknown. Unfortunately he did not have access to some high end flow modeling tools for the fins. It may end up being a bit of a gamble.

That's cool... Actually the Soviet N-1 moon rocket used fins identical to these, as well as the Soyuz rocket presently launching our astronauts to the ISS with their Russian hosts. On the Soyuz they remain folded up against the sides of the upper fairing just below the Soyuz capsule to minimize drag (and their effects on the vehicle stability) unless the Soyuz does an abort and fires its launch escape rocket tower... then the panels fold down into the airflow automatically to stabilize the stack. Presently the N-1's grid fins have been repurposed to create a small picnic structure near the Baikonur Cosmodrome in Khazakhstan... I saw pictures of it a few days ago on the net...

Basically they work as "drag fins" and move the CP back by creating drag... not terribly efficient in flight, but they DO work. As for sizing, no, the programs like Rocksim and Open Rocket don't really model these well that I'm aware of. I'd suppose that you could simulate or calculate the stability similar to how you would for a cone-fin rocket (one using a large flattish cone near the back of the rocket to move the CP aft and create stability). Grid fins of similar area should be stable, but always do a swing test to make sure, or a heads up flight in isolation to ensure stability before launching anywhere with crowds around...

As for the launch lugs, I PREFER a loose fit... a loose fitting lug will not cause any problems with stability, especially if you're using dual lugs (one mounted about halfway up the rocket, the other near the back end of the rocket), BUT a tight-fitting launch lug can EASILY bind or hang up on the launch rod, creating problems... Either the rocket won't lift off and simply burns up the motor on the pad or hangs up about halfway up the rod and sorta flails around up there under thrust, hopefully not turning the launch pad over and starting to "land shark" across the ground. WORST of all, is if the rocket lifts off TAKING THE ROD WITH IT! Then you have a falling piece of steel rod which is nigh impossible to see from the ground plummeting to earth at high speed!

If in doubt, sand until everything is nice and loose... or replace the launch lugs! Better safe than sorry... OL JR

There's also a bomb or missile that uses those type "grid fins"

[GregGleason]

... There's also a bomb or missile that uses those type "grid fins"

MOAB.




Greg

[luke strawwalker]

MOAB.




Greg

THANKS GREG!! That's the one I was thinking of...

Do you know where this museum is??

Later! OL JR

[GregGleason]

THANKS GREG!! That's the one I was thinking of...

Do you know where this museum is??

Later! OL JR

That's in Eglin AFB in sunny Florida.

http://en.wikipedia.org/wiki/Air_Force_Armament_Museum

Greg

[Aksrockets]

First, what the heck was that motor?

G250 Vmax. One of my favorites.

Alex

[luke strawwalker]

That's in Eglin AFB in sunny Florida.

http://en.wikipedia.org/wiki/Air_Force_Armament_Museum

Greg

Thanks... I found it on the wiki page right after I posted...

Hmmm.... going to Florida in a week... might have just found another attraction on the way home...

Later! OL JR