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Saturday, January 25, 2020

Introducing the mechanical rocket motor.I:rotational carbon nanotube propulsion.

Copyright 2020 Robert Clark
(patents pending)

 SpinLaunch has been funded to launch rockets to space electromagnetically from the ground:

Stealth space startup SpinLaunch snares another $35 million from investors.
By Mike Wall
The company's total investment haul is now $80 million.
https://www.space.com/spinlaunch-launch-startup-investment-funding.html

 Though some launch proposals for slinging rockets to space have proposed nearly getting to the full orbital velocity of 7,400 m/s, this proposal seems more modest in proposing getting in the range of 1,500 m/s. The rest of orbital velocity would be provided by onboard rockets.

 However, it may be possible to get the entire propulsion for the flight to orbit by mechanical means. That is, rather then using chemical energy, by using stored mechanical energy. Because of their ultra high strength at lightweight, carbon nanotubes have the capability of serving as the propellant for such an orbital rocket. Below are some earlier discussions on the topic.

 There are several methods discussed of using the nanotubes as the propellant. It is notable they each  would allow SSTO vehicles.

 Also, notable that at a 10,000 m/s rotational rim velocity of a carbon nanotube flywheel, it would provide energy density at 100 times that of lithium batteries.

 So far however, carbon nanotubes only have been made at most at about centimeter lengths. However, this would be no impediment to the propulsion application since you could use very many of the flywheels at the microscale, which when they fly off at off in unison would provide sufficient thrust for a large scale rocket.

 And for the energy storage application very many of the carbon nanotube flywheels could be arranged in arrays to act as large-scale batteries.



*************************************************
From: "Robert Clark" <rgrego...@****.com>
Newsgroups: sci.astro,sci.physics,sci.energy,sci.materials,sci.space.policy
Subject: The Mechanical Rocket Motor. I
Date: 8 Sep 2006 17:54:39 -0700

This page gives the energy storage capacity for a flywheel given the
tensile strength of the material and its density:
 The energy storage per weight is best when the mass is concentrated as
a thin hoop of rotating material, though the energy stored per volume
is less in this configuration.
 If you want to maximize the energy stored per weight criterion, then:
 (energy stored)/mass = (1/2)*(tensile strength)/density .
 For the thin hoop configuration this is also equal to
(1/2)*(velocity)^2. So
 velocity = sqrt(tensile strength/density).
  The tensile strength of multiwalled carbon nanotubes has been
measured to be 150 GPa:
Direct mechanical measurement of the tensile strength and elastic
modulus of multiwalled carbon nanotubes.
B.G. Demczyk et al.
Materials Science and Engineering A334 (2002), 174, 173-178.
http://www.glue.umd.edu/~cumings/PDF%20Publications/16.MSE%20A334demczyk.pdf
 This was for micro-scale samples. It is not known if this strength
will still hold for macro-scale nanotubes, but it has been confirmed at
the micro-scale.
 The density of carbon nanotubes is in the range of 1300 kg/m^3. Then
the possible speed of the hoop could be:
 velocity = sqrt(150 GPa/1300 kg/m^3) = 10,740 m/s.
 This is a tremendously high speed. This raises the possibility they
could be used for rocket propulsion. What you would want to do is
convert this rotational velocity to linear velocity to be able to
impart momentum to the rocket.
 However, you also want the material to all exit in a single direction
to impart the momentum of the rocket in the opposite direction. But if
you induced the hoop to fly apart the materials would fly off in all
directions in the plane of rotation.
 So another possibility would have the nanotube material rotate around
as a thin rod attached at one end. Then if we broke the connection at
this end the rod could be made to fly off in the desired direction by
breaking the connection at the right time in its rotation. However the
rod will not attain the full velocity of its rotational speed at the
free end. The reason is the rod will still retain some rotation because
of conservation of angular momentum. So some of the energy will go into
rotation and some will go into linear translational motion.
 This report by Jerome Pearson calculates the velocity possible at the
tip of a thin uniform rod according to its tensile strength and
density:
 The speed calculated is U = sqrt(2σ/ρ) , σ the tensile strength and
ρ the density. Pearson refers to this as the materials characteristic
velocity. For the carbon nanotube material it would be U = sqrt(2*150
GPa/1300 kg/m^3) = 15,200 m/s.
 However, as I said this would not be the linear speed of the rod
flying off because some of the energy will be retained as rotational
motion. The linear speed of the rod when it flies off should instead be
the speed of the center of mass, which is at the midpoint for the
uniform rod, because of conservation of linear momentum.
 The speed of this midpoint half-way along the rod will be half the tip
speed or 7600 m/s, giving an ISP of 760 s. This compares to the best
liquid hydrogen/liquid oxygen chemical rockets of 450 s. The carbon
nanotubes would also take up much less space since they are denser than
liquid hydrogen. However, the volume would not be found simply from the
density of the carbon nanotubes. This is because you would need space
for the rods to rotate freely before they are released. So the
effective density would be less than 1300 kg/m^3, but still better than
that of liquid hydrogen. You also would not have the volume of the
liquid oxygen to carry.
 You could probably also design the rod in a tapered configuration to
maximize the linear translational velocity. Pearson in his report
calculates the degree of tapering to attain the maximum tip velocity.
The intent of his report was to propose a method of propulsion in the
form of a large 'sling' that could propel mass from an asteroid as a
means to retrieve the asteroid. But the calculations still work for a
tapered rod at the micro-scale.
 This page describes the idea in the form of a launching method for
payloads from Earth:
 To use this idea instead to calculate the speed the tapered rod would
fly off if released, you need to calculate the position of the center
of mass. Using this you can find the speed for the center of mass from
the proportion of its distance from the fixed end to the tip. From
conservation of linear momentum this will be the speed the tapered rod
will receive when released. The center of mass calculation for the
center of mass is rather complicated but I wind up with a speed for the
released rod of v =(characteristic velocity)/sqrt(π) = 8570 m/s, an
ISP of about 860 s. I would like to receive some confirmation on this
calculation though.
 This tapered rod does give a better ISP but you would have the problem
of binding the nanotubes together to result in the right taper. It is
not known whether nanotubes bound together will retain the same
strength of individual nanotubes.
 One possibility would be to use the single atomic layer graphene
recently produced. This has been made in micron-scale sizes which is
sufficient for the purpose. You could cut this in the shape to have the
right taper. I've been informed by one of the scientists who produced
it that it should have the same strength as individual nanotubes.
 This speed though is still less than the speed of the thin rotating
nanotube hoop at 10,400 m/s. One possibility to convert this rotational
motion fully into linear motion in a single direction might be to have
a fixed low friction flat slab with one end very close to and
tangential to the rotating hoop. You cut the hoop at the point closest
to the slab. This point will fly off in a tangential direction then
will move linearly along the surface of the slab. But we want the rest
of the hoop to also move linearly along the surface of the slab. To
insure this you might have the hoop be rotating inside another hoop
kept fixed of a slightly larger diameter.
 This though would increase the weight of the material that has to be
carried along, thus effectively reducing the ISP. However, if this
material is also made of strong nanotube material you might be able to
get a higher velocity of the rotating hoop thereby cancelling out the
effect of the increased weight that has to be carried.
 Both the tangential slab and the containing hoop would have to be made
of very low friction material at the velocities to be used. Carbon
nanotubes remarkably have been found to have very low friction:
Low-Friction Nanoscale Linear Bearing Realized from Multiwall Carbon
Nanotubes.
Science 28 July 2000: Vol. 289. no. 5479, pp. 602 - 604.
http://www.sciencemag.org/cgi/content/abstract/289/5479/602
 Then the rotating hoop, the containing hoop and the fixed slab could
all be made of nanotube material or perhaps of single atomic layer
graphene.

   Bob Clark
*************************************************
From: "Robert Clark" <rgrego...@****.com>
Newsgroups: sci.astro,sci.physics,sci.energy,sci.materials,sci.space.policy
Subject: Re: The Mechanical Rocket Motor. I
Date: 9 Sep 2006 16:10:01 -0700
The spinning of rotors at the micro-scale and the nano-scale has
already been accomplished so this is within the range of what is
currently possible:
Nanomotors realise visionary's dream.
Thursday, 30 October, 2003
"Researchers at Berkeley at the University of California created the
world's smallest electrical device earlier this year - one hundred
million of which could fit on the end of a pin."
http://www.scienceagogo.com/news/20030627221223data_trunc_sys.shtml
Using Nanotubes and Etched Silicon, UC Berkeley Physicists Build
World's Smallest Motor.
Berkeley, CA. July 23rd, 2003.
http://www.nanotech-now.com/ucb-release-07232003.htm
 The spinning motion of the rotor is initiated by electrostatic charge.
You would have to have quadrillions to quintillions of them though at
the nanoscale if they were to amount to thousands of kilos of reaction
mass.
 An automated process would be needed for making the rotors, such as
for example used for integrated circuits.
 Another problem is that the spin of the rotors would have to be
maintained in vacuum. So additional mass for the vacuum chamber would
have to be carried along.

  Bob Clark
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Sunday, January 5, 2020

Pumping pressurized fluids to high altitude for the space tower and for fighting forest fires, Page 5: further on laminar flow.

Copyright 2020 Robert Clark
(Patents Pending)

 In the blog post, Pumping pressurized fluids to high altitude for the space tower and for fighting forest fires, Page 3: achieving ultimate laminar flow, I discussed using laminar flow to achieve long distance water streams, either with piping or without. 

 If we are to achieve it without piping over kilometer distances then we can use the staging idea to have laminar flow inducing devices along the flow path along with the staged pumps. This would reduce the length the water had to remain laminar while not constrained within a pipe.

 For the method that does use piping, some notable recent research is that flow that has turned turbulent can be converted back in laminar flow:

New approach can save up to 95 percent of energy used for pipelines.
Study proves that turbulent flow can be destabilized so that it turns to an energy-saving laminar flow
Date: January 8, 2018
https://www.sciencedaily.com/releases/2018/01/180108121612.htm

 Also, notable is that flow in microfluidic channels are commonly laminar:

CHEM-ENG 590E: Microfluidics and Microscale Analysis in Materials and Biology

 Then you would use large numbers of the microchannels bundled together to get the needed water flow.


   Bob Clark

UPDATE, 1/11/2020:

 The possibility of raising the pipe or the free water stream to high altitude in stages, also raises the possibility of covering the needed horizontal distance in stages. This is important because of having to keep a pipe aloft horizontally over kilometer distances raises a significant weight issue.