Re: Big dumb rockets vs. small dumb rockets

From: Andrew Nowicki (andrew_at_nospam.com)
Date: 02/09/05 

Date: Wed, 09 Feb 2005 20:09:51 +0100

Andrew Nowicki wrote:

> The cost of fabrication and integration depends on fabrication
> technology. The engine cluster is made of small rocket engines
> which are already integrated into rather large engines.

Jeff Findley wrote:

> If this is what you're proposing, then you need to tell us the thrust to
> weight ratio of these "engine clusters", not of the engines themselves.

About 10 to 15 for the entire launcher
if you keep low pressure in the tanks
(about 1.5 MPa).

> My guess is that you'll quickly find that the mass you need to add to make an
> engine cluster (structure, plumbing, control systems, sensors, and etc) will
> quickly make the thrust to weight ratio of your cluster look bad compared to
> a single engine with the same thrust.

If you are comparing 10 cm dia. engine with a cluster
of 10 cm engines carved from a slab having diameter
of one meter, the small engine weighs only about
half as much as the cluster equivalent. The cluster
is relatively heavy, but it can be easily carved by
a milling robot and it is already integrated. If you
make the small engines separately using conventional
methods, the entire launcher including piping is
lighter than the engine cluster, but it is much more
expensive to fabricate, weaker, and it needs
additional means to protect it during reentry.
(The engine cluster is thick and heavy, so it can be
used as the heat sink during reentry.)

I believe that we have exhausted the topic. The big
dumb boosters are not dumb, but the small ones also
look interesting because some fabrication techniques
favor small engines. The big tanks are cheaper to
make than small tanks, but they are difficult to
handle due to their enormous size.

The Optomec LENS-850 laser sintering systems have been
used to make high quality, thin walled parts from
aluminum alloy, so they certainly can produce small,
cheap, pressure-fed engines. The largest objects made
by the LENS-850 have dimensions of 18"x18"x42":
http://www.optomec.com/downloads/Optomec-LENS-850-System.pdf
The LENS-850 makes new part at a rate of 0.5 cubic
inches per hour, so it would take about two months to
make 18" diameter engine. Its powerful 1000 W laser
can be used to make small, tapered injector holes.

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