The "rule of thumb", fwiw, is a maximum of 20:1; otoh, the sounding rocket
folks--from whom I learned this--violate this "rule" all the time. But
sounding rockets, being solids, tend to be very stout; the issue there is
making the interstage(s) over-strong and taking very great care to spin
balance the whole contraption to avoid coning. Very precise fin alignment
is also required.
For your vehicle, you might want to consider a larger diameter and thus a
shorter length.
Bill
On Wednesday, September 14, 2016, Ben Brockert <wikkit@xxxxxxxxx> wrote:
You're not required to blow the tanks down to atmospheric through the
engine. You can shut the run valves when the chamber pressure drops
off and/or you run out of fuel.
If you go high fineness there is a concern of inertial coupling, and
as such the straightness of the fins becomes more critical. The
failure mode is sometimes called "coning" in HPR.
Falcon 9 has a fineness ratio near 20; I'd say people are less
hesitant about high fineness than they were in the past.
On Wed, Sep 14, 2016 at 7:51 AM, Stephen Daniel <swd@xxxxxxxxx
<javascript:;>> wrote:
Max stress on the airframe of such a rocket may be when the fuel runs out<javascript:;>> wrote:
and the rocket decelerates sharply. At this time there is no pressure in
the tanks.
On Wed, Sep 14, 2016 at 12:29 AM, Zachary Martinez <znm3m8@xxxxxxx
and
Troy,
It is a pressure fed liquid I had opted for skin and stringer air frame
with supports throughout (front and back of each tank, top and bottom
quickmay put a couple more depending on results from FEA). Attached is a
ofand dirty model for reference. I could pretty easily make the structure
get tothe rocket the tanks and it is looking better and better the closer I
assumefinal numbers. I would just need to figure out what to do about the
connections between the tanks if I went that route.
Zachary Martinez
Aerospace & Mechanical Engineering
Missouri S&T
On Tue, Sep 13, 2016 at 10:55 PM, Troy Prideaux
<troy@xxxxxxxxxxxxxxxxxxxxx <javascript:;>> wrote:
Depends on more details like… is it an HPR solid rocket or is it a
pressure fed liquid or hybrid? If it’s a pressure fed liquid (which I
most ofit is), you could design it as essentially a monocoque structure so
mostthe structure will be propellant tankage which will be pressurized for
part ofof the flight including the moments of max Q. Pressurized tankage for
pressure fed designs are pretty damn tolerant of bending and buckling
moments – like you seriously don’t have to worry about them on that
thethe structure.
Such rockets (high fineness with much of the propellant forward of
wereCenter of pressure point) generally tend be over stable. This can be
manageable, but could make active dynamic control challenging (if you
arocket-bounce@xxxxxxxxxxxxx <javascript:;>]doing that) or the rocket more prone to issues like windcocking.
Troy.
From: arocket-bounce@xxxxxxxxxxxxx <javascript:;> [mailto:
WhatOn Behalf Of Zachary Martinez
Sent: Wednesday, 14 September 2016 1:30 PM
To: Arocket List <arocket@xxxxxxxxxxxxx <javascript:;>>
Subject: [AR] Fineness Ratio
Hello All,
I know that amateur rocketry very often uses high fineness ratios in
comparison to industry and understand some of the reasons why (drag).
rocketare the technical challenges I should be aware of as you scale up a
able towith high fineness ratios? PSAS mentioned something about not being
thismodel the rocket as a rigid body during flight and I am not sure if
diameter ofwould affect stability. I am looking at a height of 5.1m and a
can't.2m for a fineness ratio of 25.5. Any insight would be valuable as I
gutseem to find many resources. I am assuming this is still "small" so my
feeling is that I should be good.
Note: I am considering all types of buckling as failure modes and
designed the air frame accordingly.
Thank you,
Zachary Martinez
Aerospace & Mechanical Engineering
Missouri S&T
