cluster question

[bobt85]

Ok, I messed up. I put a standard Estes engine mount in a Delta II rocket that I built, but now I find that the weight of the rocket w/ a C-6-5 in it is just about 7 oz. Way over liftoff weight for the C-6. I DO have a cluster of BT-5 tubes around the main body doing nothing but looking cool - could I get enough thrust by adding, say, 3 engines to the mix? (Are the A series engines that small?) How do you figure things like clustering and how it affects max liftoff weight? Thanks for the info -- I'm feeling pretty stupid right about now...

Bob

 

[hokkyokusei]

Clustering several motors together will increase the max possible lift off weight.

Estes mini-motors are designed for BT-5. Do a search on clustering to find lots of hints and tips!

 

[dtomko]

Sounds like the perfect opportunity to use the plugged mini engines for the Blurzz rocket cars; they will fit in a BT-5. You should check the CG with the added weight, though.
Drew Tomko

 

[Elapid]

add more motors!
*woohoo*

a group of a10-pt motors would not only look way cool, but it would solve your problem also. like was suggested, make sure your CG is in the stock location before you launch it. a stouter launch rod might be necessary because of the increased mass.

 

[bobt85]

Right. So I'm going to cluster and my cg will shift back. If the rocket becomes unstable, I may need to add some weight up front, which might get me back to my original question: Is there a way for me to compute the max liftoff weight when clustering? There must be some cost/benefit type analysis for adding engines...

Thanks

 

[qMaxx]

According to an old Estes catalog that was within reach, the max lift weight of a C6-5 is 4 oz and the max l.o. weight of an A10-3T is 3 oz. If you cluster 3 A10's with a core C, you're looking at a max liftoff weight of 13 oz.

Total weight of the 4 motors is 1.75 oz. Add that to the weight of the rocket (you said 7 oz, and I'm guessing that's dry weight?) and you get 8.75 oz. That leaves 4.25 oz that you can add in nose weight, but unless you did a really bad design job, you shouldn't need that much.

I designed and built one in the same scale (I used BT-5's for the strap-ons as well) years ago, that flew okay on a single B, and went a bit unstable on a C (to be honest, I think it was actually the motor clip that deflected the exhaust). I only used about 0.75 oz of nose weight, I think. Of course, I cheated a little bit and had fairly large (but smaller than an Alpha), swept back balsa fins that totally ruined the scale appearance, but it was a test model anyway.

 

[adrian]

Yes, but according to the old Estes catalogue I'm looking at, the maximum lift weight for a C6-3 is also 4 oz.

I tend to work by the rule that a thrust/weight ratio of 5:1 is good and a thrust/weight ratio of 3:1 is permissible. So, I'd calculate that 7 oz rocket's thrust/weight ratio as 6 / (7 x .454/16 x 9.81) = 3.08, put a C6-3 in it, and fly it on a really calm day to minimise weathercocking.

That's the theory. In practise, I built a model of India's GSLV-D1, intending to fly it on a cluster of a C6-3 and four 13mm A's, but hoping I could get away with just the C6-3 sometimes. It turned out too heavy even by my 3:1 rule, so I tried loading a couple of A's for the first flight. They didn't light, so it went up on just the C6-3 anyway. It flew straight enough, just not very far, and did eject the parachute well before landing (although significantly after apogee too). More recently, after overhauling my launch controller, I launched it on a full load of a C6-3 and four A10-PT's, and not surprisingly it flew much better.

 

[Micromeister]

Bob:
I'd add 3 A10's to the model, I've found by adding nose weight equal to the weight of any added thrust rings, clips or other components and 1/2 the weight of the additional motors all have flown wonderfully. Estes SR-71, Geo-Sat HLV and Deep Space Transport is name just a few

 

[bobkrech]

Adrian

A 5:1 thrust:weight should be the minimum ratio considered safe for most rockets using standard length rods on a calm day.

With a 5:1 ratio, the rocket accelerates at 4 G's, and requires a 3.5 ft long launch rod length if the minimum stable velocity is 30 ft/s (20 mph), and the minimum length almost doubles to 6.25 ft if the minimum stable velocity increases to 40 ft/s (27 mph).

With a 3:1 ratio, the rocket accelerates at only 2 G's, and in each of the above situations, the minimum required rod length doubles to 7 and 12.5 ft respectively. If you are not using a rod of this length, the slightest breeze can upset the rocket (stall the fins) and create a dangerous situation.

On a windy day, experienced folks fly a minimum 10:1 thrust to weight ratio to prevent unwanted aerobatics 10s of feet above the pad. I have witnessed an underpowered longburn rocket do 4 complete rotations at 50 ft on a windy day. That's a scary sight.

I have attached a simple Excel spreadsheet that calculates the minimum required launch rod length vs the thrust to weight ratio for a given minimum stable airspeed.

Bob Krech

 

[adrian]

An Estes motor has an initial thrust spike considerably higher than its sustained thrust to get the rocket off the launch rod quickly. Thus, although the rocket's thrust/weight ratio may be 3:1 for most of the thrust phase, it is probably about 6:1 at lift-off. Which is just as well - requiring a thrust/weight ratio of 5:1 during sustained flight would ground most of my collection permanently! Very few of my models are as bad as 3:1, but probably none of my scale models manage 5:1 either...

 

[bobkrech]

Adrian

You are absolutely correct, but you have to make sure that the rod length is long enough allow the rocket to reach the minimum airspeed where the fins become effective before it leaves the rod. The previous spreadsheet does this calculation for an averaged thrust to weight ratio, but you have to go to the manufacturers or certifcation data to get the time resolved initial thrust data and average it yourself.

It's a real shame to see a nicely finished rocket, particularly scale, crash or strip a chute due to being underpowered. If your rocket has a low thrust to weight ratio and you don't want to use the spreadsheet, you should run a simulation program to insure that you will indeed reach a reasonable airspeed before your rocket leaves the rod. You can use rocksim (demo free) or wrasp (free) to do this for the actual motors you are using.

The failure to sim an underpowered rocket will likely result in a wreaked rocket. Having witnessed several thousand rocket flights over the past few years, I don't think I've seen a flight succeed with < 5:1 T/W ratio on a windy day, and many of those with less than 10:1 were iffy. A few crashes could have been prevented if a longer launch rod was used, but the majority simply needed to use a higher thrust motor.

Bob Krech

 

[adrian]

Originally posted by bobkrech I don't think I've seen a flight succeed with < 5:1 T/W ratio on a windy day, and many of those with less than 10:1 were iffy. [/B]

I have.

Although I've never had a crash of this nature, I've seen someone else launch an Estes Saturn V on a D12-3 in quite high wind and it went surprisingly vertical. On one occasion I launched my Mean Machine, modified to break in the middle, from a regular length rod, on a D12-5 which should give it a thrust/weight ratio of around 5:1 and certainly nowhere near 10:1. I hoped it would weathercock a bit so that the wind would bring it back to me; in fact, it went straight up, then drifted a long way. So I don't launch my heavier models in high winds, and prefer not to launch my less heavy models in high winds either!

I said I've never had a crash of this nature. My A-9 did crash on its first flight, but that's because its strakes were doing odd things to its CP. Extra nose weight reduced its thrust/weight ratio even further but improved its stability, and it now flies perfectly.

 

[bobt85]

I know, I know. Where's the pic? But believe me, it was beautiful! After a long day of doing experiments w/ Alpha's with a bunch of kids from my homeschool group, I pulled out my newly finished delta - C65 in the middle, and 3 A10PT's to get it off the ground. We were launching those Alpha's on B64's and they were racing out of sight. It was a true pleasure to see the Delta go slow enough to actually see it, and see it nose over at apogee. I was a little nervous as it started down, but the ejection charge did fire the nosecone off, and it came down on dual 18" chutes.

Thanks for all your help. Hoping to someday get some pics posted. It really was something... I guess you had to be there.

Bob