[AR] Re: Propellant tank positions?
- From: "Anthony Cesaroni" <anthony@xxxxxxxxxxx>
- To: <arocket@xxxxxxxxxxxxx>
- Date: Thu, 4 Mar 2021 23:19:20 -0500
Henry wrote;
"SpaceX probably has a milder version of the same issues on "Starship"
(stupid name...)."
Please define "stupid".
Jest.
Anthony J. Cesaroni
President/CEO
Cesaroni Technology/Cesaroni Aerospace
http://www.cesaronitech.com/
(941) 360-3100 x1004 Sarasota
(905) 887-2370 x222 Toronto
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx <arocket-bounce@xxxxxxxxxxxxx> On Behalf
Of Henry Spencer
Sent: Thursday, March 4, 2021 10:59 PM
To: Arocket List <arocket@xxxxxxxxxxxxx>
Subject: [AR] Re: Propellant tank positions?
On Fri, 5 Mar 2021, Charlie Jackson wrote:
Just a question that came to mind when looking at some booster
designs; what determines the positions of the fuel and oxidiser tanks?
For example, the Saturn V features its Lox tank placed above the
Kerosene tank, however on SpaceX?s starship it?s fuel tank is above the
oxygen.
Is this to do with density and propellant mass?
A lot of it is mass distribution. With LOX-based combinations, the LOX is
usually a lot heavier than the fuel, and substantially denser too.
(Mildly so for kerosene, more so for methane, hugely so for LH2.) Putting
the LOX at the bottom usually reduces structural mass, because the heavy
part of the mix is right above the engine bay.
On the other hand, lots of mass aft is bad for aerodynamic stability, and
can also mean wider gimbaling range to cope with the center of mass moving
around. Most of the mass of a loaded Shuttle ET was in the LOX tank, almost
spherical, up in the ET nose, so the center of mass of the ET was near there
for essentially the whole powered flight, and since it was far away from the
engines, they didn't have to gimbal very much to stay lined up with it. (If
the LOX tank had been down near the bottom, right beside the engines, the
required gimbaling range would have been very wide, and the stack would have
been flying nearly belly-first toward the end.)
Putting the smaller tank on the bottom shortens the feed lines from the
upper tank, which reduces various problems of running them through or around
the lower tank. On the other hand, long feed lines may be an advantage for
LOX, because the added hydrostatic head at the bottom makes pump design
easier.
I think those are the high spots...
For the Saturn V first stage, I believe it was stability issues that
dominated, with a bonus for hydrostatic head. For the upper stages, with
the LH2 immensely bulky but not very heavy, structural mass and feed-line
issues dominated, and LOX on the bottom was the only practical choice.
SpaceX probably has a milder version of the same issues on "Starship"
(stupid name...).
...And would this be relevant to small-scale liquid fuelled rockets?
Probably the only one of these issues that would matter a lot for small,
simple rockets would be aerodynamic stability... while could be important.
Henry
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