Interchangeable motor mounts

[Doug Sams]

(For the first time ever, my ADHD @$$ is starting a new thread instead of hijacking one.)

Tim posted this pic of his 24mm to BT-70(?) motor mount.
http://www.doug79.com/BT70mms/tbzep-pic-p.jpg


The first time I ever saw a coupler used as part of a motor mount was probably the old Estes Saturn V system which used BT-60 as a sleeve for interchangeable 1x24mm and 3x18mm mounts (and required the custom BT-63 tube to receive it).
http://www.doug79.com/BT70mms/estes-int-Dp.jpg


During my BAR-dom, I have built many motor mounts, but have always avoided the use of sleeves (ie, couplers) around the mounts relying on the rings themselves to provide the seal as well as load structure of the mounts. For example, this 29mm-BT70 mount has only the fore and aft rings.
http://www.doug79.com/BT70mms/29-70p.jpg

So, why do folks use the sleeves? What do you like about them? How do they improve the mount? Is it just a paradigm left over from the old Estes catalog? Or is there something else?

Doug

.

[tbzep]

I've done them both ways. When I built my Centuri Saturn V, they didn't have 24mm motors and the kit was for 3x18mm only, so I made my first interchangeable mount after reading that it could be done in the Estes catalog. I didn't use a coupler with it. I used flimsy cardboard centering rings so I had to permeate the corrugations with glue to make sure the ring was strong enough to be retained at ejection.

The recent ones we've made have couplers/sleeves because......just because. :p

I had couplers on hand. I had scrap cardboard for centering rings on hand. I didn't have to go out to the shop and cut plywood for the non-sleeved type. Maybe that's why I did it. Maybe it's because I liked the slick clean look of the Centuri Power System mounts. http://www.clicksmilies.com/s1106/fragend/confused-smiley-013.gif

[tonypv]

I did the same thing with my Semroc Saturn 1B. Using a BT-70 and couplers. I had them on hand at the time so thta's what I used.

[GregGleason]

I think it's "dealer's choice".

Using a tube will reinforce the rings if they are prone to flex, but will add more weight.

If the centering rings are stiff enough, then the tube is unnecessary.

On my MPR/HPR project, I opted to use rings only.

Greg

[blackshire]

I wonder if the use of stage couplers on such motor mounts began as Estes' way of burning through an oversized inventory of couplers? I once read that their SPEV ("Spare Parts Elimination Vehicle") kit was the result of a company inventory audit that showed that they had up to a 50-year supply (at their then-current sales rates) of some rocket parts.

[AFlyingMonkey]

I'm going to do this for my Saturn V, but I'm wanting to do a 5 engine cluster. 4x18mm for the outboards and a 24mm for the core. I'd love to be able to switch those out for a 29mm or a 38mm so I can level 1 with this one. I hope that will allow me for few different flying options

[mojo1986]

Estes originally used the centering rings/coupler arrangement on all of their models that required an engine mount (i.e. BT-50 and up). This structure gave the strength necessary for the G forces the model would experience. It was the flimsy diecut cardstock centering rings that Estes used during the early years that mandated the use of the coupler in the mount. In later years Estes went to a heavy centering ring and adopted the mount design that Doug favors. However, the largest Estes tube to use this type of mount was, as far as I recall, the BT-55. I don't think Estes ever made the heavy ring in a '60' size or larger, so they stayed with their original design for larger models such as the Big Bertha, at least for a while. I know that they never used the coupler for an engine mount in their really big models such as the Maxi Brutes (unless you include the Saturn V in that group) but they did provide heavier cardstock rings in those kits.

Joe

[blackshire]

That raises two interesting questions, Joe. Kits with BT-50 body tubes such as the Astron Farside and (if memory serves) the 18 mm motor Honest John used the "flat rings 'n stage coupler" motor mounts, as did many of the Design Of the Month winners in the early years.

[1] However, did the Alpha ever use this earlier type of motor mount? All of the ones I've ever seen (which go back to 1969, and this kit first appeared in 1966) have had the thick spiral-wound CR-2050 Centering Rings that are standard today. The model rocket guide in the 1969 Estes catalog (the "yellow pages section"), which features the Alpha, also shows the modern motor mount in the exploded view drawing of the Alpha.

[2] Did later production runs of the Farside, Honest John, and other Estes kits with the older-type motor mount have their motor mounts updated to the newer design?

[Mark II]

That raises two interesting questions, Joe. Kits with BT-50 body tubes such as the Astron Farside and (if memory serves) the 18 mm motor Honest John used the "flat rings 'n stage coupler" motor mounts, as did many of the Design Of the Month winners in the early years.

[1] However, did the Alpha ever use this earlier type of motor mount?

No. I had an Alpha from the very first run of the kits (purchased in spring, 1967). It had, as I recall, the AR2050 rings, just as you described.

Having never owned any Centuri kits back in the day, I didn't see that type of mount until after I became a BAR.

I believe that the "coupler and rings" type of mount was more of a Centuri thing. None of the Estes kits that I had used that style.

MarkII

[blackshire]

No. I had an Alpha from the very first run of the kits (purchased in spring, 1967). It had, as I recall, the AR2050 rings, just as you described.Thank you for confirming this--I suspected it, but I wasn't sure.Having never owned any Centuri kits back in the day, I didn't see that type of mount until after I became a BAR.

I believe that the "coupler and rings" type of mount was more of a Centuri thing. None of the Estes kits that I had used that style.My brother Richard's Centuri Nike-Smoke had the "flat rings 'n stage coupler" motor mount. My father's 1968-vintage Estes Big Bertha also had this type of motor mount, but I don't know of any other Estes kits besides the Astron Farside and (I think) the 18 mm Honest John that used it.

The Estes Model Rocketry Manual (see: http://www.ninfinger.org/rockets/nostalgia/estesmrm.html ) that first appeared in the 1969 Estes Catalog (see: http://www.ninfinger.org/rockets/nostalgia/69estcat.html ) shows the "flat rings 'n stage coupler" motor mount on page 52 of the catalog (see: http://www.ninfinger.org/rockets/nostalgia/69estp52.html ). However, the only motor mount of this type that is shown is one for BT-60 size rockets, so BT-55 and smaller kits may all have been using spiral-wound motor tube centering rings by that time.

[Mark II]

So, why do folks use the sleeves? What do you like about them? How do they improve the mount? Is it just a paradigm left over from the old Estes catalog? Or is there something else?

Doug

.I suspect that one purpose of this mount style, perhaps even the primary purpose, was to stiffen the fin can area in order to provide a firmer, more solid base for the surface-mounted fins. Four of the 1960's-era Estes kits that had it were the Farside-X, the Big Bertha, the Thor-Agena B and the Gemini-Titan. The body tubes for these were, respectively, BT-50, BT-60, BT-60 and BT-70. When the Gemini-Titan was introduced in 1965, it was the largest kit in the Estes catalog; the Big Bertha, also released in 1965, was the largest single-engine Estes kit. (The Ranger, first introduced in 1963, was the same size as the BB, but it had a 3-engine cluster.) I suspect that there was concern that the body tube in the area where the fins were attached in the Bertha and the GT-3 could buckle when the fins of these larger, heavier rockets contacted the ground during recovery. Both the Gemin-Titan and the Thor-Agena also had clear plastic fins which were troublesome to bond to the body tubes. Bertha's fins had long tip edges and a long sweep; the combination would provide great leverage for tearing the fin away from the body tube when the fin struck the ground. And even though BT-60 and BT-70 had thicker walls than the smaller sizes of tubes, their larger diameters made them less rigid laterally.

(I occasionally read complaints that BT-80 is cheap and flimsy, when in fact it is actually a sturdy, well-made airframe tube. The notion that it is flimsy comes from the fact that it has a large diameter, which makes it, especially in shorter lengths, easy to ovalize. To some extent, that "complaint" could also be made about BT-60 and BT-70, even though they too are well-made tubing stocks.)

If the airframe in the area of the fins buckles even slightly, the rocket is much more likely to lose a fin. Adding an internal coupler to the area makes it very rigid so that it provides a firm base for mounting the fins. Neither the Ranger nor the Cobra, which also had BT-60 fin cans, needed the stiffening because the three BT-20 motor tubes inside each of these models provided plenty of internal support.

Even though the Farside-X used BT-50 for its airframe, all three stages had reinforced fin cans. The two booster stages needed them because the combination of large fins and tumble recovery would exert great stress on the fin attachments when they landed. The sustainer needed to have its fin can stiffened because of the long fin sweep and the increased mass of the large payload section.

Another kit that had the sleeved motor mount was the Honest John. It is not as easy to see a reason for its use in this model, but it might have had something to do with the need to add the paper nozzle cone at the aft end of the mount and the adhesive paper "expansion" in the fin can. There may have also been a perceived need to add strength to the airframe immediately above the "expansion."

Bonding plastic fins to thin-walled paper airframes was troublesome back then, because the adhesive that was used softened the kraft paper tube. Adding a thick layer of fish paper (the coupler) right underneath the airframe skin would have addressed this issue for the Gemini-Titan and the Thor-Agena B.

Another kit from that era that had clear plastic fins was the K-39 Semi-Scale Saturn V. It had a thin, clear plastic tube coupler installed in the tail end of the BT-60 lower airframe. To attach the plastic fins, the builder was instructed to cut narrow fin slots through just the BT-60 but not through the plastic coupler that had been bonded inside of it, and then use clear butyrate dope to adhere the fin root directly to the coupler. I had one of these kits. The fin attachment method would have been quite nifty if it had worked. One or more of the fins always popped loose upon landing and needed to be reattached. In my model's third and last flight, my SS Sat V lost a fin during boost. It veered violently when that happened but the three remaining fins kept it pointed up and it had an otherwise normal flight and recovery. But that was the end of its flight career. Although I didn't know it then, that flight in the spring of 1971 would also be the last time that I launched a rocket for the next 33 years.

MarkII

[blackshire]

I think you nailed all of the reasons for the use of stage couplers in motor mounts. I had conjectured that this type of motor mount may have been used in the Farside and Farside-X for weight-saving reasons, but your explanations for its use in the Farside and Honest John kits (as well as in the larger models you mentioned) make much more sense.

[mojo1986]

Estes' Astron Alpha did, in fact, use the old style motor mount (i.e. the RA-2050 ring set) but did NOT use the coupler. Later on, Estes substituted a pair of AR-2050 rings for the old RA-2050 ring set and modified their plans accordingly. I suspect that Estes never included the coupler in the engine mount for the Alpha because, being a lightweight model, it didn't need the extra strength that the coupler would give to the structure. I have three sets of older Alpha plans in my collection...............the earliest set clearly shows the old thin rings in the exploded drawing and describes use of the RA-2050 rings. A later plan is nearly identical (same exploded drawing) but includes an insert describing substitution of the 'new' AR-2050 rings for the RA-2050 ring set. A still later plan has a new exploded drawing showing the AR-2050 rings in use and makes no mention of the old RA-2050 rings.

Joe

[ghrocketman]

In-flight POWR-VEER of rockets due to loss of a fin is ALWAYS interesting and adds additional heads-up "FLAVOR" to flights. "Nominal" flights are BORING; the ones with anomalies are the good ones.

[Mark II]

In-flight POWR-VEER of rockets due to loss of a fin is ALWAYS interesting and adds additional heads-up "FLAVOR" to flights. "Nominal" flights are BORING; the ones with anomalies are the good ones.Yup, it certainly was impressive! :D Back then I really took stability for granted, and I never worried that any of my rockets would ever loop or power-prang. (What did I know...?) So that really added to the experience, because I hadn't even realized until then that a model rocket could do that! (Well, from reading stuff like the Model Rocketry News, I knew that it was possible, but I never thought that I would ever see it.) I was quite annoyed and frustrated with the plastic fins on the Semi-Scale Saturn V, even though otherwise I liked the kit. Much of my problems with it were due to my rather crude model-building skills and my lack of good tools, which were issues that I recognized at the time. I'm sure that they contributed more to my problems than the fin attachment method itself. But it had never occurred to me until I wrote that post yesterday that those cursed plastic fins did keep the model going straight up even after one of them came off. So they did their job admirably well. I would never deliberately set up a rocket to strip a fin during launch, but the effect, at least that time, was definitely exciting. Concern, stark terror and then relief, all in a couple of seconds! :chuckle:

MarkII

[blackshire]

An Estes Gyroc whose fin/rotor elevons deploy at maximum velocity will accomplish the same end, while also *completely* disassembling the model in mid-air. My father built one and painted it with flat black lacquer (I still remember the smell of the paint). On its first and only flight, the elevons suddenly deployed at maximum velocity, just as the propellant burned out. The motor must have CATOed as well, because in the blink of an eye I heard a loud sizzling noise accompanied by the sight of the fins flying sideways, the body tube tumbling upwards, and the nose cone arcing upwards above the body tube, with all of the parts moving rapidly away from an expanding ball of dense blue-gray smoke.

[Mark II]

An Estes Gyroc whose fin/rotor elevons deploy at maximum velocity will accomplish the same end, while also *completely* disassembling the model in mid-air. My father built one and painted it with flat black lacquer (I still remember the smell of the paint). On its first and only flight, the elevons suddenly deployed at maximum velocity, just as the propellant burned out. The motor must have CATOed as well, because in the blink of an eye I heard a loud sizzling noise accompanied by the sight of the fins flying sideways, the body tube tumbling upwards, and the nose cone arcing upwards above the body tube, with all of the parts moving rapidly away from an expanding ball of dense blue-gray smoke.Epic fail! More details, please, so that we can recreate it. :chuckle:

(Just kidding.)

MarkII

[blackshire]

The only reason I got to see it in such detail was because I was standing about 40 feet away from the launch pad, perpendicular to its trajectory. While building it he commented that the Gyroc's elevon retention tab system seemed "hair trigger" and easy to trip at the wrong moment.