USS Andromeda, 24mm, 6' Upscale "right sized for mid power" (ORK)

[stormbringergrey]

I previously posted a 29mm/10' version which I had no intention of building (too big, too expensive, no field), on the other hand, this is a version I am giving serious consideration to building and flying. Built around a 24mm thick-walled central tube and designed to operate on mid-power. Note that the crew compartment is a bit oversized, a BT-55 would be more to scale but I like the dramatic transition to the BT-60 better (the reactor radiator rings are also a little oversized, those I might scale back).

 

[lakeroadster]

Looks extremely "fragile".

One thing to remember about Open Rocket... it won't tell you if there will be a fun flutter issue or a body tube collapse... those sort of things.

A cursory review reveals that, due to fin flutter, the fins on your rocket aren't good for more than 78 mph... and the sim shows the max velocity of the rocket is 174 mph. Even switching to basswood isn't strong enough.

 

[tsmith1315]

Looks extremely "fragile".

Indeed, that’s part of the Andromeda experience!

 

[stormbringergrey]

Back in the prehistoric days before rocket sims... ie: late 90's ;-) I built what I called the Andromeda II. It featured a flattened hexagonal engine bay with a recessed, in-line cluster of 29mm-38mm-29mm. I fancied it looked a bit like the aft end of a Star Destroyer. The thick 38mm motor tube also formed the central tube (much as the 24mm does here), the crew compartment was a 3" tube with a custom nose cone. It did not have the diagonal conduits or the fin-on-fin antenna array. I detailed the Ram Jets to include stator fins and added a central 18mm nacelle but never powered them for fear of steering the rocket. No compression issues in the body and no issues with fin flutter although to be fair, they were plywood.

In this case, not shown in OpenRocket, I intend to paper skin the fins. I don't want to move away from balsa as I'd have no way to cut the curved fin profile in a harder wood, also, the diagonal conduits would be hardwood dowels much as they were in the original Estes kit. They are modeled in OpenRocket as extensions of the conduit mount but that was only for convenience.

 

[lakeroadster]

Back in the prehistoric days before rocket sims... ie: late 90's ;-) I built what I called the Andromeda II. It featured a flattened hexagonal engine bay with a recessed, in-line cluster of 29mm-38mm-29mm. I fancied it looked a bit like the aft end of a Star Destroyer. The thick 38mm motor tube also formed the central tube (much as the 24mm does here), the crew compartment was a 3" tube with a custom nose cone. It did not have the diagonal conduits or the fin-on-fin antenna array. I detailed the Ram Jets to include stator fins and added a central 18mm nacelle but never powered them for fear of steering the rocket. No compression issues in the body and no issues with fin flutter although to be fair, they were plywood.

In this case, not shown in OpenRocket, I intend to paper skin the fins. I don't want to move away from balsa as I'd have no way to cut the curved fin profile in a harder wood, also, the diagonal conduits would be hardwood dowels much as they were in the original Estes kit. They are modeled in OpenRocket as extensions of the conduit mount but that was only for convenience.

Plywood is roughly 5 times stronger than balsa...

Don't get me wrong, this seems like a way cool build. I'd just hate to see you build it and then shred the fins on the 1st launch.

Anybody know how thick the big elliptical fins were on the original Estes kit? I'm striking out finding that data on the rocketreviews site.

 

[stormbringergrey]

Anybody know how thick the big elliptical fins were on the original Estes kit? I'm striking out finding that data on the rocketreviews site.

The Semroc reproduction is the same size as the original Estes but with a thicker 18mm central tube. The fins are still balsa but I can't find a spec either. I don't recall the thickness of the original being unusual. (btw.. my upscale is about a 70% increase).

I'm OK with difficult first flights. If I can recover it, I can rebuild it.

Any thoughts on the strength gained by paper skinning? I've never tried that before but it's an easy and probably prudent precaution.

I have other options as well; I could apply a thin spar and play it off as "surface detail".

 

[lakeroadster]

The Semroc reproduction is the same size as the original Estes but with a thicker 18mm central tube. The fins are still balsa but I can't find a spec either. I don't recall the thickness of the original being unusual. (btw.. my upscale is about a 70% increase).

I'm OK with difficult first flights. If I can recover it, I can rebuild it.

Any thoughts on the strength gained by paper skinning? I've never tried that before but it's an easy and probably prudent precaution.

I have other options as well; I could apply a thin spar and play it off as "surface detail".

I did a strength test on papered basswood.. I saw an increase in bend strength of 8:1.

But it depends on how you paper the fins, and what paper you use.

https://www.rocketryforum.com/threa...er-1-16-basswood-with-a-strength-test.172313/

 

[Bluegrass Rocket]

The Semroc andromeda fins are 3/32” thick. Pretty sure that was what the Estes version was.

 

[Bluegrass Rocket]

For the BT-20 to heavy BT-50 (or equivalent), the measurements should be times 1.345, around a 35 percent increase. I’d get 2 heavy wall tubes from BMS and cut them to scale length (24.21”) along with a 4” tube coupler, so that the tube seam would be in the exact same place. Then the fins for the dowels cross the seams like the original. My 2 cents.

 

[stormbringergrey]

I actually did a 35% scale up, starting with a BT-50 Thick Wall body the closest crew compartment would be a BT-55, the problem was the OD for the BT-50TW was a little oversized and the OD of the BT-55 was a little undersized, the combined result was that the body transition was too subtle and just didn't have the "Andromeda look", in addition, the motor would be difficult to service with the narrow gap between the BT-50TW and BT-55. Also, the final design was light and best powered in the D-E range, my target was F-G so I knew I could go a little bigger ;-)

The scale up shown is 70% but it's not true scale. All dimensions are +70% including the BT-60 crew compartment, except, the OD of the central body tube should be a BT-55. I considered double-walling a BT-50TW inside a BT-55 but I like the exaggerated transition to the BT-60 and I think double-walling is overkill for mid range power so I left the BT-50TW exposed even though the OD is out of scale.

As for the conduit mount, as you mentioned, it should be at the midpoint of the total central tube length, including the portions inserted into the forward and aft compartments. The 70% scale up length requires 3 full BT-50TW tubes and a partial fourth tube which is why the fin doesn't fall on the seam. I could of course make it fall on the seam by trimming both the fore and aft tubes evenly but I wanted to keep cutting to a minimum.

 

[lakeroadster]

The Semroc andromeda fins are 3/32” thick. Pretty sure that was what the Estes version was.

The numbers show about a 140 mph fin flutter speed at that thickness. Per Rich DeAngelis's USS Andromeda Flight Log - 2012-11-23 -the rocket powered, by a C6-3, logged a maximum speed of 69 mph.

That's a nice safety factor, and thus, all is right with the world .

 

[neil_w]

the rocket powered, by a C6-3, logged a maximum speed of 69 mph

 

[lakeroadster]

It's a 4 ft long rocket... with all kinds of draggy bits, powered by a C6-3.

Here's another.. 74 mph on this one.

In regard to the slow speed... If you're wanting mind blowing performance... don't drive a "stock" Pinto.

Maybe go with the 24mm motor upgrade option like Bob Glidden used to drive.

Rich DeAngelis has written some great reports over on the Rocketreviews site. Definitely fun reading those..... On his Estes USS Andromeda he informs us: "During initial string stability tests, the rocket buckled and folded in half!"

 

[stormbringergrey]

Fun reports. I had one as a kid in the 70's, never had a buckle due to acceleration but I know well that an Andromeda starts at a factory designed length and grows shorter through use due to hard landings and "hanger rash"

As for performance, my preference has always been to put on a good show; tons of draggy detail in the design, lots of noise and smoke, and the slowest possible safe liftoff. Never really cared how high as long as I had safe deployment of the chute... that said, I am a data junky and after reading the reports I might invest in a data recorder.

 

[stormbringergrey]

Parts ordered. Design refined with 1/4 launch lugs (I don't see a practical way to equip rail buttons). Added a mass object for engine retainer. All fin surfaces will be papered balsa for strength/weight (basswood was too heavy). Oversized parachute with spill hole spec'd to soften landings. Approximately $110 in parts + $20 shipping, paint and decals not included. About 40% of the cost is the parachute alone (going to use a quick release clip to reuse on similar sized rockets).

Ordered four F44W-4's which were on sale and seem to be good performance match and economical at about $17.50/motor. Love the performance profile of the long burn Cesaroni F30 but too much hardware investment for initial flights (and a single reload is about the same cost as two SU F44's).

Wildman rockets has custom fit decals, will order after test flights.

 

[lakeroadster]

Parts ordered. Design refined with 1/4 launch lugs (I don't see a practical way to equip rail buttons). Added a mass object for engine retainer. All fin surfaces will be papered balsa for strength/weight (basswood was too heavy). Oversized parachute with spill hole spec'd to soften landings. Approximately $110 in parts + $20 shipping, paint and decals not included. About 40% of the cost is the parachute alone (going to use a quick release clip to reuse on similar sized rockets).

Ordered four F44W-4's which were on sale and seem to be good performance match and economical at about $17.50/motor. Love the performance profile of the long burn Cesaroni F30 but too much hardware investment for initial flights (and a single reload is about the same cost as two SU F44's).

Wildman rockets has custom fit decals, will order after test flights.

Looking forward to this build...

Some questions and a couple comments:

• Will the 48" chute you selected, and it's wadding, fit in the 9" long space you've allocated in the BT-60 recovery bay?
• Changing those big fins to Basswood only changes the apogee by 25 feet, and the off the rail speed by 0.4 mph. Why are you saying it's "too heavy"?
• Why is the fin thickness 0.118" in the simulation?
• Make sure you adjust the launch rod length in the simulation to reflect the actual length you are using, adjusted for lug placement. That's critically important to determine off the rail speed, especially on a looooong rocket like this one.
• Balsa Machining Services can provide the BT-50H tubes in a 34" length... and 34" C-50 couplers. Making a full length internal coupler makes this rocket much more durable. Food for thought.

 

[neil_w]

Why is the fin thickness 0.118" in the simulation?

(Raises hand) Pick me, pick me!

0.118 (3mm) is the OR default value.

 

[stormbringergrey]

Looking forward to this build...

Some questions and a couple comments:

• Will the 48" chute you selected, and it's wadding, fit in the 9" long space you've allocated in the BT-60 recovery bay?
• Changing those big fins to Basswood only changes the apogee by 25 feet, and the off the rail speed by 0.4 mph. Why are you saying it's "too heavy"?
• Why is the fin thickness 0.118" in the simulation?
• Make sure you adjust the launch rod length in the simulation to reflect the actual length you are using, adjusted for lug placement. That's critically important to determine off the rail speed, especially on a looooong rocket like this one.
• Balsa Machining Services can provide the BT-50H tubes in a 34" length... and 34" C-50 couplers. Making a full length internal coupler makes this rocket much more durable. Food for thought.

• Recovery System: actually, I ordered a PML 54" with spill hole because the 48" was out of stock. It was right in OR sim, not sure why I wrote the comment wrong. Will it fit? I am not sure, I am trusting OR's packed dimensions which suggests I have plenty of room. I am also using 12' of 2.5mm braded Kevlar for a shock cord, the OR part is not an exact match. For wadding I'll be using loose insulation fill, shouldn't take too much given the entire system is about 5 feet from the ejection charge. I am going to test fit the parts before cutting, if the recovery system is tight I am not opposed to using more of the BT-60 to extend.
• Wood Selection: yeah, I don't know what happened, I originally spec'd basswood and the off rail velocity suddenly dropped to 12mph... but this morning both balsa and basswood are showing just over 30mph. Basswood would be the better choice but I only have a modeling knife and good Dremel, not sure if I'd be able to cleanly cut the harder wood (back in the day I had a scroll saw).
• Fin thickness: yeah, what Neil said, now corrected. No significant change in performance. BTW, a rant from a returning newbie here... inconsistent use of metric and imperial units, sometimes mixing both on the same product spec is driving me crazy. This is no way to run an amature space program ;-)
• Launch Rod: here I did override the default correctly. The launch field has a 6' 1/4 rod, I set the effective rod to 5.5'. The mid-ship lug is about 3' up but the aft lug will travel the full effective distance. Our launch site has two 8' rails but a mid-ship button would require an ugly offset to align with a button on the sail fin.
• Double Walled Main Tube: I went back and forth on this in OR... ultimately decided to save the weight but I might reconsider when my package arrives. I've worked with larger thick walled tubes and they would be plenty strong, this is my first experience with 24mm... will just have to "feel" it to decide.

 

[lakeroadster]

• Recovery System: actually, I ordered a PML 54" with spill hole because the 48" was out of stock. It was right in OR sim, not sure why I wrote the comment wrong. Will it fit? I am not sure, I am trusting OR's packed dimensions which suggests I have plenty of room. I am also using 12' of 2.5mm braded Kevlar for a shock cord, the OR part is not an exact match. For wadding I'll be using loose insulation fill, shouldn't take too much given the entire system is about 5 feet from the ejection charge. I am going to test fit the parts before cutting, if the recovery system is tight I am not opposed to using more of the BT-60 to extend.

OR doesn't have a packed dimension calculation to my knowledge? Perhaps @neil_w can confirm or deny.

You basically have to look elsewhere to determine volume of body tube needed.

And based on PML's chart, for their PAR-54, they don't recommend a tube smaller than 2.1" dia....

 

[neil_w]

OR doesn't have a packed dimension calculation to my knowledge? Perhaps @neil_w can confirm or deny.

Yes it does, as of latest release:

I confess I am a bit light on understanding of exactly how it works... @H. Craig Miller worked extensively on this feature and should be able to give more detail.

 

[lakeroadster]

Yes it does, as of latest release:
View attachment 580103
I confess I am a bit light on understanding of exactly how it works... @H. Craig Miller worked extensively on this feature and should be able to give more detail.

Looking forward to learning more about this.

 

[stormbringergrey]

Unless I'm doing something wrong, OR seems to be at odds with PML's chart. I checked Rocketman Chutes and the packing volume aligns with the PML chart. 54" feels excessive for an MPR rocket (I thought I was using something in that size range for HPR). 36" shows an 8 mph decent but even that's too big to fit?!? Still investigating but I'm going to call Rocketarium tomorrow and see if I can change the order.

Edit: Apogee sells a 48" parachute that is verified to fit easily into a BT-60. It's half the cost of a similar sized PML, I suspect its not the same quality but probably "good enough" for a 1 pound rocket (they claim the same drag coefficient). I can get it in 2 days via Amazon Prime so a good plan B if the PML doesn't fit.

Edit 2: Rockerarium confirms, the PML 54" will not fit in a BT-60, not even close. Unfortunately, it is also already on the way and $12 to return so I'll just hold onto it for a bigger project. Looks like it will be the cheap 48" chute from Apogee for this project. I'm going to wait until I have the Kevlar shock cord in my hands before ordering... I don't have a good feel for its bulk in proportion to the BT-60, might need to replace it as well.

 

[jqavins]

(Raises hand) Pick me, pick me!

0.118 (3mm) is the OR default value.

 

[Tobor]

View attachment 589164

Ha! Good ol' Arnold Horshack. Now THAT brings back a flood of memories...........

 

[neil_w]

Ha! Good ol' Arnold Horshack. Now THAT brings back a flood of memories...........

A noble name, which means "The cattle are dying".

 

[jqavins]

Edit: Apogee sells a 48" parachute that is verified to fit easily into a BT-60. It's half the cost of a similar sized PML, I suspect its not the same quality but probably "good enough" for a 1 pound rocket (they claim the same drag coefficient). I can get it in 2 days via Amazon Prime so a good plan B if the PML doesn't fit.

I doubt it's lower quality, but rather that it's thinner material Far from the same thing.

Also, since you've got plenty of length below the BT-60 recovery compartment (and that's putting it mildly) you could use a little of that space for a baffle, and then not need any wadding. The wadding shouldn't take up much space, but none at all is better than not much.