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#1 2018-02-28 07:41:13

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,501
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Rickety Skiff

I don't know about you guys, but to me the idea of a bunch of spaceships cobbled together from space junk and used parts flying hither and yon in explored space (and sometimes further) really excites the romantic in me.

I have googled it and in general a "skiff" is smaller than the kind of boat I have in mind.  I'm thinking something more on this scale:

zwc8eu.jpg

The point of this thread is to ask ourselves:  Who are our enterprising captains, where/when/why could it make sense to have a bunch of smaller ships zooming around, and how do these ships work?

I think this kind of ship would be best-suited for the relatively low delta-V and difficulty of non-interplanetary, on-orbit transportation.  This could be in the Earth's Hill Sphere, between various orbiting stations, the Earth-Moon LaGrange Points (maybe Sun and Earth L-1/2 also), and Lunar orbit.  It could be in the Jupiter/Saturn systems.  It could be the asteroid belt (You could imagine some sort of pony-express kind of thing with ships taking zig-zag paths from rock to rock, carrying cargo to and from each) plus possibly very local hauling of large objects between different parts of the same asteroid or station.

The big question seems like propulsion to me, per usual.  The delta-V requirements aren't particularly high.  2-3 km/s can get you just about anywhere in Earth's vicinity, and less in the asteroid belt if you have time (or are only travelling a short distance).  Jupiter and Saturn are higher, with 6-10 km/s and 4-6 km/s respectively being more representative, but these systems also have plentiful fuel resources so if necessary you can stop along the way to drop off/pick up cargo and refuel.


-Josh

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#2 2018-02-28 10:13:40

JoshNH4H
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From: Pullman, WA
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Re: Rickety Skiff

So for the delta-V capacities we're looking at here, chemical propulsion is actually decently well-suited to the task (as long as you're willing to accept a decently high fuel fraction).  3 km/s with Hydrogen looks like a mass ratio around 2 and a volume ratio around 3 cubic meters of propellant per tonne of dry mass.  With Methlox the mass ratio is closer to 2.25 and the volume ratio is 1.25 cubic meters of propellant per tonne dry mass.

Depending on how you feel about giving nuclear reactors out to anyone who can afford them, nuclear thermal could also be an option.  If you run on LH2 (Isp 950) your mass ratio will be 1.4 and your volume ratio will be 5.7 m^3 propellant per tonne.  With Ammonia (Isp 650) your mass ratio will be 1.6 and your volume ratio will be 0.9 m^3 per tonne.

One good thing about this is that low accelerations are totally fine.  0.1 m/s^2 or less is totally acceptable.  While there is a delta-V penalty for slow accelerations, it could be worth it compared to the cost of more engines.  So for example a 100 tonne dry mass craft running with an ammonia NTR at an efficiency of 70% would need a total thrust of 16 kN and an engine power of 75 MWt (roughly 100,000 horsepower).


-Josh

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#3 2018-02-28 12:24:31

Terraformer
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From: Lancashire
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Re: Rickety Skiff

So, rockets for ring raiders?

Obviously aerodynamics aren't going to be an issue, unless they rely on aerobraking to save fuel. But will the need to have the rockets thrusting through the centre of mass lead to symmetric spacecraft?

I imagine the fuel tanks will need to be covered to shield against solar illumination and keep the propellent cold. This would probably also be used as a whipple shield, which would bulk up the craft. As was pointed out on TV Tropes, this shield would develop a scarred and pitted look, providing the used spacecraft look that is popular in space opera.

How big do you think the spacecraft would range? Something used to go between local stations doesn't have to be that big; it's being used more as a plane than a boat. One plying the rings of Saturn, on the other hand, will have journeys measured in days, and will need the life support to match. The former can get away with stored oxygen and a small cockpit for the pilot to sit. If we even put someone in it...


"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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#4 2018-02-28 13:21:52

Oldfart1939
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Re: Rickety Skiff

I would anticipate that some backup system would be imperative, and I would fall back on the UDMH/NTO system for a chemical fail-safe option. As a propellant gas for use in Nuclear-thermal, I'd tend to favor ammonia. Otherwise the size of tankage becomes an eater of mass when the cryo insulation is taken into account for H2.

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#5 2018-02-28 14:34:17

JoshNH4H
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Re: Rickety Skiff

One way to handle that is to "calibrate" the ship before firing.  By fitting it out with strategically placed ion engines that create a known thrust, test-firing them, and seeing how the ship reacts in 6 dimensions (Rotation and translation in X, Y, Z) you can determine just about all you need to know about the mass properties of the ship.  If you have 3+ engines that can be dialed up and down (alternative: One non-throttleable main engine with 3+ subsidiary engines that can be throttled) separated in space you can get the center of thrust at pretty much any point.

No idea really how big a craft you'll need.  I'd imagine the biggest ones would be in the Earth Zone and the smallest ones in the asteroid belt.  Any individual captain probably wants his or her ship to be as big as they can afford, so as to get the most business (assuming that there are ports big enough to fill the hold).

The biggest ships will probably be the equivalent of oil tankers, bringing fuel resources to stations that are big waypoints in the middle of space (for example, EML-1/2/3/4/5) on a slowboat trajectory (I'm thinking solar or nuclear electric propulsion).  I imagine personnel transportation generally won't exceed a dozen.

It seems to me that you're always going to want to have at least one person aboard a ship (especially if it's a bucket of bolts) but maybe not more than that


-Josh

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#6 2018-02-28 15:09:05

Terraformer
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Re: Rickety Skiff

There's also legal issues to consider. Such as speed limits. Putting a person onboard *at the least* gives them skin in the game when it comes to ramming space stations, as well as a pilot who can be commanded to change course. There might be good reasons to have a human in control even on small trips, and why that human has to be onboard (unreliable radio signals?). Life support shouldn't be too onerous for short trips, 'just' a pressurised bubble with an oxygen tank and CO2 scrubber.

Depending on the lengths of the journeys, they may be able to get away with being freefall-only, if the crew can exercise every few days on stations. Though a small centrifuge could be fitted in?


"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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#7 2018-02-28 17:21:04

Void
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Registered: 2011-12-29
Posts: 2,975

Re: Rickety Skiff

I hope you don't mind, but I would like to try to add a method of how these people would get useful materials to a market to get a profit so that they can repair their rickety skiffs.

I don't require a reply, just step over.  Just now my access to the internet is limited for a few days.

This method would involve a two stage vehicle in space already, joined at the nose in a manner similar to the Apollo craft with the LEM.

The second stage would be any form you guys are dreaming of Nuclear Ammonia if you like.

The first stage will be a "Hybrid type rocket".

Involved in the first stage would be a Metal (Mostly) shell like a solid rocket.  Inside a Carbon fill.  A hollow shaft down the middle of the Carbon.

The metal shell is your payload/payday materials.  You just make it do some work along the way.

The Carbon:
https://en.wikipedia.org/wiki/Carbon
Physical properties
Phase at STP
solid
Sublimation point
3915 K (3642 °C, 6588 °F)
Density (near r.t.)
amorphous: 1.8–2.1 g/cm3[2]
graphite: 2.267 g/cm3
diamond: 3.515 g/cm3
Triple point
4600 K, 10,800 kPa[3][4]
Heat of fusion
graphite: 117 kJ/mol
Molar heat capacity
graphite: 8.517 J/(mol·K)
diamond: 6.155 J/(mol·K)

The Carbon can be heated up to thousands of degrees before launch of the 1st stage.  I will explain the method I prefer.

The method will involve a double stationary solar concentrating mirror.  (Actually in an orbit near a base I presume).

You point the primary mirror at the sun, and point the secondary mirror into the hollow shaft in the hybrid rocket through the engine nozzle.

The solid rocket engine is temporarily embedded into a cylindrical thermally insulating device.

You pump as much heat into the interior of the 1st stage as you can, without vaporizing the Carbon or causing a failure of the Metal/Ceramic shell.

You remove the concentration mirrors just prior to launch, and detach the thermal insulating cylindrical device.

Now, since this is a hybrid rocket, you can launch by pumping a fluid of your choice into the 1st stage hybrid rocket core.

Is it water, Hydrogen, Ammonia, LOX?  Something else?

In this case Hydrogen might have merit, as it's tank could also be payload.  And further, boil off prior to launch could be kept to a minimum, buy refrigeration/insulation (That is not coming with the launch).

I will be interested to find out what you think the best fluid would be.

Once the 1st stage is burned out.  You could throw it away, but it is your payload.  So, in my opinion, you keep it attached and use your 2nd stage of the method that you like to finish the mission.

......

Ceres:
https://cosmosmagazine.com/space/ammoni … s-of-ceres

https://www.nasa.gov/feature/jpl/recent … htest-area

Quote:

Recent Hydrothermal Activity May Explain Ceres' Brightest Area

The brightest area on Ceres, located in the mysterious Occator Crater, has the highest concentration of carbonate minerals ever seen outside Earth, according to a new study from scientists on NASA's Dawn mission. The study, published online in the journal Nature, is one of two new papers about the makeup of Ceres.

"This is the first time we see this kind of material elsewhere in the solar system in such a large amount," said Maria Cristina De Sanctis, lead author and principal investigator of Dawn's visible and infrared mapping spectrometer. De Sanctis is based at the National Institute of Astrophysics, Rome.

At about 80 million years old, Occator is considered a young crater. It is 57 miles (92 kilometers) wide, with a central pit about 6 miles (10 kilometers) wide. A dome structure at the center, covered in highly reflective material, has radial and concentric fractures on and around it.

De Sanctis' study finds that the dominant mineral of this bright area is sodium carbonate, a kind of salt found on Earth in hydrothermal environments. This material appears to have come from inside Ceres, because an impacting asteroid could not have delivered it. The upwelling of this material suggests that temperatures inside Ceres are warmer than previously believed. Impact of an asteroid on Ceres may have helped bring this material up from below, but researchers think an internal process played a role as well.

More intriguingly, the results suggest that liquid water may have existed beneath the surface of Ceres in recent geological time. The salts could be remnants of an ocean, or localized bodies of water, that reached the surface and then froze millions of years ago.

"The minerals we have found at the Occator central bright area require alteration by water," De Sanctis said. "Carbonates support the idea that Ceres had interior hydrothermal activity, which pushed these materials to the surface within Occator."

The spacecraft's visible and infrared mapping spectrometer examines how various wavelengths of sunlight are reflected by the surface of Ceres. This allows scientists to identify minerals that are likely producing those signals. The new results come from the infrared mapping component, which examines Ceres in wavelengths of light too long for the eye to see.

Last year, in a Nature study, De Sanctis' team reported that the surface of Ceres contains ammoniated phyllosilicates, or clays containing ammonia. Because ammonia is abundant in the outer solar system, this finding introduced the idea that Ceres may have formed near the orbit of Neptune and migrated inward. Alternatively, Ceres may have formed closer to its current position between Mars and Jupiter, but with material accumulated from the outer solar system.

The new results also find ammonia-bearing salts -- ammonium chloride and/or ammonium bicarbonate -- in Occator Crater. The carbonate finding further reinforces Ceres' connection with icy worlds in the outer solar system. Ammonia, in addition to sodium carbonate, has been detected in the plumes of Enceladus, an icy moon of Saturn known for its geysers erupting from fissures in its surface. Such materials make Ceres interesting for the study of astrobiology.

"We will need to research whether Ceres' many other bright areas also contain these carbonates," De Sanctis said.

A separate Nature study in 2015 by scientists with the Dawn framing camera team hypothesized that the bright areas contain a different kind of salt: magnesium sulfate. But the new findings suggest sodium carbonate is the more likely constituent.

"It’s amazing how much we have been able to learn about Ceres' interior from Dawn's observations of chemical and geophysical properties. We expect more such discoveries as we mine this treasure trove of data," said Carol Raymond, deputy principal investigator for the Dawn mission, based at NASA's Jet Propulsion Laboratory, Pasadena, California.

Dawn science team members have also published a new study about the makeup of the outer layer of Ceres in Nature Geoscience, based on images from Dawn's framing camera. This study, led by Michael Bland of the U.S. Geological Survey, Flagstaff, Arizona, finds that most of Ceres' largest craters are more than 1 mile (2 kilometers) deep relative to surrounding terrain, meaning they have not deformed much over billions of years. These significant depths suggest that Ceres' subsurface is no more than 40 percent ice by volume, and the rest may be a mixture of rock and low-density materials such as salts or chemical compounds called clathrates. The appearance of a few shallow craters suggests that there could be variations in ice and rock content in the subsurface.


Dawn’s mission is managed by JPL for NASA’s Science Mission Directorate in Washington. Dawn is a project of the directorate’s Discovery Program, managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible for overall Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed and built the spacecraft. The German Aerospace Center, Max Planck Institute for Solar System Research, Italian Space Agency and Italian National Astrophysical Institute are international partners on the mission team. For a complete list of mission participants, visit:

Done.

Last edited by Void (2018-02-28 17:49:01)


I like people who criticize angels dancing on a pinhead.  I also like it when angels dance on my pinhead.

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#8 2018-03-01 06:40:13

louis
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From: UK
Registered: 2008-03-24
Posts: 4,565

Re: Rickety Skiff

I quite liked the Armadillo/Scorpius rocket approach:

http://spaceref.com/onorbit/lunar-lande … rizes.html

I think such rockets might well be within the scope of a small but technically advanced Mars colony to build.  Such rockets could take people up to LMO rendezvous in ones and twos or take cargo up.  Would be useful to be able to use such craft at locations across planet Mars, rather than having to build large BFR-ready Spaceports.


Let's Go to Mars...Google on: Fast Track to Mars blogspot.com

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#9 2018-03-01 19:23:50

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,501
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Re: Rickety Skiff

Terraformer wrote:

There's also legal issues to consider. Such as speed limits. Putting a person onboard *at the least* gives them skin in the game when it comes to ramming space stations, as well as a pilot who can be commanded to change course. There might be good reasons to have a human in control even on small trips, and why that human has to be onboard (unreliable radio signals?). Life support shouldn't be too onerous for short trips, 'just' a pressurised bubble with an oxygen tank and CO2 scrubber.

+1 on all of this. 

These ships necessarily exist in a time when we've gotten life support down pretty good.  You could imagine all of the atmosphere controls being combined into a unit the size of a washing machine or refrigerator (bigger/smaller depending on the size of the ship and the crew).  Air inlet plugs in here, air outlet plugs in here, bleed valve to vacuum here, spare oxygen bottle here, spare inert gases here, water supply (for humidification) here, plugged in through a relatively standard outlet (again depending on size, either a standard 120/220V connection or one of the higher amperage ones rated for industrial use).  Program the machine with the specifics of what you want for your atmosphere (drier/wetter depending on preference, hotter/colder depending on preference, higher/lower pressure, oxygen content, etc).  Probably there will be certain standard presets with the option to customize if desired.

Terraformer wrote:

Depending on the lengths of the journeys, they may be able to get away with being freefall-only, if the crew can exercise every few days on stations. Though a small centrifuge could be fitted in?

One thing I really wonder about is how we're going to respond to lower gravity levels in the long term, especially when people start being born in space.  As far as the solar system goes, only Jupiter and the Sun have substantially more surface gravity than Earth, and neither has a real surface.  Venus, Saturn, Uranus, and Neptune have roughly the same gravity (+/- 20%) but they likewise don't have habitable surfaces.  Just about anywhere you would actually want to go has substantially less gravity than Earth, with Mars and Mercury being the closest.  As a sidenote, I am deeply skeptical of heliostats in the atmospheres of gas giants, where Hydrogen has 14 times less lifting power as it does on Earth--which also has no floating cities.

I know there's a lot of evidence that microgravity causes degradation to our biological systems, but based on my limited research it seems to me that a lot of the degradation could also be described as adaptation.  It's as if you were lifting weights all day for your whole life and then suddenly stopped: You get out of shape.

I suspect that the effects are some combination of real degradation and adaptation.  (This is sort of an empty statement because I don't want to speculate on what proportion).

In any case, I don't think anyone is going to want to be stuck in Zero G because their bones and muscles are too weak for gravity, so you'll probably want some kind of spin gravity.  I think tether-based is probably best, with the main body of the rocket (and possibly also the cargo?) being used as a counterweight.  If anyone is interested, you can view a .pdf here describing my thoughts on the matter.

Void-

I think the biggest problem with your idea is that for any useful ship you need to lug all that carbon along with you.  Very roughly speaking, Carbon at 3900 K only has about 2.5 MJ/kg of usable energy--not really enough to be work lugging around.

Louis-

I like the armadillo designs as well and I think they're really well-suited to vacuum use


-Josh

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#10 2018-03-01 19:28:19

JoshNH4H
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Re: Rickety Skiff

To elaborate a bit, Void your proposal reminds me of something that was posted on Newmars a while back.  I can't seem to find it (it may have been lost in the Great Crash), but if I recall correctly the person was suggesting to make a thermal rocket based on thermite instead of nuclear power.  We ultimately settled on the fact that it would work and could even have a high Isp but was not a good idea because of how much thermite fuel you would need to carry.

For reference, Thermite has roughly 7.5 MJ/kg so it's actually a better energy store than hot carbon.  H2/LOX would be best of all, at 15 MJ/kg, but at that point you're basically running a diluted chemical rocket with its exhaust diluted with Ammonia.


-Josh

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#11 2018-03-02 08:11:18

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 1,003

Re: Rickety Skiff

Gerard O'Neill explored the idea of small, private spacecraft in his book 'The High Frontier'.  His propulsion solution was the mass driver engine.  This is basically a coil gun, which accelerates solid materials to velocities up to 10km/s using magnetic fields.  The power supply would have been solar PV.
This is a simple system and can be relatively low tech and easy to build.  It can also use raw materials gathered from asteroids as reaction mass without any processing.
The downsides are low acceleration, large physical size and multiple failure points.
Another option would be an arc-jet or VASIMR, using dust as propellant.  This would be smaller and more compact.

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#12 2018-03-02 11:40:06

JoshNH4H
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Re: Rickety Skiff

I love railguns as an idea but I'm a little more skeptical about them in practice.  These are my two concerns:

  1. A railgun is basically a space junk machine, pushing high-velocity solid objects out the back, with junk concentrated in the biggest shipping routes

  2. A railgun has the same power consumption issues as other forms of electric propulsion

Space is big but it's not infinite, and its inevitable that mass drivers would eventually create a hazard for spacecraft.  A way around this is to use ices as driver fuel instead of rocks.  Water (in the inner solar system), Ammonia, Methane, etc. will sublimate quickly enough that they won't be hazards to crafts that follow.

The second problem is the more serious of the two.  Let's consider a mass driver with a firing velocity of 30 km/s mounted on a 100 tonne (dry mass) cargo craft.  This craft has a mission delta-V of 3 km/s and an acceleration capacity of 0.1 m/s^2.  The mass ratio is very very low, at just 1.11.  Propellant usage is also quite low, around 0.37 kg/s (total thrust is 11.1 kN).  However, power consumption is still large: Assuming an 80% efficiency, power consumption is 200 MW.  If you can get 100 W per kilogram, you'd still need 2000 tonnes of power source to get that kind of acceleration

If you choose an exhaust velocity of 7 km/s things are better but still not really that good.  For reference, mass ratio is 1.5, propellant usage is 2.15 kg/s (total thrust 15 kN), power consumption is 53 MW, and powerplant is 530 tonnes.

Power density is limited mostly by material properties, so it's something we definitely can improve upon in the future, but unless I'm off by roughly two orders of magnitude this seems like a tough one.


-Josh

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#13 2018-03-02 16:16:43

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 1,003

Re: Rickety Skiff

The mass driver would only appear to be workable at very low acceleration.  At an acceleration of 0.001m/s2, a vehicle will achieve dV of 10km/s in 116 days.  Under this scenario, with a 30km/s Vex, your power supply would need to be 2MWe and would weigh 20 tonnes.  That's more doable.

Reaction mass would become a serious pollutant if it accumulated in close orbit around Earth, Mars or Lunar.  Using a liquid or ice is one way around this.  O' Neill discussed the use of liquid oxygen, since huge amounts would have been produced as a biproduct of metal production from Lunar ores.  Another option discussed was grinding materials to micron sizes, such that they would not penetrate a spacecraft hull.  Or perhaps carefully adjusting exhaust velocity such that reaction mass intercepts the Earth's atmosphere.

P.S.  I wonder how small a dust particle needs to be in order for sunlight pressure to accelerate it to say 1km/s over the course of a year?  This would ensure that pollution did not accumulate within planetary orbits.

Last edited by Antius (2018-03-02 16:23:34)

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#14 2018-03-04 04:14:06

elderflower
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Registered: 2016-06-19
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Re: Rickety Skiff

Railguns might be useful as fixed, but steerable, installations on a large body for launching mined products in vehicles that will withstand the massive accelerations. These vehicles have to be able to make orbital corrections so that they can reach their destinations, and if there is no atmosphere at the destination, they have to be able to brake by reverse thrust. Still you would have nearly halved the propellant mass to be launched along with the vehicle and the product. This removes the objections due to the size and mass of energy plant required.

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#15 2018-03-04 06:15:49

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 1,003

Re: Rickety Skiff

elderflower wrote:

Railguns might be useful as fixed, but steerable, installations on a large body for launching mined products in vehicles that will withstand the massive accelerations. These vehicles have to be able to make orbital corrections so that they can reach their destinations, and if there is no atmosphere at the destination, they have to be able to brake by reverse thrust. Still you would have nearly halved the propellant mass to be launched along with the vehicle and the product. This removes the objections due to the size and mass of energy plant required.

Rail guns or coil guns for launching lunar materials are only desirable if the payloads are strictly dumb (i.e. ore packages).  Building technology into the payloads such that they can make course corrections undermines the point.  The mass driver (coil gun) was supposed to be a means of very cheaply launching large quantities of raw material payloads, that could be collected in a large bag, transported to an ore refinery and processed into metals to support space manufacturing.  If the payloads need to make course correction then that idea doesn't really work.

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#16 2018-03-04 08:28:34

JoshNH4H
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From: Pullman, WA
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Re: Rickety Skiff

Antius wrote:

The mass driver would only appear to be workable at very low acceleration.  At an acceleration of 0.001m/s2, a vehicle will achieve dV of 10km/s in 116 days.  Under this scenario, with a 30km/s Vex, your power supply would need to be 2MWe and would weigh 20 tonnes.  That's more doable.

Reaction mass would become a serious pollutant if it accumulated in close orbit around Earth, Mars or Lunar.  Using a liquid or ice is one way around this.  O' Neill discussed the use of liquid oxygen, since huge amounts would have been produced as a biproduct of metal production from Lunar ores.  Another option discussed was grinding materials to micron sizes, such that they would not penetrate a spacecraft hull.  Or perhaps carefully adjusting exhaust velocity such that reaction mass intercepts the Earth's atmosphere.

P.S.  I wonder how small a dust particle needs to be in order for sunlight pressure to accelerate it to say 1km/s over the course of a year?  This would ensure that pollution did not accumulate within planetary orbits.

That does sound more reasonable! 

If you're looking at a typical delta-V of 3 km/s instead of 10 km/s, that's closer to 35 days' firing time, which is on the high side.  Lowering Vex down to 7 km/s but keeping power consumption constant, you have your same mass ratio of 1.5 from before, propellant flow of 81.6 g per second, and a thrust of 571 N.  Net acceleration is 0.0038 m/s^2, so that's 9 days for a 3 km/s mission which seems pretty reasonable (of course you always want higher acceleration).

I guess another question is what kind of T/W you can get out of a railgun.  I don't know the technology very well so I feel like I can't really speculate. 

Then another question is whether 100 W/kg is really a reasonable power level or we should expect higher.  The SAFE-400 reactor produces 400 kWt (roughly 100 kWe) and weighs just 512 kg.  This is a specific power of 195 W/kg, but does not include power conversion (which is sorta important!).  The Saturn 20 microturbine produces 1240 kWe and weighs 10,530 kg.  Let's say you could halve that if you really tried (these gas turbines are not designed for space after all--Aluminium, Carbon composite, and Titanium can probably replace steel for many parts, and the total metal volume could also probably be reduced by thinking carefully about how much metal you actually need to use), and reduce the mass per kW of the SAFE-400 core by 25% by scaling up.  Mount two units, each producing 1,240 kWe, on your ship, and you have a total powerplant mass of 20 tonnes with a rated power of 2.5 MWe.

I think we'd be selling ourselves short if we thought this was the best we could do.  This is like a 10-year project (fully funded), not a 100-year project (and it seems to me that's how long it'll be until our rickety skiffs are zooming around the sky).

As far as light pressure, I think it actually tends to bring dust particles in towards the sun.  This is called the Poynting-Robertson Effect.


-Josh

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#17 2018-03-04 08:42:56

JoshNH4H
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Re: Rickety Skiff

Follow up: At 7 km/s or higher I suppose you would expect propellant particles to leave the Earth Zone entirely and fly out in all directions in interplanetary space (not so for Jupiter or Saturn), but imo that's not a good reason not to use a volatile fuel that will not pose a collision hazard.


-Josh

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#18 2018-03-04 10:00:26

Terraformer
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From: Lancashire
Registered: 2007-08-27
Posts: 3,017
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Re: Rickety Skiff

I would expect skiffs to be used by small colonies and independent tramp freighters, though. I can't image an asteroid colony of 10,000 having a nuclear industry.

If you have volatiles, why use a mass driver anyway? You get more thrust and efficiency, and a simpler engine, by using an NTR.

If you don't have nuclear, you could still use a solar thermal rocket, maybe even out at Jupiter's orbit (trojan skiffs?). The distance would reduce your thrust, but not your Isp. Solar Moth claims 4000N for 100kg. Say that means 160N out by Jupiter, and the spacecraft is 10% engine by mass, then the acceleration would be 0.16 m/s^2. About 14 km/s per day. That assumes hydrogen; water is more likely, and would give much better thrust. They might even be viable out around Saturn. Provided you have water, why not use STR?


"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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#19 2018-03-04 16:52:24

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,501
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Re: Rickety Skiff

I was mentally kicking around an idea for a sort of hybrid thermal-electric propulsion, where reactor waste heat would be dumped into the propellant as a preheater and then an electric arc (which can theoretically get higher temperatures and therefore Isp than a solid heat transfer element) as the main heating.  The benefits are theoretically higher Isp than regular thermal propulsion and higher thermal efficiency than traditional electric propulsion.  The downside is that this technology would not actually make sense for any reactor working at reasonable temperatures.

What could work would be having a main NTR reactor, with a main electrical reactor whose waste heat was used as a preheater and electrical output used as an afterburner.

But anyway, you're quite right that tramp freighters probably won't have access to reactors.  It's just asking for all sorts of issues from nuclear weapon proliferation to massive radiation health issues (which you'd imagine spacers to be sensitive to in a practical way).

I'd be willing to stand up for a mass driver from a maintenance perspective.  Electrical reactors operate at much lower temperatures (and probably with much longer lifetimes) than a NTR core could.  Mass drivers are relatively simple to build and fix (you could probably make one in your garage on a shoestring) so they are the sort of thing you conceivably could find aboard a little skiff.  They can also have pretty high exhaust velocities for something so simple so there is a lot of upside.

The downside is low accelerations, and it looks like the solar moth is much better on that score.  If you can build a 3000 K heat exchanger and turbopumps that world reliably (or as times goes on, a fused quartz heating chamber seeded with Carbon or Tungsten Carbide to do direct heating).  You'll probably start off with Isp around 575-600 with Ammonia if you're using a heat exchanger, and be able to get it up maybe as high as 800 if you figure out direct heating.  Aiming and calibrating the mirrors is going to be pretty tough but it's doable I guess.


-Josh

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#20 2018-03-05 06:47:45

Antius
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From: Cumbria, UK
Registered: 2007-05-22
Posts: 1,003

Re: Rickety Skiff

Electrically powered propulsion always comes off poorly from a power (thrust) - weight perspective and thus always has poor acceleration.  The power conversion and generating equipment is heavy and to get a passable Carnot efficiency, the a power cycle must operate at very high temperature (challenging for materials) or dump heat at low temperature (huge radiator mass).  Energy transitions are very expensive in terms of mass.  Project Neptune in the 1980s was a US attempt to push nuclear-electric propulsion to its limits by developing a boiling potassium reactor with an MHD generation cycle.  Whether a boiling potassium reactor is something you want to trust to the likes of Malcolm Reynolds and Kaylee Fry, even in the emptiness of interplanetary space, is questionable.

The best option in terms of power-weight is always to generate huge amounts of heat within the fuel and use the products as reaction mass without any heat transfer.  Heat transfer and energy transitions are rate limited and imply a lot of extra mass.  Hence, the enormous power to weight ratio of chemical rockets and the huge thrust-weight and ISP of the nuclear salt rocket - which is basically a torch ship.  High ISP x High thrust = Very High Power.  The solar rocket and nuclear thermal rockets are the next compromise in terms of thrust to weight and ISP, because energy transitions are kept to a minimum, heat stays as heat and the equipment involved is light-weight in comparison with the energy flux it receives.

Last edited by Antius (2018-03-05 06:51:30)

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#21 2018-03-05 10:40:52

JoshNH4H
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From: Pullman, WA
Registered: 2007-07-15
Posts: 2,501
Website

Re: Rickety Skiff

No contest on that score.  I think as far as cargo and personnel transport are concerned, low acceleration is fine as long as it's not so low that it ends up affecting the total time needed to go from A to B.  At a guess, for most low-delta-V missions, a maximum acceleration of 0.1 m/s^2 is roughly as good as 10 m/s^2 (and maybe even better, since it puts less stress on ship components).

I was thinking about solar thermal rockets earlier, and it occurred to me that there is one material that is clearly superior to all other known materials if you want to build a solar thermal engine chamber.

This material is diamond.

Diamond has an extremely high temperature tolerance, extremely high tensile strength, high thermal conductivity, and great optical transmission properties.  This combination makes it absolutely ideal for the high temperature, high pressure, optically transmissive characteristics of a solar thermal engine using direct heating.

I found a book called Optical Properties of Diamond which seems to contain this data, but I can't really make sense of it.  If you want to buy it you can pay $200, or you can download it for free here

You could imagine that the engine manifold could be carved from a big diamond block using lasers (we're getting better at making big diamonds all the time) or that it could be created directly with the correct shape in something like a 3D printing process.

The engine manifold would likely be the most expensive part of the ship, which seems reasonable to me.  The diamonds are likely to turn yellow and perform worse (lower Isp) as they get older which also seems pretty reasonable/normal.  At the end of its useful life you would probably sell the engine manifold for whatever crazy carbon compounds have evolved over time within it.


-Josh

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