Tuesday, August 26, 2014

Fixing my vintage air gun

One of my recent obsessions has been watching various DIY videos. A lot of them focus on making compressed air guns. Deep down there is a part of me that wants to make some, however, the sane part of me knows I would just get into trouble. This got me thinking about my old Benjamin Model 132 .22 cal pump air pistol.




I got it in the late 1960s and used the heck out of it. The last time I had used it to any extent was in the mid 80's. About a year ago I found it and thought I'd plink a bit. To my horror I found it wouldn't hold pressure. I learned you could buy kits to replace the seals but didn't pursue the issue. About a week ago, I started doing more research. Near as I could tell and without asking for a quote, the kits plus valve tools would run at least $60. That's assuming I could get it fixed and reassembled on my own. And not get a spring in my eye. I also found that a new model, now made by Crosman, can be had for less than $60. Finally, I checked what I might be able to sell mine for. In its worn condition, I'd be lucky to get $40 for it.

I found some random threads where airgun repair was discussed. They noted that the seals in these old guns were leather and they would dry and shrink if not oiled, which mine wasn't for over 20 years. Some people said they had success just oiling them heavily and letting the leather absorb the oil. There was discussions of gun oil, 10W30, and even airgun oil. I started with what I had: 3-in-1. For a few days, I'd add some oil in the lube hole, tip it on end and wait. Then I'd repeat and tip it the other way. No joy. Next, I decided to try WD40. After a day, I could get a couple of pumps before I heard it leak. That was progress so I continued. This morning I gave it a try and, to my amazement, it was holding some pressure!  I seem to remember it being harder to pump after about 10 pumps. Since I haven't gotten stronger, I suspect it may still be leaking at some pressure, although I can not hear the leak as before. So, while it may not be up to full power, it will shoot a 22 pellet through my foam target at 20'. I'm continuing the oil treatment and will make a better target.



Monday, August 25, 2014

Interesting oddroc from LDRS 33

These came from Liberty Launch Systems/ROCKETS Magazine's LDRS-33 Saturday album. The rocket didn't seem to fly 100% straight but appeared to recover just fine. I'm hoping Neil will talk about it in the launch report. (Photos by Neil McGilvray, used with permission.)





Friday, August 22, 2014

Guidelines for an induction stabilized rocket

Dean Black put together this nice diagram describing the design of an induction stabilized rocket of constant diameter. As they say, a picture is worth a thousand words. It appears that I can shake the lead shot out of the Inductor's NC. Woohoo! E9, baby!

Thursday, August 21, 2014

Inductor, the build, part 3

It appears the inductor may have become the latest victim of weight bloat. The last comment from Dean is that I should probably move the CG a tad further forward. After a few milliseconds of regret about shortening the upper tube, I poured approximately an ounce of shot into the base of the cone. The CG is now about 3.75" ahead of the air gap and the loaded mass is 9.4 oz. Going to my trust thrust-weight graphs, I find the E9 will not be sufficient if this rocket adheres to the common 'rules' regarding said ratios. Bummer.

As with the Cohete Gigante balloon rocket, I am left with a hard decision regarding the motor choice. I hope this is not a sign of things to come!

My only consolation is that, for some rockets, there is no 'correct' motor choice. My closing thought is I need to build and 3/4FNC rocket and give up this oddroc nonsense.

Wednesday, August 20, 2014

Inductor, the build, part 2

Note to self: Launch lug has been installed. I used a 1/4" lug so I can fly it from a high power pad (i.e. further from people). If it flies and survives, I'll try a 3/16". If there is no rod whip and it flies well, I may build a rod-less platform to fly it from.

Now that I got confirmation that my CG is located appropriately, I'm saying this one is done (except for maybe a tad of touch painting around the lug).  I didn't put in much effort finishing the rocket but I didn't want to fly her naked.

The nose cone is loose. Since this is expected to go unstable after burn out, it will slow down fast. Losing the cone may not be the worst thing that can happen.







Inductor, the build, part 1

I proceeded to build my finless, induction stabilized rocket, the Inductor.

The phenolic liner from a Loki 75mm motor is the perfect size to accept 12oz aluminum soda cans...just a tad off from a 2.5" body tube. I didn't want to use the liner and briefly pondered building an 18mm rocket from BT-60. However, I ended up rolling a tube from poster board, 3M "77" spray adhesive, and using Loki casting tube as a mandrel. Quick and sticky. The spray adhesive seemed to work well...until I sprayed on some primer, which promptly caused the tube to start peeling. A little epoxy and a strip of tape took care of that.

I cut a 7.5" length for the induction tube. This fits the rule of having that tube over 2 calibers in length and allows two cans to fit. Two cans with the top and bottom lopped off, that is. I cut one end down on the taper so it would overlap the 2nd can slightly.  These photos show the induction tube with the cans installed. The two small screws allow the cans to be replaceable, will ensure they stay in place and, hopefully, won't affect the airflow. On the other end are 4 popcicle sticks that will hold the upper and lower body pieces together across the air gap. The tie wraps were only used to dry fit the tubes.





This is a quick and dirty build so it required a quick and dirty nose cone. This one is made from a cap from some spray paint, a ball from a ball pit, a cardstock shoulder and some braided twine.



I didn't photograph the upper section. I made it 11" long and ended up using a 24mm mount because I found the required parts i.e. didn't have to make any rings. An E9-4 should be a good motor choice. I added a Kevlar leader and will use some good old Estes rubber band cord.

I should have some photos of the finished rocket later today, complete with its fugly paint job. I think I'll actually need some mass on the bottom to get the CG just above the air gap. Note to self...glue on the launch lug!

Friday, August 15, 2014

A new approach to finless (updated)

A few days ago, one Dean Black posted a unique finless design on Facebook (members can join the NAR group and look for Dean's name). He describes the design as "induction stabilized." He also has a .pdf describing the rocket (again, you'd have to join and ask for it). There are several criteria which must be met for this to have any chance of working. Thus, I will generally describe the rocket but will not provide details until I've flown one and can personally vouch for it.

His design is a two piece body with a central gap and no fins. The motor is in the top section. He claims * two things about this design. First, the rocket will be stable as long as the motor is burning, even if it is a slow speed launch. In fact, he says it is stable with no launch rod. Second, if the lower body is shiny (i.e. made out of aluminum), it will promote the after burning of the CO in the exhaust of a a BP motor. This creates a large, more impressive exhaust plume.

Update: * He sent me a video showing his finless, induction stabilized rocket flying from a flat platform with no rod. So, I guess this is more than a 'claim'.

I am now thinking of using hand rolled 2.5" ID tubing with a 29mm mount. I will have a recovery system (his are very light and don't have one) and will use a lug (just not to press my luck any further). Why the odd tubing? Well, I want to see that afterburn and, apparently, that won't work of the inner surface is crudded up with residue. So, I will cut up soda cans and shoot for a replaceable liner.

My plans may change up to and including their cancellation.

Here's a RockSim model I built just to get a feel for the CG.

Thursday, August 14, 2014

The last Shuttle stack

Or should I say the stacking of a Shuttle?



Caption via Flickr:
JSC2014-E-077299 (14 Aug. 2014) --- Space Center Houston's space shuttle replica Independence is hoisted for placement atop the Shuttle Carrier Aircraft 747. The aircraft and simulated spacecraft tandem will remain on display at the Houston facility where many millions of tourists will be able to visit it in the future. Photo credit: NASA/James Blair