Large rocket engines cannot be reusable

Large rocket engines cannot be reusable because they are
damaged by large Reynolds number. The Reynolds number =
Re = V*D*S/N whe
V = gas velocity
D = diameter of the chamber
S = gas density
N = gas viscosity

Gas viscosity is primarily a function of temperature. The
impact of pressure is minor and the viscosity correction for
pressure is less than 10% for up to 3.5 MPa. This means that
the Reynolds number is proportional to the chamber's diameter
and to the gas density, which is proportional to its pressure.
The large Reynolds number is the primary cause of turbulence,
combustion instability, and cavitation. Turbulence disturbs
the protective layer of cool gas adjacent to the chamber's wall.
Cavitation and vibration damages turbopumps. All of these
fatal diseases are associated with large combustion chambers
and large turbopumps. They do not exist in small combustion
chambers and well designed, small turbopumps because their
Reynolds number is small. This means that large rocket engines
cannot be reusable and they are suitable for nuclear missiles
only.

Small rocket engines have another important advantage: they
have superior thrust-to-weight ratio due to the Cube-Square
Law which states that as scale is reduced, properties which
are a function of volume (mass) will decrease faster than
those which are a function of area (thrust and strength).

Small, simple engines can be made by robots, so they can
be very cheap. Large engines must be made by rocket plumbers.

A rocket launcher made of a few hundred small engines is
very reliable because the failure of a few engines out of
a few hundred is not a catastrophe.

The small engines have a small flaw: their small combustion
chambers do not mix fuel and oxidizer well. This means that
only volatile propellants (e.g., oxygen and methane) can be
used in those engines, and the injector holes must be small.
Small holes are more difficult to make than large holes and
they may plug up with dirt.

The best example of small, robust engines that can be made
by industrial robots are engine clusters. They are robust
enough to survive reentry, splashdown, and handling on a
bobbing ship:
http://www.islandone.org/LEOBiblio/S...engine_cluster

In article ,
Andrew Nowicki wrote:
...This means that large rocket engines
cannot be reusable and they are suitable for nuclear missiles only.

This would surprise the people who built the F-1, whose specs demanded
that it be reliably capable of 20 starts and a total run time exceeding
half an hour, even though its operational use required one start and a run
time of 2.5 minutes. Demonstrating the rated life required six test
engines to achieve more than double it without incident, which they did.

Your "fatal diseases" of large chambers and turbopumps are indeed
problems, but they can be avoided, and repeatedly have been.

Small rocket engines have another important advantage: they
have superior thrust-to-weight ratio due to the Cube-Square
Law which states that as scale is reduced, properties which
are a function of volume (mass) will decrease faster than
those which are a function of area (thrust and strength).

It is not that simple, alas. You don't make a small engine by just
scaling down a big one. In particular, achieving adequate chamber
residence time for efficient combustion, other things being equal, tends
to require scaling only two of the three dimensions, which makes a hash of
your proposed application of the square-cube law.

Small, simple engines can be made by robots, so they can
be very cheap.

So can large, simple engines. Ask the Russians, who invested much more
heavily in production automation (and in design for easy production) than
the US ever did.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |

Andrew Nowicki wrote:
Large rocket engines cannot be reusable because they are
damaged by large Reynolds number.

Of course, considering how large some reusable engines have gotten (2
million pounds thrust), I suppose that large engines only hit that
"non-reusable" level of size at some point beyond the largest engines
that have seen service.

Mike Miller

(Henry Spencer) :

In article ,
Andrew Nowicki wrote:
...This means that large rocket engines
cannot be reusable and they are suitable for nuclear missiles only.

This would surprise the people who built the F-1, whose specs demanded
that it be reliably capable of 20 starts and a total run time exceeding
half an hour, even though its operational use required one start and a run
time of 2.5 minutes. Demonstrating the rated life required six test
engines to achieve more than double it without incident, which they did.

Question, are not the engines of the Space Shuttle considered large engines?
It is my understanding that they are presently up to 4 firings before being
refurbished. And the inspections after each flight are just that,
inspections. They are not needed, but NASA being a government organizations
knows that if there is a failure and they had not done an inspection that
heads will roll so they spend the extra money to cover thier butts not
because they are needed.

Earl Colby Pottinger

--
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the time? http://webhome.idirect.com/~earlcp

Henry Spencer wrote:

This would surprise the people who built the F-1, whose
specs demanded that it be reliably capable of 20 starts
and a total run time exceeding half an hour...

That is how long you can run a car engine after
you have drained its oil...

Winged rockets like the Shuttle and the Baikal
are too heavy and too expensive. An economical
rocket launcher is a wingless launcher that can
survive splashdown and rough handling, and can
be reused many times without expensive maintenance.

If all the launcher engines are small, you do not
have to test and repair them as thoroughly as the
Space Shuttle Main Engine. If they are simple,
pressure-fed engines, all they need is ultrasonic
cleanup and new propellant filters after every flight.
If one percent of your small engines fell apart,
and your technicians are too drunk to replace them
with new engines before the next launch -- you go
ahead with the launch and replace the engines later.
If your technicians are half sober while replacing
the engines, they can do no harm because the engine
replacement is a no brainer.

If all your small, simple engines have the same
design, any machine shop can make the engines in
a week -- you just post their design on the web,
call a few machine shops on the phone, and you will
have the new engines delivered in a week.

Andrew Nowicki wrote:

Small, simple engines can be made by robots, so
they can be very cheap.

Henry Spencer wrote:

So can large, simple engines. Ask the Russians, who
invested much more heavily in production automation
(and in design for easy production) than the US ever did.

Yes, but it takes a small, cheap robot to make a small
engine, but a big, expensive robot to make the big engine.
Furthermore, it takes a big piece (stock) of aluminum
to make the big engine, so you have to custom order
the aluminum piece and wait until the foundry makes it.

Big things are dinosaurs.

Andrew Nowicki wrote:
Henry Spencer wrote:

This would surprise the people who built the F-1, whose
specs demanded that it be reliably capable of 20 starts
and a total run time exceeding half an hour...

That is how long you can run a car engine after
you have drained its oil...

Go on then-- try it. 30min not a chance and thats a plain car. High
perfomance engines like Nascar or F1 would not even last 10's of
seconds.

Big things are dinosaurs.

this really small engine thing lacks substance. I have made small
rocket engines for amature rockets, and guess what- its not at all easy
and high perfomance is still next to imposable untill you size them up
a bit.

The truth is there are scaling laws, and relabity issues as well as
cost issues that means there is some kind of "optimal size". Its not
really really big but then its not really small either.

Greg

Thus spake Earl Colby Pottinger unto the assembled multitudes:

Question, are not the engines of the Space Shuttle considered large engines?
It is my understanding that they are presently up to 4 firings before being
refurbished.

So why is it that the SSMEs need refurbishing after a launch is aborted
after they have fired and run for just a few seconds?



--
Andy Clews University of Sussex IT Services
(Remove DENTURES if replying by email)

In article ,
Andy Clews wrote:

Thus spake Earl Colby Pottinger unto the assembled multitudes:

Question, are not the engines of the Space Shuttle considered large
engines?
It is my understanding that they are presently up to 4 firings before being
refurbished.

So why is it that the SSMEs need refurbishing after a launch is aborted
after they have fired and run for just a few seconds?

Operational policy.

Andy Clews wrote:
So why is it that the SSMEs need refurbishing after a launch is aborted
after they have fired and run for just a few seconds?

Got a link to support that need for refurbishment after a few seconds
of operation?

Mike Miller

Andy Clews wrote in
:

Thus spake Earl Colby Pottinger unto the assembled multitudes:

Question, are not the engines of the Space Shuttle considered large
engines? It is my understanding that they are presently up to 4
firings before being refurbished.

So why is it that the SSMEs need refurbishing after a launch is
aborted after they have fired and run for just a few seconds?

They don't need it. They need inspection. Refurbishment is needed once
every ten flights. That NASA does it more often than needed is based on
conservatism and a desire to gather trend data on the engines (as implied
by the recommendations of both accident review boards to treat the shuttle
as an experimental vehicle).


--
JRF

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