You could design the tanks to be self-supporting under 1 g and be able to
withstand ground handling, but then use the advantage of pressurization to
stiffen them for flight loads. However, be careful about what happens at
burnout (assuming no active throttling)--the rocket is at maximum load
factor so if you let the tanks vent to ambient as they run out of
propellant, there goes the pressure stiffening--crunch!
The NASA Astronautic Structures Manual (NASA TM-X-73305) discusses pressure
stiffening to resist buckling.
On Mon, Sep 5, 2016 at 5:18 AM, William Claybaugh <wclaybaugh2@xxxxxxxxx>
wrote:
Zachary:
I assume you are planning on a "skin and stringer" airframe with the tanks
set inside and non load bearing. This is both heavy
and--importantly--expensive.
Assuming you use sheet stainless to roll wrap the barrel section and buy
standard semi-elliptical domes, your wall thickness-for a typical 12"
diameter tank--will be way more than strong enough to carry flight loads.
There is no reason to burden yourself with the expense of a built-up
airframe.
Bill
On Friday, September 2, 2016, Zachary Martinez <znm3m8@xxxxxxx> wrote:
Sent from my iPhone
On Sep 2, 2016, at 16:41, George William Herbert <
george.herbert@xxxxxxxxx> wrote:
Bill is correct, but you get very fragile rockets and minimum gauge
problems this way.
Not impossible, just... Be careful.
I am not going to have tanks be load bearing for obvious reasons, but I
will still have to check to make sure I still have a decent factor of
safety against buckling during expected g loading. Using aluminum rather
than stainless should help as it increases the wall thickness.
Sent from my iPhone
On Sep 2, 2016, at 12:04 PM, William Claybaugh <wclaybaugh2@xxxxxxxxx>
wrote:
Zachary:
You can do as little as 50 psia against vacuum and 150 psia to sea level
air; look into Bob Truax's Sea Dragon work for reference.
That said, note that he was planning on using Lox / LH2 to recover the
lost Isp. But as a general rule mass fraction improves as the operating
pressure of a pressure fed liquid drops...and mass fraction has an
exponential effect on performance, whilst Isp is merely linear.
Bill
On Friday, September 2, 2016, Zachary Martinez <znm3m8@xxxxxxx> wrote:
Hello all,
Are there any issues operating at a relatively low chamber pressure for
an engine ~180-220psi other than the poor Isp. For a pressure fed rocket
with a relatively high thrust to weight ratio the benefits of using a lower
chamber pressure are really tempting, but I imagine there are some design
challenges that I am unaware of. My main concerns are difficult ignition or
possible flame out but I don't really know.
Thank you,
Zachary Martinez
Aerospace Engineering | Missouri S&T
