Newsgroups: sci.space.shuttle
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Soviet shuttle
Message-ID: <1988Oct14.170639.1828@utzoo.uucp>
Organization: U of Toronto Zoology
References: <1988Oct1.224801.11041@utzoo.uucp> <1109@cfa237.cfa250.harvard.edu> <1988Oct5.232401.15176@utzoo.uucp> <4938@hplabsb.UUCP> <198 <4945@hplabsb.HP.COM>
Date: Fri, 14 Oct 88 17:06:39 GMT

In article <4945@hplabsb.HP.COM> dsmith@hplabsb.UUCP (David Smith) writes:
>>Proton does, reportedly, give its payloads a fairly rough ride.
>Do you know whether "rough ride" means high-G or vibration? ...

Vibration.

>It would seem to me that an all-liquid rocket should be pretty smooth.

I don't think the solid-vs-liquid difference has much effect on things
like combustion-chamber vibration.  Certainly liquids make their own
contributions like pogo effect (which is rumored to be a particular problem
on Proton).

Okay, since somebody is sure to ask what pogo effect is...

Consider a momentary slight rise of pressure in combustion chamber.  This
produces back pressure in the fuel lines coming in, which will tend to
slow fuel flow a bit.  Slower fuel flow means less coming into combustion
chamber, which will tend to reduce chamber pressure.  This reduces back
pressure, which increases fuel flow, which raises chamber pressure again.
Now, if the natural frequency of this oscillation happens to match the
resonant frequency of the structure, it can become quite strong.  All the
more likely because a rocket's structural resonances sweep over a range
of frequencies as the tanks empty.  A familiar problem in new rockets.

The fix is to add some damping somewhere.  For the Saturn V first stage,
von Braun's engineers added small air cavities (well, helium-gas-filled
cavities) in the valves in the fuel lines; turned out the cavities were
already there because of the valve design, and all they needed to do was
fill them with helium.  This added enough "spring" in the fuel lines to
considerably reduce the changes in fuel flow caused by fluctuations in
chamber pressure.

(NB in the above, "fuel" is used generically to include "oxidizer" too.)

>>Proton's first stage *is* kind of a weird design.
>... is there something beyond the cluster-look that you have in mind?

It goes deeper than that.  The things that look like strap-ons are not;
each of them contains one fuel tank and (at the bottom) one engine.  The
central core contains a single oxidizer tank, and no engines.  [I just
might have fuel and oxidizer reversed here.]  It's all one, oddly-shaped,
piece, not a central stage with strap-ons.  Bizarre.
-- 
The meek can have the Earth;    |    Henry Spencer at U of Toronto Zoology
the rest of us have other plans.|uunet!attcan!utzoo!henry henry@zoo.toronto.edu