The potential problem could be the practical limits to how low you can
realistically run the chamber to maintain stable combustion. If there's enough
of a shortcoming of density, the difference in tank mass between the very low
tank pressure of a pump fed system (just enough to avoid inducer cavitation and
whatnot) and a bit higher pressure fed system (enough to avoid combustion
instability and enough to maintain enough of an injector & plumbing pressure
drop) can add up quickly and swamp the associated pump mass quickly (with lots
and lots of provisos).
One potential advantage of say a non regulated blowdown pressure fed system
over pump fed for a 1st stage is the former allows the potential to increase
the level of thrust at minimal dry mass penalty to assist in minimising gravity
losses provided the payload and upper decks can tolerate the extra kick. The
injectors and chamber would need to cope with substantial feed pressure loss
over the course of the burn, but accompanying reduction in atmospheric pressure
(with altitude) should go some way to complement the drop in chamber pressure
from the tank(s) depletion. Or, you can do what everyone else does and just add
boosters to a pump fed stage :)
Troy
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of David Weinshenker
Sent: Monday, 14 December 2015 12:40 PM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Hydrogen and oxygen used as pressurizing gasses
On 12/13/2015 03:30 PM, Wyatt Rehder wrote:
Wouldn't using a pressure fed system with hydrogen on an upper stage
be optimizing for the worst characteristics of each system? You have
the least dense fuel possible making the largest tanks, in a system
where your tanks have to be rated for high pressures and very heavy,
on the stage where every pound counts.