Hello Kyle,
I’ve been following the TCCrockets threads for a year or so now, but I
have not had the opportunity to attended a launch, before all this
happened with COVID, and I haven’t participated much in the threads,
except once or twice. So, most of you don’t know me at all.
I am a member of NAR and a member of Livermore LUNAR club and I have
attended launches there.
When I saw your thread Kyle regarding launch rail button placement, I
thought I’d share with you what I do when it comes to rail button
placement on my high-power rockets.
I know people are concerned about Cp (center of pressure) and Cg (center
of gravity) in relation to button placement.
In my opinion, as long as the Cg is above your Cp by 1 to 2 calibers,
where your place your buttons is not that important, although I like the
top button to be above the Cg.
What I do think is important, is the answer to this question:
“Assuming you are using two rail buttons (not three), is the rocket
velocity when the top button leaves the top of the rail sufficiently
high to cause the fins to take over the stabilization of the rocket?”
That answer is, in my opinion, the only consideration.
The rule-of-thumb for kit-built rockets launched with no wind is above
50 feet per second (fps). If your velocity is at least 50 fps when your
top button leaves the rail, then the location of your rail buttons
relative to each other and relative to the rocket (other than Cg) is not
important. (The rail buttons ought to straddle the Cg and Cf but other
than that, the exact location is not very important.)
If you launch in the wind, the rule-of-thumb velocity is to be higher
than 50 fps because your fins now have to compensate for a cross-wind
vector. I have no good rule-of-thumb for wind velocity versus launch
velocity, except to believe that your velocity when the top button
leaves the rail ought to be higher than 50 fps.
Attached to this email you will find a MS Excel spreadsheet (Mac
construction) that will calculate the velocity (V) when the top rail
button leaves the top of the rail for a given launch rail length, a
given motor average thrust, and a given distance from the rocket top
rail button to the top of the rail.
If anyone has any suggestions, or finds an error, or thinks I'm missing
something, please let me know.
I hope this helps answer your question Kyle, regarding rail button
placement.
Be well,
Steven Cerri
Steven Cerri, B.S. Aero, M.S. Geophysics, MBA
Author of: "The Fully Integrated Engineer: Combining Technical Ability
and Leadership Prowess",
published by Wiley and IEEE
STCerri International
231 Market Place, Suite 320
San Ramon, CA 94583-4743 USA
Office: 925-735-9500
Email: steven@xxxxxxxxxxx
Website: www.stevencerri.com
Attachment:
Rail Velocity Auto Calculator-Steven Cerri-v9.xlsx
Description: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
