Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@zoo.toronto.edu (Henry Spencer)
Subject: Re: For All Mankind [was Re: center engine out]
Message-ID: <1990Sep29.233359.6067@zoo.toronto.edu>
Organization: U of Toronto Zoology
References: <15953@wpi.WPI.EDU> <1096@tsdiag.ccur.com> <5633@mace.cc.purdue.edu> <1990Sep28.151756.3973@infonode.ingr.com>
Date: Sat, 29 Sep 90 23:33:59 GMT

In article <1990Sep28.151756.3973@infonode.ingr.com> drudetb@infonode.UUCP (Ted B. Drude) writes:
>Ahem.  I beg to differ.  The Atlas DOES use LH2 for its booster fuel.

Sorry, you are misinformed.  LOX and kerosene.  The Centaur upper stage,
when it is used (it wasn't for, e.g., Mercury), does burn hydrogen.

>In fact,  it was the use of hydrogen fuel that made it possible for the Atlas
>to have such high payload weights for its day (late 50's-early 60's).

Sorry again.  Hydrogen had nothing to do with it.  Hydrogen engines were not
sufficiently well developed to be used for Atlas; NASA had considerable
trouble getting them ready in time for Centaur and Saturn.  Atlas's high
payload mass with a 1.5-stage launcher was due mostly to the use of
"balloon tanks", relying on tank pressurization for most of the structural
strength of the rocket.  Exploiting the extra strength of pressurized
tanks was not novel, but relying on it to the extent that the rocket
would collapse unless pressurized was.  This gave outstandingly low
structural weights, and a plain Atlas (no Centaur) is the closest thing
to a single-stage-to-orbit launcher the US has ever built.

(For those who are mystified about how you get half a stage:  Atlas drops
two of its three engines, and some associated structure and equipment,
partway up.  The third engine burns from launch to orbit.)

>Until Energia (which also uses hydrogen) the Soviets had used nothing
>but kerosene/LOX for fuel...

Well, not strictly true.  They also use the UDMH/N2O4 combination in
some of their boosters, notably the first stage of Proton as I recall.

>BTW,  when NASA  started to put Mercury capsules on Atlases (instead of
>Redstones),  there was a lot of concern about having humans ride on hydrogen-
>based boosters...

No, there was a lot of concern about having humans ride on Atlas in
particular.  Its reliability record was dismal.  NASA lost a lot of
lunar and planetary probes to Atlas failures in the early days, and
of course it was NASA that got blamed for it.

>...(Which may partly explain why the Saturn V had a kerosene first
>stage.)...

No, the reasons for the kerosene first stage are quite simple and well
known.  The single biggest reason for hydrogen's high exhaust velocity
(which makes it a lightweight fuel for upper stages) is the low molecular
weight of the exhaust.  This works *against* it in first stages, because
low molecular weight also means low thrust for a given engine size,
other things being equal.  First stages are not too sensitive to weight
but care a lot about thrust, and being big they also prefer high-density
fuels (which hydrogen isn't).  All of this might have meant kerosene in
any case for the S-IC stage... but it was academic, because there simply
was no prospect of developing a sufficiently large hydrogen engine in time.
Getting the J-2 ready for the second and third stages was hard enough.
-- 
Imagine life with OS/360 the standard  | Henry Spencer at U of Toronto Zoology
operating system.  Now think about X.  |  henry@zoo.toronto.edu   utzoo!henry