Estes "Thermonuclear" Ejection Charges

[Tau Zero]

Since Estes is making its rocket motor ejection charges (exponentially?) stronger than 30 years ago, it seems only prudent to take that into account when we design rockets for the 21st century. (rolleyes) (sad)

This all started coming together for me when I put an engine hook on an ST-7/BT-20 18mm minimum diameter rocket, and the A8-5 ejection charge tore an inch-long gash in the body tube. Then Bill Eichelberger had a (*much!*) more destructive experience with a Centuri "Taurus" clone, which also had a hollowed-out transition:

http://www.rocketreviews.com/reviews/kits/sem_taurus.html

That's why I'm wondering if engine hooks are even a good idea in rockets that have low-volume, high-pressure interiors. My Semroc "Javelin" (a minumum-diameter 18mm rocket) survived the Estes charges nicely, because it DIDN'T HAVE AN ENGINE HOOK.


Here's the question. Currently I'm working on a design (pictured below, minus fins) inspired in part by the "Intrepid" from the Fall 1975 Centuri "Rocket Times":

http://www.ninfinger.org/~sven/rockets/catalogs/centuriRT75FallSpecial/75FallScenRT2.html

This design has the parachute in the ST-7 tube, while the BR-710 reducer/transition has been drilled out to "port" the ejection gases.

I started building a pre-release Semroc "Aphelion" clone last spring. The 10" long ST-10 body tube and EM-710 motor mount made it through the Great Basement Flood of 2005 with only a dent, and I could easily salvage it for my new design by cutting off the front 4" of the body tube. But the motor mount HAS AN ENGINE HOOK!


In Dennis McClain-Furmanski's EMRR review of the Semroc "Mark II" (which has a 6" long ST-10):

http://www.rocketreviews.com/reviews/kits/sem_mark_ii.html

...he mentioned that it was tough to stuff all of the recovery system into that limited area. That's why I want to drill out the middle 1/2" of the BR-710 reducer (see below). But given the "thermonuclear" ejection charges from Estes motors, I *don't* want to risk blowing a major hole in the ST-10 body tube. In cases like this, I don't mind friction-fitting motors (which I think I would need to increase the chances of the ejection charge pushing the parachute and nose cone out of the ST-7 tube.


So here's my point: It looks like some of our current designs/builds would survive *more* flights IF WE LEFT OFF THE ENGINE HOOKS. (I hate to be an Ultra-Conservative Rocketeer, but maybe this idea will help lots of you guys.)

Any feedback?


Thanks, and cheers,

--Jay

[John Brohm]

So here's my point: It looks like some of our current designs/builds would survive *more* flights IF WE LEFT OFF THE ENGINE HOOKS. (I hate to be an Ultra-Conservative Rocketeer, but maybe this idea will help lots of you guys.)

Any feedback?


Thanks, and cheers,

--Jay

Hi Jay;

I never have understood the reasoning behind putting an engine block in front of the engine hook, yet nothing behind it. Presumably the thinking was/is that the forward thrust is way more powerful than the rearward thrust from the ejection charge, and I'm sure this is the case. But my experience over the years has been the same: eventually those engine hooks work loose because of the rearward ejection force. One possible manufacturing solution might be to make the hooks a quarter of an inch longer, so that the end can be installed in front of, not behind, the engine block.

In any case, I stopped installing the hooks the way Estes recommends a long time ago. I use an engine block for the forward thrust, and I cut the hook, place a reverse bend (an outside bend) and then secure it on the outside of the MMT with another ring (don't like the modern thumb tabs the present hooks have anyway, and this method gives me an excuse to cut them off). Obviously this method can't work on a minimum diameter rocket.

I'm in Vegas on business just now so I can't upload a picture, but I'll shoot you one at the end of the week once I get home.

Hope this helps;

[A Fish Named Wallyum]

These days I rarely bother with a hook. Masking tape has yet to let me down. Some of the kits still get a hook, but my clones are less likely.

[CPMcGraw]

Currently I'm working on a design (pictured below, minus fins) inspired in part by the "Intrepid" from the Fall 1975 Centuri "Rocket Times":

http://www.ninfinger.org/~sven/rockets/catalogs/centuriRT75FallSpecial/75FallScenRT2.html (http://www.ninfinger.org/~sven/rockets/catalogs/centuriRT75FallSpecial/75FallScenRT2.html)

This design has the parachute in the ST-7 tube, while the BR-710 reducer/transition has been drilled out to "port" the ejection gases...It looks like some of our current designs/builds would survive *more* flights IF WE LEFT OFF THE ENGINE HOOKS. (I hate to be an Ultra-Conservative Rocketeer, but maybe this idea will help lots of you guys.)

Any feedback?

You need to do more than just drill out the transition. When you remove that much material from the piece, you've given that pressure build-up a place to break free from the expansion chamber, should there be any obstruction of the nose cone at deployment. You have to reinforce that passage with a length of ST-3 tubing. ST-5 would be too large, because it would not leave you enough of a wall thickness in the shoulder to prevent a stress failure.

Second, the ST-7 body tube really is too small for any parachute over about 10" in diameter. Remember you still have to stuff wadding into that tube, and it's the wadding that can get stuck. This happened to me on a Quest Gamma Ray.

I know you're wanting the recovery components to be in the ST-7, but I really think the safest place is down in the ST-10, using a mid-body breakpoint. The transition shoulder can be adjusted to fit without binding, and the system has a better chance of working. Would it be too much of a change to bump up to an ST-8 for the upper tube?

As for the engine hook, as long as the ejection gas pressure is relieved within the "normal span of deployment time" (a few milliseconds) there should not be enough retro force to cause a problem, as long as the hook is reasonably secure to begin with. If it wiggles side-to-side, it's too loose. It's when you get "ejection constipation" that motors start ripping things out...

[Initiator001]

One possible manufacturing solution might be to make the hooks a quarter of an inch longer, so that the end can be installed in front of, not behind, the engine block.



The AeroTech kits incorporate this feature.

I can't take credit for it, Scott Pearce came up with the idea. ;)

Bob

[Tau Zero]

You need to do more than just drill out the transition. When you remove that much material from the piece, you've given that pressure build-up a place to break free from the expansion chamber, should there be any obstruction of the nose cone at deployment. You have to reinforce that passage with a length of ST-3 tubing. ST-5 would be too large, because it would not leave you enough of a wall thickness in the shoulder to prevent a stress failure.What about slitting an ST-5 so it would fit into the 0.5" passage? :confused: Or not?


Second, the ST-7 body tube really is too small for any parachute over about 10" in diameter. Remember you still have to stuff wadding into that tube, and it's the wadding that can get stuck.Yeah, and there's still the 600 lb. Kevlar cord attached to the engine mount, as well.


I know you're wanting the recovery components to be in the ST-7, but I really think the safest place is down in the ST-10, using a mid-body breakpoint. The transition shoulder can be adjusted to fit without binding, and the system has a better chance of working.
Then I run into having to extend that particular body tube so everything will fit. (heavy sigh) But I'm thinking I'll have to put the parachute, etc., in the same section of the piecemeal Semroc Taurus I haven't built yet, to avoid what happened to Bill's Taurus.


Would it be too much of a change to bump up to an ST-8 for the upper tube?(mock "offended" look, stuffy British accent) "But of *course.* Then I wouldn't be able to use the PNC-76 counterpart." ;)


As for the engine hook, as long as the ejection gas pressure is relieved within the "normal span of deployment time" (a few milliseconds) there should not be enough retro force to cause a problem, as long as the hook is reasonably secure to begin with. If it wiggles side-to-side, it's too loose. It's when you get "ejection constipation" that motors start ripping things out... I'm more concerned with the "hooked" motor blowing out the side of the ST-10, or another structural component...

--Jay

[CPMcGraw]

What about slitting an ST-5 so it would fit into the 0.5" passage? :confused: Or not?

And thus introduce a big seam for the gasses to find? :( The purpose of the tube is to keep the gasses from finding the weakened balsa in the first place. SEMROC sells BT-3. It'll be big enough...

Then I run into having to extend that particular body tube so everything will fit. (heavy sigh)

Point taken. Keep it as a last-gasp alternative...

(mock "offended" look, stuffy British accent) "But of *course.* Then I wouldn't be able to use the PNC-76 counterpart." ;)

I take it you've already got this part, right? (Upper-crust London-style harrumpffing sounds...) I guess we'll just have to live with that...:rolleyes:

I'm more concerned with the "hooked" motor blowing out the side of the ST-10, or another structural component...

Take a strip of computer printer paper, about 1/2" wide and about 4" long. Apply glue to one side of the paper, and start wrapping it around the top end of the motor tube such that you cover the front end of the hook twice. While the glue is wet, run your fingernail alongside the front and side edges of the hook, and press the paper down tightly into the corners. When this dries, you'll have a seal around the slot in the motor tube, and the hook will have better anchoring to the tube. The two centering rings should be notched only enough to allow the hook to pass without creasing the motor tube out of shape. There should be no play in the hook at all. The paper strip and the rings themselves will keep the hook secure to the tube, and the notches in the rings will prevent excessive side-to-side play.

The real problem is still having a tight-fitting recovery system. As long as the nose cone comes off cleanly, and the wadding pushes out completely in just a few milliseconds, there should be no damaging retro pressure against the motor mount.

Possible alternative method -- use a piston in the ST-7 that slides easily instead of wadding which can "bunch" and jam.

[A Fish Named Wallyum]

Then I run into having to extend that particular body tube so everything will fit. (heavy sigh) But I'm thinking I'll have to put the parachute, etc., in the same section of the piecemeal Semroc Taurus I haven't built yet, to avoid what happened to Bill's Taurus.
--Jay


Not to let Estes off the hook, but I think my problem was at least partially caused by thinning the balsa sides of the transition too much. Too thin at a critical point.

[Tau Zero]

And thus introduce a big seam for the gasses to find? :( The purpose of the tube is to keep the gasses from finding the weakened balsa in the first place.
Yeah, that's why I thought it wasn't such a good idea, on second thought. :(


SEMROC sells BT-3. It'll be big enough...I got a sample of it in my one and only "Totally Tubular" order about 5 years ago. Although that *still* looks awful skinny and pressure-inducing.


I take it you've already got this part, right? (Upper-crust London-style harrumpffing sounds...)"Yes, well, one can only make *so many* Vector-V clones, *can't* one?" (incomprehensible sounds in kind)


Possible alternative method -- use a piston in the ST-7 that slides easily instead of wadding which can "bunch" and jam.*Ooh.* --*That's* a cool idea! I'll have to look into that! :D :cool:

But then where would I attach the Kevlar? (thinking furiously) Hmm... :p

Thanks, Craig!

--Jay

[CPMcGraw]

Not to let Estes off the hook, but I think my problem was at least partially caused by thinning the balsa sides of the transition too much. Too thin at a critical point.

You might want to try the liner tube if you decide to do another Taurus that way. Also, try using the paper strip over-wrap I mentioned to Jay. It's a cheap reinforcement in a critical area.

[CPMcGraw]

*Ooh.* --*That's* a cool idea! I'll have to look into that! :D :cool:

But then where would I attach the Kevlar? (thinking furiously) Hmm... :p

Thanks, Craig!

--Jay

Run the Kevlar through that liner tube and attach it to the motor mount as you normally do. Make the Kevlar thread long enough for the piston to fully escape the tube. Then attach the other end to a screw eye at the base of the piston. Attach another screw eye to the top of the piston, and your elastic to that. You just "splice" the piston in between.

[Tau Zero]

Make the Kevlar thread long enough for the piston to fully escape the tube. Then attach the other end to a screw eye at the base of the piston. Attach another screw eye to the top of the piston, and your elastic to that. You just "splice" the piston in between.Overnight, I also considered the idea of drilling a small hole through the (sanded-down balsa tube connector?) piston, and running the Kevlar through it, then tying the elastic to the "forward" end of the Kevlar. Hmm...


Cheers,

--Jay

[CPMcGraw]

Overnight, I also considered the idea of drilling a small hole through the (sanded-down balsa tube connector?) piston, and running the Kevlar through it, then tying the elastic to the "forward" end of the Kevlar. Hmm...


Cheers,

--Jay

Another suggestion for reinforcing that transition...

Drill out the entire center to the diameter of the ST-7 tube, then run a very long ST-7 through the transition and all the way to the rear edge of the ST-10. Use one centering ring about 0.5" in from the edge to keep the tube from shifting around. Shove a TR-7 up 2.5" in, and you've just eliminated all of the issues. The forward part of the ST-7 (exposed length) becomes the upper body, while the transition and ST-10 lower body just hang on for the ride, and you attach the fins to it. One central core tube that becomes both the upper body and the motor tube all-in-one.

Come to think of it, you could eliminate the balsa transition and just use a paper shroud with one additional centering ring. Less costly, and easier to construct...

[Tau Zero]

Come to think of it, you could eliminate the balsa transition and just use a paper shroud with one additional centering ring. Less costly, and easier to
construct...Y'know, I had a 13mm design using a BR-58 between an ST-5 motor tube and an ST-8 streamer compartment... but the biggest I could drill out the center was something like 5/32". :mad: So I altered the design by making the ST-5 body tube longer (Figure 1 below), and using a paper reducer (transition) with two centering rings. (RockSim thought the revision was less stable, but oh well. :rolleyes: )

So while using one big long honkin' ST-7 all the way through this other design's ST-10 is probably overkill (IMO), all I have to do is turn the solution above around 180 degrees. Shove the ST-790 3/4" into front of the ST-10 , use a couple of centering rings (these could be lightweight and thinner than the standard ones) and a 3/4" long paper transition, and... (Figure 2 below)

"--We got 'er done!" ;) :D :cool: Thanks, Craig.


(Talk about needing to think *outside* the box...)


Cheers,

--Jay

[CPMcGraw]

"--We got 'er done!" ;) :D :cool: Thanks, Craig.


(Talk about needing to think *outside* the box...)


Cheers,

--Jay

The only drawback is that area between the top of the motor mount and the bottom of the forward tube. It is going to be impossible to clean out the crud that accumulates from ejection charges.:confused:

(Added) Of course, I've used the same layout in a number of my own designs, so this is not a big issue...

[Tau Zero]

The only drawback is that area between the top of the motor mount and the bottom of the forward tube. It is going to be impossible to clean out the crud that accumulates from ejection charges.:confused:

(Added) Of course, I've used the same layout in a number of my own designs, so this is not a big issue...Larry the Cable Guy would probably say, "Crud. 'At's what you need, right there. Git-R-Done!"

Which reminds me of an amusing story Bruce McLawhorn told me in the Vendors Room at NARCON involving grass seed, ducks, and firearms. (It had to do with "Stuff That Brings Out the Redneck in Us.") :o :eek:


Cheers,

--Jay