Rapid manufacturing

From: Andrew Nowicki (andrew_at_nospam.com)
Date: 02/12/05 

To: sci-space-tech@moderators.isc.org
Date: Sun, 13 Feb 2005 00:02:48 +0100

Rapid manufacturing (also known as direct manufacturing)
is an additive fabrication technology that makes complex
parts automatically from a CAD STL file. There are several
terms similar to rapid manufacturing: 3-Dimensional Printing,
additive fabrication, freeform fabrication, solid freeform
fabrication, and stereolithography. Most rapid manufacturing
systems make objects from flimsy materials (plastic, paper,
starch...) but a few can make strong metal parts. Some of
them can make inexpensive rocket engines.

Here is a brief review of rapid manufacturing:
http://www.triz-journal.com/archives/2003/10/l/12.pdf

The best source of information about rapid manufacturing
is the annual Wohlers Report ($390,
http://www.wohlersassociates.com)

The cheapest rapid manufacturing method is a process
similar to inkjet printing. The fastest printer of this
kind is ProMetal R2. It makes aluminum 6061-T6
(45 KSI = 310 MPa) parts having the maximum size of
8"x8"x6". It can make one such part in 24 hours.
ProMetal web site: http://www.prometal.com/equipment.html

Rapid manufacturing is limited to small parts because
the technology is still expensive. The 8"x8"x6" size
is rather small for a rocket engine. The engine would
probably have four small exhaust nozzles. The metal
parts made by the ProMetal R2 printer are dirty --
they are covered with unbound aluminum powder which
must be removed manually. The clean part is coated
with bronze to make it impervious and then sintered
in a small oven. I imagine that a large oven could
fuse the small parts into one large engine.

Metal parts produced by any sintering process have
poor corrosion resistance, so they should be coated
with a refractory metal if they are going to be in
contact with the hot exhaust gas. A laser-melting-
-powder method used by Optomec LENS-850 machine makes
corrosion-resistant parts, but the process is slow
(0.5 cubic inches per hour) and the machine is
expensive. The maximum part size is 18"x18"x42".
Arcam EBM S12 electron-beam-melting-powder machine
(http://www.arcam.com) also makes corrosion-resistant
parts having the maximum size of 8"x8"x7". Arcam is
three orders of magnitude faster than Optomec, but
it makes only steel and titanium parts. (Its web
site claims that aluminum alloy powder will available
soon.) 3D Systems Sinterstation selective laser
sintering machine is as slow as the Optomec LENS-850.
Parts made by the Sinterstation are of the same quality
(poor corrosion resistance) as parts made by the
ProMetal R2 printer. EOS machines are slower than
Sinterstation.

The best materials for regenerative rocket engines
(aluminum and copper) reflect the laser beam (albedo
up to 98%) rather than absorb it. The high albedo and
poor energy efficiency of lasers (typically less than
10% of electric energy is converted to laser beam
energy) strongly favor the Arcam electron beam system.
A powerful electron beam is easier to generate and
deflect than a powerful laser beam. Laser beams are
deflected by moving parts which cannot match the
scanning speed of the electron beam and require too
much maintenance.

It is theoretically possible to make an oversize
Arcam machine that can fabricate one big (3 ft. dia.)
aluminum, pressure-fed rocket engine in a week. The
speed of Arcam fabrication is now limited by the
speed of delivering thin layer of metal powder to
the spot where the electron beam melts it. If this
bottleneck is eliminated, the Arcam-like machine can
fabricate several big rocket engines in one day...