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#126 2017-12-07 08:26:03

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 974

Re: Un- conventional ways to LEO

JoshNH4H wrote:

Here's an unconventional way to LEO: Initial launch with a water gun.

The basic idea is that the speed of sound in water is much faster than the speed of sound in air, at roughly 1500 m/s (vs 340 m/s for air and 1240 for H2).

I would put this into motion by having a giant tank of water with a piston at one end (or alternatively maybe just an inflatable balloon, perhaps inflated by vaporizing dry ice using heat exchange with seawater).  The top of the tank would be a converging nozzle that accelerates the water to its speed of sound.  A rocket would get caught up in this flow and come out moving at 1500 m/s, which will have the effect of substantially reducing the required delta-V of the rocket.

That's an excellent idea.  An ocean launch gun would eliminate the need for digging a tunnel or maintaining a high tower, although an excavated barrel could take advantage of the compressive strength of rock.  It all comes down to economics.

For ocean launch: One could use a long steel or concrete tube that is floated into position and then tilted vertical by flooding a ballast tank at the base.  The hydrostatic pressure at depth would partially compensate internal pressure during launch.  Maybe reinforced concrete could be used for the lower parts of the barrel, with steel liners, with either carbon steel or pre-stressed concrete used for the upper parts close to or above the surface.

In terms of propulsive power, a simple gas-driven hydraulic ram would appear to be the cheapest option.  Compressed air would be the technically easiest way of powering this, but would require quite a lot of electrical energy to charge before launch, so maybe not the cheapest.  Why not simply ignite a natural gas air mixture above a sea water hydraulic piston?  That way you achieve a very rapid transient to a pressure of ~10bar (if starting at atmospheric) and the energy cost of natural gas is <1US¢/MJ.

A 5km long ram, with acceleration of 10g, would give a barrel velocity of 1km/s.  That's enough to push the rocket to edge of the stratosphere.  At this point, the engines are firing in true vacuum, so a ram launch like this should allow the use of a Lox/methane SSTO with an acceptable mass ratio.

Last edited by Antius (2017-12-07 08:33:03)

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#127 2017-12-07 08:56:13

JoshNH4H
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Re: Un- conventional ways to LEO

It seems to me that if you're using a gun like this it would be tricky to modulate the acceleration well.  What you might do would be to put your payload inside some kind of mostly-empty capsule that would act as a sail and jettison the capsule once you've freed yourself of the water stream.  You might also be able to gain additional velocity after leaving the gun itself by riding in the stream of water.

We definitely want laminar flow for this, so let's say we want a Reynolds number of 2000.  If the water speed exiting the cannon is 1,000 m/s, density is 1,000 kg/m^3, and viscosity is 8.9e-4, the diameter of the pipe would need to be less than 2 micrometers for laminar flow.  (This is a non-physical scenario; practically speaking we're dealing with highly turbulent transonic flow with Re>1,000,000,000).


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#128 2017-12-07 09:56:08

Antius
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From: Cumbria, UK
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Re: Un- conventional ways to LEO

JoshNH4H wrote:

It seems to me that if you're using a gun like this it would be tricky to modulate the acceleration well.  What you might do would be to put your payload inside some kind of mostly-empty capsule that would act as a sail and jettison the capsule once you've freed yourself of the water stream.  You might also be able to gain additional velocity after leaving the gun itself by riding in the stream of water.

We definitely want laminar flow for this, so let's say we want a Reynolds number of 2000.  If the water speed exiting the cannon is 1,000 m/s, density is 1,000 kg/m^3, and viscosity is 8.9e-4, the diameter of the pipe would need to be less than 2 micrometers for laminar flow.  (This is a non-physical scenario; practically speaking we're dealing with highly turbulent transonic flow with Re>1,000,000,000).

You would definitely need a capsule and would need a closely fitting seal between the capsule and barrel of your gun.  At the sort of acceleration we are talking about, the forces acting on the rocket would be much greater than buoyant forces and the water column would overtake the rocket exerting horrible shear stresses unless a closely fitting seal is provided.

I do not know how an incompressible liquid like water would actually behave if accelerated close to its sonic velocity.  At relatively low velocity, the static pressure of the liquid would drop to the point where any diffused gas would come out of solution.  That happens at much less than sonic velocity.  Centrifugal pump speeds need to be carefully controlled to avoid cavitation, as bubble surface tension shock blasts the interior of the pump.  Would these sorts of effects place limitations on the maximum velocity of a hydraulic ram?  I do not know.  But the forces acting on individual components at a flow speed of 1500m/s would be huge.  The Bernoulli equation indicates that dynamic pressure would be ~1GPa, but it is questionable that this applies at these flow speeds, as the water would be subject to significant density change.

Maybe some sort of chemically propelled canon or gas gun would be more practicable.  Using cordite, there is an approximate limit of 5000fps (1540m/s) on the muzzle velocity of a gun.  This is imposed by the rms speed of the expanding gas, which is mostly CO2, SO2 with some steam.  To accelerate bullets to much more than 1km/s starts to get increasingly inefficient, and for muzzle velocity of 4500fps, the cordite charge has about 50 times the volume of the projectile.  One way of improving upon this would be to use superheated steam as the propellant gas.  At 3000K, this has rms speed of 2000m/s.  One could generate steam at this temperature by igniting a hydrogen-oxygen mix within the gun chamber.

For muzzle velocity of 1km/s, it is easy to chemically propel a projectile – many rifles have muzzle velocity in this range.  Using H2/O2 mix, that speed could be increased to 2km/s, with declining energy efficiency as that limit is approached.

With a 20% energy loss from atmospheric drag, a 1km/s muzzle velocity would propel your spacecraft to 130,000feet.  That is outside of the sensible atmosphere as far as rocket engines are concerned.

Last edited by Antius (2017-12-07 10:00:36)

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#129 2017-12-07 10:07:56

JoshNH4H
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Re: Un- conventional ways to LEO

Notionally, the idea with using water was to leverage its higher speed of sound to keep the flows purely liquid, mechanical, low-temp, and laminar, but I think we've shown between our analyses that this isn't a realistic proposal and we've ended up back at the light gas gun.  The light gas gun is a perfectly reasonable idea, but it's a little disappointing the water gun probably wouldn't work.


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#130 2017-12-07 11:38:36

Antius
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Re: Un- conventional ways to LEO

JoshNH4H wrote:

Notionally, the idea with using water was to leverage its higher speed of sound to keep the flows purely liquid, mechanical, low-temp, and laminar, but I think we've shown between our analyses that this isn't a realistic proposal and we've ended up back at the light gas gun.  The light gas gun is a perfectly reasonable idea, but it's a little disappointing the water gun probably wouldn't work.

It is uncertain whether a water ram would work at speeds that would be useful.  I wouldn't dismiss the idea completely, but can certainly see problems.

I think one potential problem with the gas gun idea at the scale we are talking about is that over a length of 5km, it would take a steam molecule ~ 2.5 seconds to travel from one end of the barrel to the other (even at 3000K), whilst the total acceleration time of the projectile takes only 10 seconds.  This suggests to me that there could be stratification effects within the barrel, with the gas close to the bottom remaining quite hot and the gas at the top being cooler, having done work on the projectile.  This could result in a less than expected muzzle velocity and lower energy efficiency.  In a gun, we are generally relying upon the fact that gas temperatures are homogenised by conduction and diffusion, so we exploit the full thermodynamic expansion energy of the gas.  That is reasonable on the small scales of a normal gun barrel, but it becomes a bit of a stretch when the gun barrel is kilometres long.  Again, it is difficult to say exactly how much of a problem this would be without some really impressive CFD analysis.  It may be no problem at all, or it could introduce significant inefficiencies.

One way around this would be to mount multiple steam generators along the length of the tube, each discharging as the projectile passes.  The problem with this idea is that we are then building complexity and cost into a concept whose main advantage is simplicity and low cost.

One concept that does interest me is the plasma gun.  This works in much the same way as the gas gun, but is pumped by a plasma arc generator instead of a chemical explosion.  One advantage is that the much high rms speed allows much greater muzzle velocity.  Significant disadvantages are high electric power consumption, electrode erosion and barrel erosion in the superheated plasma.  But theoretically, such a gun could launch dumb payloads to escape velocity.

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#131 2017-12-07 12:24:49

Antius
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From: Cumbria, UK
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Re: Un- conventional ways to LEO

Perhaps an all-round better option would be a rail gun, using a sled that can be recovered and reused.

https://en.wikipedia.org/wiki/Railgun

The limiting factor with a rail gun is friction with the conductor rails.  This limits top speed, as the rails suffer progressively greater erosion as speed increases.  The wiki article notes that railguns readily achieve muzzle velocity of 3km/s.  Achieving a muzzle velocity of 1km/s or even 1.5km/s would appear to be technically well beneath state of the art, as this is high end rifle bullet speed and should therefore be achievable using ordinary materials like steel and aluminium alloy.
A clear disadvantage over the gas gun is the additional complexity and capital cost added by so many thousands of electromagnets along the track.

As with any static launcher concept, the economics depend upon frequency of use.  The more traffic it sees, the cheaper it becomes.

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#132 2017-12-07 13:01:11

JoshNH4H
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Re: Un- conventional ways to LEO

Well I know a bad idea when I see one, and in this case it's mine.  There's just about no conceivable reason why you would use my water gun over a more traditional cannon or light gas gun for orbital launch, and that's fine.  Bad ideas are part of the process smile

As far as light gas guns are concerned, gases cool as they expand so you might keep the gas expanding by installing microwave units in the sides of the cannon.  This will work especially well with Hydrogen.  The big benefit to this is that they're contact-free, so you don't need to pump anything or expose your electrodes to the high heats/damaging effects of high current flow.  I believe they use microwaves to heat fusion plasmas, so it's almost definitely good enough for our applications.


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#133 2017-12-07 14:25:32

RobertDyck
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Re: Un- conventional ways to LEO

Did this thread discuss a Gerald Bull super gun? Not something for people, or delicate equipment, but could be used for bulk cargo, supplies, and hardened satellites.

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#134 2017-12-07 14:27:14

JoshNH4H
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Re: Un- conventional ways to LEO

I don't think it was mentioned specifically, but we've talked a lot about space guns


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#135 2017-12-07 15:19:19

Antius
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Re: Un- conventional ways to LEO

I like the microwave idea.  It would be especially elegant if you could mount the magnetron into the sled and feed it with power through rails mounted in the barrel.  The barrel can then be technologically simple - two long pipes containing compressed hydrogen with solenoid valves every several metres and perpendicular conductor rails.

Last edited by Antius (2017-12-07 15:20:07)

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#136 2017-12-07 15:35:17

JoshNH4H
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Re: Un- conventional ways to LEO

Sounds like we're getting closer now to a railgun-type design


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#137 2017-12-07 16:10:27

Antius
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Re: Un- conventional ways to LEO

JoshNH4H wrote:

Sounds like we're getting closer now to a railgun-type design

Yes.  But I think capital cost will likely dominate the total cost of launch from a device like this.  One problem I can see with a space launch rail gun is the capital cost of several thousand large electromagnets and the power switching mechanism needed to activate them at sufficient speed.  The only reason we are considering a gun type launch system is the potential to do away with the capital and operating costs of the lower stage.  The lower the amortization cost of the gun, the greater cost reduction you achieve.  I think a sled equipped with a single magnetron will probably be cheaper than a long line of electromagnets.  As the muzzle velocity is only 1000m/s or so, there are a number of options that could be made to work.  It is all about finding the one that is cheapest to build.

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#138 2017-12-07 16:19:16

JoshNH4H
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Re: Un- conventional ways to LEO

Can't really argue with that.  I do wonder what kind of power-to-weight you can get with a microwave.  I don't have any numbers on this really but it would probably need to be in the MW/kg range to get sufficient acceleration


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#139 2017-12-07 16:47:51

louis
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Re: Un- conventional ways to LEO

Damn - they beat me to it! lol

Terraformer wrote:

louis,

That idea was proposed a couple of years ago - http://www.cbc.ca/news/technology/space … -1.3194738 I don't know what's happening with it.


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#140 2017-12-08 10:51:36

JoshNH4H
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Re: Un- conventional ways to LEO

I was thinking about louis's suggestion for a kind of inflatable space tower a little more.  These towers won't go all the way to LEO, but they can make a pretty big difference.  In addition to getting you all the way out of the atmosphere, a 100 km tower gives you (sort of) 1,400 m/s in delta-V*.

Rather than a bunch of inflatable objects stacked on top of each other, my tower is more like an upside down space elevator.  It's circular in cross-section, largest at the bottom, and narrowest at the top. 

I wanted to do some math to see how high you reasonably could go, so I pulled out a pen and paper.  The results I got seem absolutely crazy to me, so I would appreciate it if you guys would have a look at my analysis to check my work.

***Skip this section if you aren't interested in the math behind the results***

The structure is assumed to be in static equilibrium.  The pressure inside is higher than the pressure outside. 

The top of the structure is flat, and can support a desired mass due to the pressure differential between inside and outside.

For the supports of the structure, the difference between internal and external pressure is cancelled out by gravity and tension.  Force diagram below:

4RyNV5r.jpg

By summing the forces in the horizontal direction, I found that:

t=π/2*r/σ*(Pi-Po)

Where:

  • t is material thickness

  • r is the radius of the structure at a given height

  • σ is the effective material strength (i.e. the tensile strength of the material divided by your safety factor)

  • Pi is the internal pressure at height h

  • Po is the external pressure at height h

By summing the forces in the vertical direction, I found that:

(Pi-Po)tanθ=ρgt

Where:

  • θ is the angle by which the structure deviates from vertical at a given height

  • ρ is the density of the material

  • g is gravity, assumed to be constant at 9.8 m/s^2

I plugged the first equation into the second one, cross-multiplied, and cancelled terms, and found that:

tanθ=π/2*ρgr/σ

Using a bit of trig, we can see that tanθ=dr/dh (h in this case being positive as you move towards the ground).  Cross multiplying a bit more, integrating, solving for r, and determining constants, we find that:

***Usable equations and explanation of results below***

R=exp(π/2*ρg/σ*h)

Where:

  • R is the taper ratio of the tower, i.e. the ratio of the radius at the bottom to the radius at the top

  • exp(X) is the exponential function, e^X

  • h is the total height of the tower

So, for example, a 100 km tall tower made from a material with a density of 2000 kg/m^3 and an effective tensile strength of 2 GPa would have a taper ratio of just 4.8. (Area ratio is the square of this number, so the area at the bottom would be 23 times larger than at the top).

The thing about this that is so crazy to me is that the pressure difference doesn't show up in this equation at all.  This seems like it maybe makes sense, until you consider that this means the internal and external pressures could be the same, or that the internal pressure could even be lower than the external pressure.

Thinking about it a little more, Pi=Po implies a structure with 0 thickness, in other words a structure that does not exist.  Pi<Po implies a funnel-shaped structure with an open top, whose pressure would equilibrate with the surroundings and then collapse.

Either way, check my work!  Links to my full derivation are below:

Page One
Page Two

*It takes 1,400 m/s of delta-V to reach 100 km altitude, but launching from there may or may not reduce Earth-to-LEO delta-V by that much relative to ground launch.


-Josh

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#141 2017-12-08 15:28:36

JoshNH4H
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Re: Un- conventional ways to LEO

Between the formula in the post above and the Barometric formula, we can start doing some real design work on our space tower.

Let's say our goal is to get to 100 km, the edge of Space.  Using the barometric formula, I calculate that the atmospheric pressure at that altitude is 0.36 Pa.  However, running the barometric formula using Hydrogen, I find that the pressure with H2 would be 42 kPa, meaning that you can support 4.2 tonnes over each square meter.

If you want to support 1,000,000 tonnes (why not, right?) you need an area of 238,000 square meters (~24 hectares).  This would be a circle with a radius of 275 meters, so about 550m across.  With a taper ratio of 4.8, it will occupy a space 2,640 meters across at ground level.

In addition to all the stuff at the top, you're going to need cables to hold the thing in place, because it's will probably fall over sideways if it's not held steady.  Let's say this makes it 750 m across at the top and 5 km across at the base.

You could also use a track to accelerate the rocket as it rises up through the 100 km.  If the acceleration is 4 gs (5 gs experienced by passengers/cargo) it will be moving at 2800 m/s by the time it reaches the top.

If the cable is not vertical but angled up at 30 degrees with respect to the ground, it will be 200 km long and you can accelerate at almost 5 gs without passenger/payload experiencing more than 5 gs (lets say 47.5 m/s^2).  This would have you moving at 4,350 m/s at 100 km.

This breaks down to 2,180 m/s in the upward direction and 3,775 m/s in the horizontal direction.  That upwards velocity alone will result in a peak height around 340 km (which is pretty ideal) and the horizontal velocity is just 4 km/s short of orbital velocity.

If all you need is 4 km/s, you can easily do that with a reusable rocket burning methlox propellant. Since 100% of operation will be in a hard vacuum, you can get Isp=375 s and mass ratio=3.

So, er, mission accomplished?


-Josh

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#142 2017-12-08 16:34:36

Terraformer
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Re: Un- conventional ways to LEO

You could also do it in the reverse direction. Worlds longest and most fun zipline.

Where would the best place be to build such a track? Is there a suitable desert in the US? What spans would we need between the support towers? At what point can we really open the throttle up without being wrecked by aerodynamic drag?


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#143 2017-12-08 21:09:41

JoshNH4H
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Re: Un- conventional ways to LEO

I guess it depends what we think is safe as far as being "under" parts of the structure.  The 200 km long cable will have a ground track 173 km long, but the vast majority of it will be more than 1 km up (so practically invisible).  If you want to be really exclusive with it and say nobody can be within 175 km of the centerpoint (so a circle 350 km across) you'd probably want somewhere in the Southwest.  Places like western Texas, southern Utah, and central Nevada seem like good candidates based on population density maps.  Alaska is also sparsely populated enough.  Incidentally I forgot to take the rotation speed of the Earth into account so it'll actually be a few hundred m/s easier to get to orbit from anywhere in the mainland US.  You could also do it in the ocean, I suppose, if you are willing to deal with the extra complications.

I'd be a little more permissive than creating a zone of exclusion.  If you build it somewhere agricultural, for example, grazing and farming seem like they should be allowed in the "shadow" of the tower as long as they're not too close to one of the grounding points.

I envision the main tower being held in place with 12 cables.  6 are equally spaced around the top and might be supported midway by subsidiary towers that are 50 km tall.  Of these 6, one has the track on it.  Because the track is going to be very heavy relative to a normal cable (probably?) it will be supported by three towers, with heights of 25, 50, and 75 km (I suppose each of these will have to be cable-supported as well). 

At the 50 km level there will be a second set of cables.  These 6 cables go unsupported (ideally) all the way down to the ground.

As far as speed vs. height, see below:

U743TPI.png

You hit supersonics pretty low, which is unfortunate.  It wouldn't be crazy to put the track inside an evacuated tube that is opened at the 100 km level right before launch starts (meaning that, for safety reasons, you verify that the door has opened before the vehicle even starts moving).  Gas won't enter the tube too quickly when ambient pressure at the top is 4e-6 atm.  This reduces the friction the vehicle experiences and contains the sonic boom

An evacuated tube might add a lot of weight though, so you could also make the track steeper at the beginning and flatten out as you climb.  This might require a couple of strategically-placed towers to get the shape right, since that's sort of the opposite of the shape cables like to hang in.


-Josh

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#144 2017-12-09 03:55:10

kbd512
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Re: Un- conventional ways to LEO

The General Idea

I'm in favor of using microwave-powered thermal rockets to deliver small lifting bodies to LEO.  It provides NTR Isp and could realistically deliver a 10% payload mass fraction to orbit using a single stage.  We already have the microwave and aerospace technology, thanks to plasma physics experiments and the series of hypersonic lifting bodies we've developed over the past several decades.  All the other reduced cost launch concepts I've read about here require construction of gigantic structures or use technologies that simply don't exist.  This proposal uses existing technologies with significant development, testing, and operational use behind it.

Common gas turbines like the GE 7F can provide the intermittent power required for launches.  The spin-up time is only 15 minutes.  The 1MW to 2MW gyrotrons with the correct output frequency range have already been built for ITER and X-7.  Those gyrotrons are catalogue items that can be ordered out of a catalogue.  I know this because I checked.  This is merely a question of designing and fabricating the appropriate wave guide hardware and lifting bodies.  We have lots of experience with hypersonic lifting bodies and pushing hot flowing H2 through fission reactors and rocket engines.  There's nothing fundamentally new here, it's just a novel use for microwaves and lifting bodies.

Power Requirements

The power requirement is about 300MW per ton of total vehicle mass, so roughly 3GW to put 1,000kg class payloads into LEO using a single power facility.  That power requirement can be reduced to about 1GW if two power facilities provide the microwave power instead of just one facility.  A pair of power stations, each equipped with a pair of CC GE 7F's, could provide enough power for the microwave array and ground facilities, simultaneously enabling vehicle G loading in the fighter jet range by reducing the range requirement for the microwave array.

Payload Vehicle Mass Class Comparisons

I think fleets of 100kg, 500kg, and 1,000kg payload class vehicles should be put into serial production like the General Atomics series of military drones.  The 100kg payload vehicle is the same mass class as a Predator, the 500kg payload vehicle is the same mass class as a Reaper, and the 1,000kg payload payload vehicle is in the same mass class as an Avenger.

Concept of Operations

1. StratoLaunch's Roc would orbit the power facility at high altitude and drop a payload drone off its rack to initiate a launch sequence.
2. Roc pulls away from the drone.
3. The first power facility fixes the drone's exact location, calls a "shot", and then turns on its microwave beam.
4. Initial upwards acceleration clears the sensible atmosphere and then the first station turns off its beam.
5. The second power facility fixes the drone's exact location, calls a "shot", and then turns on its microwave beam to accelerate the drone to orbital velocity.
6. The drone circularizes its orbit using AF-M315E monopropellant thrusters.
7. The drone docks at ISS, the crew removes the payload container, potentially replaces the payload with trash or damaged components, and then the drone de-orbits.
8. The drone glides back to Earth and lands at the factory for refurbishment, return payload removal and payload replacement, and refueling.
9. Roc flies back to the factory to retrieve the next batch of drones to launch.
10. This cycle repeats on a daily basis.  Roc is not completely necessary for this concept to work, but Roc can fly numerous drones per mission as a function of its carrying capacity and onboard fuel, so perhaps we can achieve our tonnage numbers even though we don't fly every day.  Basically, this is like UPS for astronauts.  If it fits, then it ships direct, in a week or less.  It's a 24/7 operation that keeps the operators of the service on their game all the time.

Concept Benefits and Optimizations

Using gas turbines for microwave power means 24/7 launch services can be provided.  Whenever something is ready to ship, it can be shipped.  A nuclear, solar, or wind power plant would be inappropriate in this case, absent a reason to build related to grid peaking power.

Using AF-M315E means no costly and dangerous Hydrazine propellant loading facilities or related human health hazards.  No additional microwave power facility is required to circularize the orbit, either.

A stratospheric launch means the rocket nozzle can be optimized for high altitude to improve Isp and some of the fuel required to vertically accelerate the drone off the ground and out of the lower atmosphere is eliminated.  A non-cryogenic propellant can still provide SSTO in a smaller vehicle, so costly and dangerous LH2 storage and transfer equipment is not required.  Roc has more than enough payload capacity for this job.  The payload mass fraction is reduced, but so is the vehicle size.  Small is beautiful in the space launch business.

Apart from initial development and construction costs, the only ongoing costs are facilities and aircraft maintenance.  A standing army is not required for this to work, but even if it was, its the sort of standing army that a shipping business has.  Nobody is sitting on their butts waiting for months at a time.  Everyone involved is actively engaged in the shipping business.  As a function of the shipment tonnage and frequency possible with this launch scheme, even a high school should be able to afford to send their robotics team's cube sat into space.

Big Ticket Items

Some of the major component costs are known to me, but some are not.  I know that brand new GE 7F's cost $30M per copy.  I don't know what the 1MW gyrotrons cost, but CPI sells various kinds.  I'm guessing they cost $1M to $2M per copy.  We need 2,000 of those for 2 power stations.  That's $2B if they cost $1M per copy and $4B dollars if they cost $2M per copy.  The major cost of this project is likely the gyrotrons, but maybe we can get a discount from the Japanese if we buy in bulk.  My wife works for the company that made these for ITER, so maybe I can get a quote.  I have no clue what Roc cost to build, but $250M is probably not an unreasonable figure.

Food for Thought

If we launched 1t payload of payload per day, that's 365,000kg per year, which is roughly three SLS payloads.  Presumably, we can launch several times per day to reduce operating costs.  Most of the cargo is just consumables and propellants to go somewhere interesting, none of which are particularly sensitive to G loading.  All of these interplanetary destinations are a question of tonnage and most of that tonnage is consumables and propellants.  You can see where this is going.  If you lose payloads here and there, it's a minor irritation rather than a major setback.

The atmosphere doesn't substantially interfere with microwaves in that frequency range, quite unlike lasers, and the conversion efficiency of these devices is already above 50%.  The development costs for Roc, the high power microwave systems, and gas turbines has already been expended, so this is just another attempt to capitalize on those technology investments.

NASA wants to spend $300M on a new mobile launch platform that can take the crushing weight of SLS.  The agency has already sunk billions into Orion and SLS, but has yet to fly it.  We also know that cost will double once the contractors start cutting metal.  It always does.  The same amount of money that would buy a new launch platform and a few SLS rockets could purchase the gas turbines, gyrotrons, and small lifting body development work (mostly just engineering at this point, rather than basic research).

We would still keep the Falcon 9 or Falcon Heavy / New Glenn / Vulcan Heavy rockets to deliver large structures for habitation or propulsion, but consumables should be delivered in batches and at a lower price point than any rocket.  This Orion and SLS debacle is bleeding NASA dry and even if it flies, it'll be every bit as expensive as Saturn V was.

If anyone here can expand on these ideas or even refute the basic economics of this concept, feel free.

Last edited by kbd512 (2017-12-09 10:46:48)

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#145 2017-12-09 12:33:39

JoshNH4H
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Re: Un- conventional ways to LEO

I'm going to restate the concept simply, just to make sure I'm understanding it right.

You're proposing a form of beamed propulsion where microwaves are used as the power source for a thermal rocket based on Hydrogen, Isp around 950 s.  While you've suggested a variety of payload sizes can be possible, you're designing for a maximum of 1,000 kg and therefore a total rocket mass of 10,000 kg.  Mass ratio for that Isp and this mission would be about 2.7, so that gives you a structural fraction of 27%.  So far so good.

950 s (I'm going to round that off to 9500 m/s) means a kinetic energy of 45 MJ/kg.  Assuming the engine is 75% efficient, that's 60 MJ/kg into the engine.  You've suggested 2 GW of power.  If that 2 GW is transferred to the craft directly without any losses (I'll get to this later in the post), that would correspond to 33 kg/s of propellant, or 317 kN of thrust.  317 kN of thrust on a 10,000 kg payload means an acceleration of 31.7 m/s^2.  That's not quite fighter jet range but it'll go up as propellant is burned, reaching a maximum level of 85.5 m/s^2 as the fuel burns out.

I guess the thing I really don't understand is how exactly you plan to transfer the power from the microwave generator to the ship.  I'm not highly familiar with this technology in particular, but the biggest problem with beamed power schemes is always power transfer.  It's hard, especially over hundreds or thousands of km, which is probably why this hasn't been more heavily researched.

I think your cost estimates may prove to be optimistic, or a minimum-cost kind of figure.  "Standing armies" result from management decisions as much as from technological requirements.  Sea Dragon was projected to have a cost of $60/kg in today's dollars, after all, but likely would not have if it had actually been built.


-Josh

If you try to talk to me about cold fusion or propellantless drives I will ignore you.
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#146 2017-12-09 13:20:51

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 974

Re: Un- conventional ways to LEO

The microwave concept we were discussing earlier, was a gun some 5km long, that would accelerate a rocket to 1.0-1.5km/s.  This is nowhere near enough to reach orbit, but it is enough to eliminate the need for a lower stage.  The rocket does not need to waste propellant in the dense lower atmosphere and can function as an SSTO using lox/CH4.

We settled on this because it appeared to be technically achievable with off the shelf technology and there are few if any technological stretches required.

The concept could cut the cost of launching to LEO dramatically, as total launch mass would be reduced by half and would be a single reusable vehicle.  But it could only do so if traffic volumes were high enough.  The build cost of the gun would be considerable.

Last edited by Antius (2017-12-09 13:27:15)

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#147 2017-12-09 14:31:51

kbd512
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Registered: 2015-01-02
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Re: Un- conventional ways to LEO

JoshNH4H,

The power transmission through the atmosphere varies considerably with frequency.  The higher the frequency, the greater the distance achieved prior to atmospheric breakdown of the beamed power.  The power density differential of a 300GHz beam versus a 3GHz beam is approximately 1,000 times greater.  In other words, actual power transmission is substantially higher over the required distance.  200km is the point of atmospheric breakdown for a 300GHz beam located at sea level.  After that distance, the atmosphere starts ionizing and either dissipates or reflects the transmitted power.  Any significant water vapor would substantially decrease the achieved distance.  Prior to 200km, the atmosphere is essentially transparent to a 300GHz beam.  The high frequency also substantially decreases the array size.

I want to put the power facility in the Rocky Mountains, but some place dry is more important than some place high.  That said, high and dry is the best of both worlds, as it pertains to factors that affect power transmission.  Roc will take off with a 100,000kg payload of 10 drones from the west or east coast at a Boeing / Lockheed Martin / Orbital ATK / SpaceX processing facility, climb to 40K to fly to the Rockies, and then release the drone approximately 100km from the power facility.  Thereafter, Roc orbits the facility and releases the rest of the drones in sequence.  The strongback and wing box on Roc have been designed to support release of 490,000kg, so 10,000kg releases shouldn't overly stress the strongback or wing box.  A fore/aft movable rack connected to the strong box can help adjust the payload position to maintain CG.

We should at least try the concept using a 100kg payload class technology demonstrator.  If that works as well as it should, then we can incrementally scale up to 1,000kg as demand for services increases.  Regarding the cost, we won't know with certainty until we do it, but we already have the gas turbines, gyrotrons, and aerial launch vehicle, so we might as well give it a go to see what we can achieve.  If all we can manage is 10,000kg per week, that's still 520,000kg per year.  The potential exists to deliver serious tonnage if some on-orbit assembly or transfer is possible.  I just want to add a new dimension to our delivery capabilities, not replace existing rockets.  Everything has its utility, it's just a matter of capitalizing on it.  All I want is a demonstration program.  If it doesn't work as well as anticipated, then we can permanently shelve the idea.

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#148 2017-12-09 17:19:32

Terraformer
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From: Lancashire
Registered: 2007-08-27
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Re: Un- conventional ways to LEO

We could even mount the beam generator on an airship. Maybe we could make the airship-to-orbit idea a reality. Up in the stratosphere there's no clouds to cause problems, and we could launch from wherever we want.

I agree with the value of small loads. Propellent, food, water etc don't need to be launched in large packages. For that matter, neither does mylar sheeting, or space frames, or clothes, or furniture. Really, the only things that need to be launched in large discrete pieces are pressure vessels and rocket engines. If we had a cheap 1 tonne launcher, we could build quite the Mars mission using a single launch of... is it still called the BFR? Launch the ship as an empty hull, just the habitat module, fuel tanks, and rockets. Outfit it with the life support and provisions once it's in LEO.


"I guarantee you that at some point, everything's going to go south on you, and you're going to say, 'This is it, this is how I end.' Now you can either accept that, or you can get to work." - Mark Watney

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#149 2017-12-09 21:41:13

JoshNH4H
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From: New York, NY, USA, Earth, Sol
Registered: 2007-07-15
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Re: Un- conventional ways to LEO

Hey kbd512,

I'm not knocking the concept as such so much as I'm trying to understand it.  The big problem with beamed propulsion in general is accurately and efficiently transferring the power from the point of production to the point of use.  What I'm getting at is aiming.  Simply generating microwaves and allowing them to radiate out and disperse according to the inverse square law doesn't do anyone much good.

How are the microwaves getting from A to B, in the form of a maser?  Is it targeted?  How are you doing the aiming?

I'm all for experimenting, and I think we have every reason to believe that this kind of system in general could work once we work the kinks out


-Josh

If you try to talk to me about cold fusion or propellantless drives I will ignore you.
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#150 2017-12-09 22:25:44

kbd512
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Posts: 1,187

Re: Un- conventional ways to LEO

Terraformer,

Lockheed Martin's LMH-1 airship could certainly take the drones to high altitude and drop them, eliminating most of the operating costs associated with using Roc, but a conventional gas turbine powered generator with the required output would be too large and heavy for an air mobile microwave power system.  Even so, there are ways to dramatically reduce the power requirements by cutting the distance to the drone.  That's why I want to drop the drone from the air on an inbound course to the power station.  That way the distance to the drone is never more than 100km or so.  The advantage to using Roc is carrying 10 drones per flight and a small initial velocity increment.

We could also build a scaled down version for use on Mars that uses MethaLox for brief power generation.  Minimal propellant mass, thus total vehicle mass, is highly desirable for use on Mars.  The atmosphere helps kill velocity on the way down for deceleration, but that 4km/s dV increment to get back to orbit is a real problem.

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