Exploring the Unknown: Selected Documents in ... - The Black Vault

Exploring the Unknown 581
calculations are sufficiently sensitive that the assumptions related to equipment
performance can change the order of the results.
This leads to the conclusion that the difference between the modes from
a mission safety standpoint as known at this point in time is the same order of
magnitude as the uncertainty of the analysis. Reliability calculations, per se, are
therefore not an adequate basis for choosing among the modes.
Major Differences Between Modes
areas:
The major technical differences between the modes lie in the following
1. Cryogenic vs. storable stages in space;
2. Weight margin;
3. Lunar landing configurations;
4. Rendezvous.
These differences will be discussed in the following paragraphs.
Cryogenic vs. Storable Stages. The question of cryogenic vs. storable
stages in space has two aspects: the reliability of the engines, and the storability of
the stage. Most propulsion experts agree that a hypergolic, pressure-fed engine
is simpler and, by implication, inherently more reliable than a pumped, regenerative
cryogenic engine. Study of engine design confirms this. However, it is also
agreed that engines reach inherent reliability only after an extended development
program. The RL-10 hydrogen-oxygen engine has been in development
for about four years; the storable engines are just starting their development
cycle. [3] Hence, at the time of the first lunar missions the cryogenic engine
(if the RL-10 could be used in all space stages) might be closer to its inherent
reliability than the storable engine. Judgment is again involved. The above
arguments nonwithstanding [sic], it is believed that storable engines will have
reached a higher reliability than cryogenic engines at the time of the initial
manned lunar attempts.
Space storability depends on the detailed thermal design of the stage.
In space, the cryogenic fuels must be insulated to prevent excessive boil-off, the
storable fuels insulated to prevent freezing. On the lunar surface, both cryogenic
and storables are subject to boil-off during the lunar day, the problem being more
severe for the cryogenics. During the lunar night, the cryogenics are subject to
boil-off, the storables to freezing. Either stage will require careful design to insure
compatibility with the environment. The problems appear to be more severe for
the cryogenic fuels, especially since the storable fuels require an environ ment
more compatible with the rest of the lunar vehicle.
The above considerations have led to the conclusion that storable propellants
should be used for the Apollo applications. Storables are also the conservative choice