MEMs rockets and jets - a fabulous opportunity
- From: Williamknowsbest <William.Mook@xxxxxxxxx>
- Date: Wed, 7 May 2008 07:45:51 -0700 (PDT)
Micro-electro-mechanical systems use techniques developed over that
past 50 years making ever tineir integrated circuits, to make ever
tinier mechanical devices. This is not nano-technology, its thousands
of times bigger. Yet, MEMs devices are thousands of times smaller
than traditional macroscopic devices.
This could be very important for rocket engines. Why? Because of
favorable scaling laws.
That's because rocket thrust is a function of nozzle area while rocket
weight is a function of part volumes. So, when you reduce the size of
a rocket by half say, the area is 1/4 and the volume is 1/8 th, so,
thrust versus weight is doubled.
So, imagine taking something like the RL-10 rocket engine which has a
nozzle diameter of about 1 meter - and reducing it to 0.25 mm - that's
about the size of a pixel cavity on a HDTV plasma screen (smaller than
can be resolved by the human eye) 0.25 mm is 1/4000th the size of a
1,000mm diameter RL-10. Its area is 1/16millionths and its volume is
1/64billionth - so, the thrust to weight of these engines if 4,000x
greater than a full-scale RL-10. So, if a full scale engine has an 80
to 1 thrust to weight ratio, the sub-scale engine has 320,000 to 1
thrust to weight ratio!!!
What can you do with tiny thrusts?
Built large numbers of engines and operate them in large arrays!
In fact, just like you can paint pictures in colorful gases by sending
signals through an array of pixels on an HDTV plasma screen, you can
paint thrust vectors across a surface made up of millions upon
millions of tiny rocket engines. This forms what I call a 'propulsive
skin'.
In fact, just as you can have three primary colors form any color in
the rainbow, and create an image plane that is made up of three
different sets of independent pixels - so too can you can form any
thrust vector relative to a surface by a triad orthogonal engines at
each point on a surface. Similarly the same signal processing and
control mechanisms that make HDTV plasma screens possible, are
directly adapted to a propulsive skin that cna produce any thrust
vector on its surface. Furthermore, unlike an imaging device that
must be flat, a propulsive skin can be shaped to achieve specific
efficiencies for a given mission or flight cycle.
There are other advantages and possibilities with this sort of engine
array
First is supreme safety. While the explosion of a 15,000 kgf thrust
engine that is 1 meter in diameter can make a bad day, the explosion
of a 1 gram thrust engine 0.25 mm in diameter is hardly noticeable.
While operating 100 million engines that cannot be serviced by any
means available today in a propulsive skin may mean a virtual
certainty that some engines will fail on each mission, those failures
need not be life threatening or even bring the vehicle to harm.
Second supreme reliability. Again, 100 million engines operated on
each flight cycle may see 100 engines fail in one way or another every
time the system is used. Howeve,r such failures are harly noticable,
and cause a performance reduction of less than 1 part per million.
10,000 flight cycles - which is typical of most aircraft today over
their service lives - mean that 99.99% thrust is available at the end
of a vehicle's life after that many flight cycles. The odds that a
sufficient number of engines fail to cause any noticible degradation
in the mission is virtually impossible.
Third - ultrasonic noise. The word rocket derives from the German
word 'racket' which means noise. Rocket engines have a lot in common
with steam whisltes, where jets of gas enter the still air at high
speeds producing violent sound waves. The principal components of
this sound wave of an engine is determined in part by its size. This
means that 0.25 mm engine's primary noise is well above the level of
human hearing. Its smaller than a dog whistle, and equally
unhearable.
Fourth - controllable thrust. Just as gray scale on a HDTV plasma
screen may be implemented by quickly switching the pixels on and off -
so too may engine average thrust levels be similarly controlled.
While such operation is not recommended in larger engines, scaling
laws favor such changes, which if cleverly exploited, can produce
radically improved reliable and less costly designs.
Fifth - controllable sound - Switching engines on and off in the
audible range produces controllable sound sources acrosss the
propulsive surface. A sonic hologram that can literally blow you away
(or alternatively lift the holographic surface) creates a number of
interesting design opportunities.
Sixth - very low mass - low cost. Imagine lifting a payload of 1 kg
can carrying it ballistically 3,000 km. this means the vehicle must
achieve a delta vee of about 3 km/sec. Assuming an engine performance
of 420 sec Isp . This implies a propellant fraction of 52%. With a
structural fraction of 3% - this leaves 45% for payload. Thus the
entire vehicle masses 2.23 kg of which 1.16 kg is propellant and
0.07 kg is mostly low cost packaging. Designing the system for 5 kg
top thrust - which is more than adequate for this mission - and a
320,000 to 1 thrust to weight ratio - implies a propulsive skin
overlaying the packaging massing only 16 milligrams !! Aerospace
hardware ranges from $5 million to $25 million per ton. That's $5 to
$25 per gram.- that's $0.08 to $0.40 for the propulsive skin in this
case. Since a kilogram of most products costs less than this figure,
and since 95% of the cost of a product stems from the logistics of
delivering that product to market, propulsive skin has a monumental
potential to change the way products are handled on Earth.
* * * EXAMPLE * * *
Imagine workers in an apple orchard harvesting apples. Instead of
dragging behind them a large gunnysack of freshly picked apples, they
have a bag of propulsive skin propelled containers. Apples are loaded
into containers, and the containers fly off to a nearby fueling
station in the field and are programmed with a destination received by
internet or cell phone or pda ordering - and fly off immediately to
their customer anywhere in a 28 million sq km area.
* * * ANALYSIS * * *
A kilo of freshly picked apples costs $1.50 the container costs
$0.08. At 1/5th cent per kWh (low cost solar panels see http://www.usoal.com)
the 1.16 kg of propellant costs $0.02 made from 1.74 liters of water.
Packaging is another $0.05. The order processing software that
programs the propulsive skin costs another $0.02 - a total of $0.20 to
deliver $1.50 worth of apples in seconds. Empties only require 5% of
the propellant to return them and propellant costs less than the
package in this analysis, so it likely that packages would be
recycled. It also gets rid of a disposal problem. Customers may
receive a $0.02 credit if their packages come back in good shape.
Alternatively, they may be charged an additoinal $0.20 if they are
not.
* * * EXAMPLE * * *
A restaurant fulfills takeaway orders. Their average food mass per
customer is less than 1 kg. Their average check size is $9.00 - they
deliver orders to anyone within a 3,000 km radius for $1.20 surcharge
and they get the 'packaging' back - with delivery in minutes.
* * * EXAMPLE * * *
A disposable (100 uses) flying cylinder carries a passenger up to
3,000 km for less than $20 in less than 10 minutes. The passenger
steps into the vehicle as they might an old-style telephone booth.
The vehicle is programmed via the customer's pda or cell phone. No
controls are present inside the vehicle - which has a low density foam
interior and a transparent ablative aluminum coated PET skin. The
vehicle takes off vertically, and as it ascends, it reorients itself
in flight to put the passenger in a reclining position, during ascent
and descent at high-gees. As gee forces lower, and soft landing is
approached, the vehicle resumes its vertical 'upright' stance and the
customer then exits the vehicle. The vehicle goes to a nearby
refueling station and thence to another caller nearby. After 100 uses
(3 flight hours) the vehicle flies to a disposal center to be
recycled.
NOTE:
MEMs based pumps heat exchangers compressors and fans and other items
used in life support and fuel handling and cryogenic refrigaration
also benefit being reduced to tiny sizes as well enhancing their
performance.
..
.
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