Booster Shot: another dubious build thread

[neil_w]

Background

I originally conceived of this design as a quasi-joke around the time of the summer build-off of 2016, then put it aside. Later, when @hcraigmiller posted his extremely impressive Anti Virus build thread I realized I needed to upgrade my design... then put it aside again. Then I started to build it... and put it aside again. And now I'm going to give it a proper go. I hope that we're now far enough past the COVID years that I can do this build without any tangential nonsense coming into this thread.

Originally this was to be a single-stage... as part of the later upgrades I toyed with the idea of making it a two stager, and that is what I'm going to build. This will be my first scratch multi-stage, and also my first gap-staged model. So whether it ends up working or not, at least it'll be interesting.

Details





This will be about 24" long and will probably end up about 3 oz dry. It will be 24mm staged to 18mm. I think it'll be stable loaded with D12 -> C6, but I doubt I'll ever fly that configuration. C11 -> C6 seems plausible, though, and plenty high for a rocket like this, as shown in the above screenshot. With not one but *two* rings, and the odd configuration of the booster, I have a bit less confidence than usual in the accuracy of the OR model, so I'm going to leave myself some extra margins of safety.

As the screenshot above shows, there's a lot going on inside this thing, and I haven't worked out all the details of the inter-stage coupling yet (mainly I don't know how long the coupling needs to be.... advice welcome).

Mercifully, for this build the paint scheme is already set, although I haven't completely sorted out the build/paint sequence.

 

[mbeels]

Looks great! Looking forward to this one. 24mm to 18mm BP gap staging is a good way to go, it works really well.

 

[MidOH]

Make sure there's plenty of contaminants in the fluid.

And maybe a bat from a Goblin kit.

 

[jmasterj]

Looking forward to this! Doesn't look nearly as complicated as some of your other builds. What are the rings, 3" and BT-80?

 

[neil_w]

Looking forward to this! Doesn't look nearly as complicated as some of your other builds. What are the rings, 3" and BT-80?

It isn't, for the most part... the rings are BT80 and a bit under 3.5".

Looks great! Looking forward to this one. 24mm to 18mm BP gap staging is a good way to go, it works really well.

I'm always a bit skittish about having underpowered boosters, particularly when there's no lockout electronics for the sustainer. According to OR, C11 -> C6 will stage at about 80' and 125 mph, with 38 mph off the rail. That seems just about right to me for baseline performance. C6 -> B6 and C6 -> A8 are tenable, but I see no reason to risk low speed off the rod when the C11 is available.

 

[jmasterj]

I have personally been a fan of the D12 to A8 combo in my son's Big Bertha. Don't have to worry about underpowered boosters with a D12.

What are you making the larger ring out of?

 

[neil_w]

What are you making the larger ring out of?

 

[lakeroadster]

Looks good Neil.

 

[neil_w]

For terminology reference:

 

[lr64]

Seems like it needs to separate at the Luer lock when letting the chute out, for scale accuracy. ;-)

Is this thing sized for inoculating whales? Probably too big for an elephant.

 

[BEC]

I love it!

 

[Cape Byron]

As I used to say to my patients, “Just a little prick with a needle”.

 

[neil_w]

Seems like it needs to separate at the Luer lock when letting the chute out, for scale accuracy. ;-)

I actually gave some thought to having the plunger separate from the barrel.... just too many problems with it. Right now the plan is for it to separate at the forward end of the piston.

Is this thing sized for inoculating whales? Probably too big for an elephant.

I dunno man, elephants are pretty big.

 

[lakeroadster]

This is a perfect candidate for rear eject, Neil. Imagine how cool it'll be floating down on a parachute... looking like a syringe..

​

 

[neil_w]

This is a perfect candidate for rear eject, Neil. Imagine how cool it'll be floating down on a parachute... looking like a syringe..

It would solve one problem for me, I guess I really should consider it. I don't love the idea of the whole rocket landing on the needle, though.

Not sure I'd want the spool to be that long, though.... it's only going to be a 12" parachute. I think I could make the spool perhaps 2-3" longer than the motor, and that'd be plenty.

I don't have to make the decision on this quite yet, but I need to figure it out sooner rather than later I think.

 

[neil_w]

Forward Ring (aka "Top Collar"), part 1

The forward ring needed to be about 3-3.5". I don't have any tubing of that size, so I had to make it. I went with my tried and true method of 2 layers of bent 1/16" balsa. For a mandrel, I used a spend tape roll, which I save for just such an occasion. I covered it in foil tape so it wouldn't stick to any stray wood glue.

First, I wet a piece and clamped it around the mandrel.


After it dried it had a pretty good starter shape.


I intentionally made the piece too long. After it was dry, I measured the overlap and cut it to size.


Then I had the bright idea of pre-shaping the piece by storing it for a while *inside* the mandrel.


With two pre-curved piece, I was ready to glue them. This is a genuine pain in the neck, because while the pieces are nicely curved, they're still not exactly to shape, and it is hard to clamp them, particular with only one set of two hands.


I decided to tape the ends of the inner piece together using label paper, to keep it in position while I wrestled with the outer piece.

It kind of worked. Ish.


I was eventually able to get it all into position, apply the TBII, and clamp it all together. It is always a sigh of relief to get to this point.


A day or two later it is unclamped and looks pretty good:


Finally the edges are sanded and label paper is applied to the outside and inside, and edges are CAed, etc.


Looks good! But I am not done with this ring, because I had an idea of an alternate way to construct it and had to try it out. To be continued...

 

[ReynoldsSlumber]

I intentionally made the piece too long. After it was dry, I measured the overlap and cut it to size.

Excellent idea. While casting about for cardboard tube-making ideas, I ran across some's idea to make the piece with even more overlap to cut off, so that the ends after trimming are fully curled. Maybe this piece was already long enough for that, dunno.

 

[lr64]

If you boiled the wood, I'd guess you could make it in one piece, twice as thick. Or you could make the ring across the grain. That would be weak until you glued it in place. Then it would be strong, assuming that it had cardboard glued inside and out.

Not too long ago, I made a 1/4" ID tube out of 1/32" balsa. That bending was across the grain, though.

Alternatively, if you had really thin balsa, you could taper both ends and then roll it onto the form until it was thick enough. You could sand out the lump if there was one.

 

[jqavins]

If you boiled the wood, I'd guess you could make it in one piece, twice as thick.

Doing it in one thicker piece wouldn't allow for offsetting the joints, which seems important to me.

Or you could make the ring across the grain. That would be weak until you glued it in place. Then it would be strong, assuming that it had cardboard glued inside and out.

With two layers of cardboard, I'm not sure you'd need the balsa. And with the sandwiched balsa rolled that way, I'm not so sure it would help the cardboard much. Two layers of balsa are lighter than two layers of cardboard, and Neil's method is one he's used before successfully.