PML Tomahawk modification

[Ian]

According to the spec sheets for this rocket, on a J350 the max altitude should be approx 5381'. My flight with it got to only 3922'. Yes the altimeter added a few ounces but nothing to explain the discrepency. The launch rail was somewhat 'dirty', and I think that contributed to the height shortfall. I am switching to rail buttons fro the rail guides and I am hoping this will give a greater performance due to less drag up the launch rail. Additionally, I am looking at taking about 12" off the airframe and an inch off each fin tip.
A couple of things I was wondering. First, taking 20% off the airframe, is this going to make any huge performance increase? And second, how best to clip the G10. I have many tools at my disposal. I am reluctant to use the circular saw for fear of shattering the G10. I am thinking a fine tooth hacksaw blade. Good/bad?
As always thanks for any advice/suggestions.

 

[Brian62]

Well, the first thing you should recheck is the coefficient of drag.
If you went to 3922' what cd does that equate to ? I can see a varience of 1000' with a cd that is way to low, some say to start with .75 or your best actual calculated number.

 

[JDcluster]

How much did the rocket weigh & was it more or less of what the catalog said it would weigh? It must of weighed quite a bit more because my Tomahawk with .093" thick fins hit 3k on an I205 CTI motor.



JD

 

[Ian]

Well I think I had excessive drag all the way up the rail due to a somewhat large amount of motor exhaust residue.
The PML rail guides are perfect up a brand new rail, but the club rails have heavy useage.
I was just wondering what cutting 20% off the airframe and an inch off each fin would do to help the performance. A little bit of weight saving yes, and a little less parasitic drag. Would we guess a 1% or a 10% increase in altitude by making these mods?

 

[robfire820]

My experiance with PML rocket is that they end up being heavier than what they were stated. For example my Endeavor is stated to weigh 71oz empty(4 lbs 7 oz) and it tips the scale at just under 6 pounds. My Miranda(heavily modified) should weigh in at just 3 pound 4 oz empty, mine weighs 6 pounds(dont ask)! I have found that most of my PML do not make the specs for altitude. By the way as long as it went up in one piece and came down in one piece, why worry about the altitude.
Good luck with your rocket, Rob

 

[Brian62]

Agreed that if u did not use your actual weight that could be a major factor. Now to answer your ? other than the insignificant weight loss on your cuts, I would say your net gain would be nearer the 1%. IMO Definately not worth doing.
Brian

 

[smurf]

PML advertises pretty light weight for their rockets...most people (me included) come out heavier than their claims.....If you want to sim out what the length difference would make try a program like rocsim (from apogee). If you are shortening your rocket and making the fins smaller take care you do not end up with an undesirabel CG/CP relationship....rockets doing loops in the sky only look cool when it's someone elses rocket.

As far as cutting the G10, I have found the diamond cutoff wheel in a dremel tool cuts the stuff like butter.....it's only 14 bucks at home depot and is a great all around shaping tool too.

 

[JDcluster]

I inquired about one of their other kits today about a confusing statement describing the parachute size. They fixed the website , but have yet to reply to my e-mail.


Some Customer service,




JD