Question re: starships

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Dale Smith
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Question re: starships

Post by Dale Smith »

Ideas are occasionally floated for ways to get spaceships up to some major fraction of light speed so that the journey to neighboring stars can be accomplished in reasonable time. For the sake of argument, let us suppose the problems at this end of the journey have been solved. There is one aspect I have never seen addressed. Once the space ship approaches its destination, how does it slow down? It will take an enormous amount of power to accelerate up to the initial speed and it will take an equal amount to deccelerate at the other end. Launching with enough fuel to accomplish the decceleration is probably weight prohibitive. Most of the journey will be too far from stars to get much energy from solar collectors. The speed would make it impractical to gather space debris along the way - it would just go through the space craft like a bullet through tissue paper. Does anyone have thoughts on how it could be done? No rush. I doubt that the answer will be urgently needed in the next few weeks.
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Dick Jacobson
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Re: Question re: starships

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One proposal that I've read is to propel the craft on a laser beam. The laser would be based in the solar system and might be solar powered. When the spacecraft approached its destination it would split into two parts, with a reflector ahead of the main craft. After hitting the reflector, the reversed laser beam would slow down the spacecraft. There have been some experiments levitating objects using photon pressure.

There was a NASA study that investigated ways to get a spacecraft up to about c/2 (half the speed of light). In addition to the solar-powered laser, there were two other ideas that seemed possible using known physics (ignoring economics and engineering). One was Robert Bussard's "ramjet" idea (collecting hydrogen atoms from space with a magnetic scoop and using them for fusion power). The other was to use antimatter, maybe a magnetically confined tank of anti-hydrogen. I read recently that a lab managed to make a few atoms of anti-helium, the largest anti-objects yet constructed.

I think any of these ideas could be applied to a spacecraft that would accelerate during the first half of the journey and decelerate during the second half. Of course this would increase travel time compared with a flyby at nearly the speed of light.

Is there a way to apply the aerobraking concept? Certainly the destination star has plenty of mass to brake our spacecraft to a stop. Could we use the star's magnetic field or corona or stellar wind to somehow put on the brakes?

I think that the subject of interstellar spacecraft will soon become a lot more important. The Kepler mission has shown that 2% of sun-like stars have an earth-like planet, in the sense that liquid water could exist on the surface. There is likely to be an Earth Two within a few dozen light years. In 10 or 20 years we should be able to build a telescope to image it and study it spectroscopically. I think it's likely that the composition of the atmosphere will imply that biology exists. Then what do we do? Sending a spacecraft might be the only way to determine whether the biology is just bacteria or something more interesting.
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Dave Venne
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Re: Question re: starships

Post by Dave Venne »

Dick Jacobson wrote:Sending a spacecraft might be the only way to determine whether the biology is just bacteria or something more interesting.
For the last few years I've been taking an annual, informal poll of my adult astronomy students. The question is whether or not humanity should be sending out radio signals to announce our emergence as a technological society. The issue of sending spacecraft to other planets (presuming someday we can) seems somewhat related.

It's discouraging to me that the polls usually come out split evenly between transmit and don't transmit. I suspect if I changed it to explore / don't explore, it would be the same if not more slanted toward don't explore. One reason given for the don't transmit votes is "It's a waste of money." So if we do send spacecraft to other planets, will cost be the deciding factor for what technology is used? Or is there a chance the flight can fire up the public's imagination and open their wallets for aggressive research into propulsion systems?

Am I right to assume that photon sails represent the least expensive forseeable way to get from star to star? If so, maybe that's all the public will will ever approve.

Maybe someone can build a steampunk Bussard ramjet?
MaryB
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Re: Question re: starships

Post by MaryB »

Ion drive and a fission power plant. The ion fuel could be scooped as you go, burn constantly until midpoint, flip and decelerate from the halfway point.
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Dick Jacobson
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Re: Question re: starships

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Dave Venne wrote:It's discouraging to me that the polls usually come out split evenly between transmit and don't transmit. I suspect if I changed it to explore / don't explore, it would be the same if not more slanted toward don't explore. One reason given for the don't transmit votes is "It's a waste of money." So if we do send spacecraft to other planets, will cost be the deciding factor for what technology is used? Or is there a chance the flight can fire up the public's imagination and open their wallets for aggressive research into propulsion systems?
We've been sending out powerful radar and TV transmissions for 60 or 70 years, so the question seems moot to me. If the aliens are out there, presumably their technology is at least as good as ours and they know all about us. I wouldn't spend a lot of money on a transmitting project unless we somehow discover an advanced civilization and want to establish communication and let them know that our TV broadcasts do not represent the summit of our culture!

Let's suppose that the Terrestrial Planet Finder gets funded and we discover an Earth-like planet at a distance of 10 parsecs or 33 light years. This seems like a reasonable prospect given the Kepler results. If that planet's star is the same brightness as the Sun, it would appear to us as 5th magnitude. Also suppose that the TPF spectroscope finds evidence of biology, a reasonable assumption given that there is evidence that microorganisms appeared on Earth shortly after it cooled off. We could point to a naked-eye star and say "there is biology circling that star". At first we would also have to say "We have no idea what form that biology has taken, and we are making no effort to find out." How long could we continue saying this?

If the only way to find out was to launch a hugely expensive spacecraft that would take many centuries to get there, I'd be all in favor of that. But I doubt whether any democratic government would allocate funds for a project that had no payoff for many generations. The Tea Party would tear the idea to shreds.

I can think of two ways to get the mission funded. One would be to finance it privately. A dying billionaire whose money was of no use to him except to buy first-rate medical care might take a long-term view. The prestige of funding mankind's first expedition to the stars could be motivating.

The other approach would be to find something useful for the spacecraft to do along the way. Even if it never reached its destination alive, it could return useful results. Measurements of fields and particles, while not very sexy to the public, would be useful. Possibly high-precision distance measurements could reveal information about dark matter, Oort cloud comet nuclei, dark planets, or whatever else might go bump in the dark out there. Gravity wave measurements would become more and more precise as the distance increased. Multiple spacecraft could form an interferometer, in effect becoming a telescope with a diameter much larger than the Solar System. Parallax measurements would benefit from the huge baseline.

I don't know if any of these ideas are practical, but we'll need some big-idea thinking if we start discovering bioplanets in our neighborhood.
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Dick Jacobson
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Re: Question re: starships

Post by Dick Jacobson »

Dave Venne wrote:Am I right to assume that photon sails represent the least expensive forseeable way to get from star to star? If so, maybe that's all the public will will ever approve.

Maybe someone can build a steampunk Bussard ramjet?
I wonder if an electrodynamic tether could be used for propulsion. This is a long wire that interacts with a magnetic field. The idea has been tested in Earth orbit as a way to raise or lower satellites, and generate power for the satellite. Could a tether work in the galactic magnetic field, or is it too feeble to be practical? Like photon sails, this would be a low-thrust, long-duration propulsion method.

I recently read a (presumably serious) proposal for a steam-powered Mars expedition. A solar collector would heat a large tank of water and generate steam for propulsion. The water would surround the astronauts and provide radiation shielding (presumably it wouldn't all be used up). I didn't see anything regarding the cost of getting all the water into Earth orbit, or a comparison of the efficiency of steam versus splitting the water into hydrogen and oxygen to power a more conventional rocket. Although I doubt that the idea is practical, I LOVE the idea of a steam-powered rocket. Maybe they could even make it go choo-choo. James Watt would be so proud.
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20-inch homemade equatorial Newtonian with periscope
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6-inch Russian Maksutov-Newtonian on Vixen equatorial mount
Too many small scopes and binoculars to mention
Reece
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Re: Question re: starships

Post by Reece »

Project Orion was a proposal to generate thrust in the engine of a space ship by use of nuclear fuels. It was scrapped because of worry about nuclear fallout, but if we assembled it in and launched it from space, I feel like there would be less worry about that. Especially if we could somehow create a build station outside of the Van Allen belt.
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Dick Jacobson
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Re: Question re: starships

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MaryB wrote:Ion drive and a fission power plant. The ion fuel could be scooped as you go, burn constantly until midpoint, flip and decelerate from the halfway point.
This is probably the best we could do with today's technology. Both nuclear reactors and ion drives have been tested separately in space but the two have never been put together. Existing ion drives use a tank of xenon as propellant; the idea of scooping ions from space would need development and the low density of matter in space might be a problem. But if nobody invented a better system, I'd be all in favor of using this even though the trip might take many centuries.

Among the three high-speed interstellar drives that I mentioned above, I really like the idea of a solar-powered laser/maser. Presumably it would include a giant parabolic reflector in space. This reflector could have multiple uses. It would beam energy to the spacecraft and receive signals back from it (maybe not at the same time). It could also function as a telescope, forming images of the target planet. The "background" image of deep space could be even more interesting. The Hubble Deep Field demonstrated how we can get a great scientific return simply by aiming a telescope in a random direction and collecting photons for a long, long time. The multiple uses of this reflector could help justify the cost of the entire mission.
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Dick Jacobson
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Re: Question re: starships

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Reece wrote:Project Orion was a proposal to generate thrust in the engine of a space ship by use of nuclear fuels. It was scrapped because of worry about nuclear fallout, but if we assembled it in and launched it from space, I feel like there would be less worry about that. Especially if we could somehow create a build station outside of the Van Allen belt.
Yes, it amazes me that they would seriously propose launching a space ship from Nevada using 800 nuclear explosions. How times have changed since the 1940's!

The only other detailed engineering study of a starship that I'm aware of is Project Daedalus. This would be powered by fusion of deuterium-helium-3 pellets using an electron beam. A two stage starship would reach 12% of the speed of light and get to Barnard's Star in 50 years without deceleration. The scientific payload would be 500 tons and the total weight of the craft would be 54,000 tons at launch from Earth orbit.

It seems to me that if you wanted to study biology it would be just about mandatory to decelerate at least part of the spaceship. The second stage could decelerate, though this would about double travel time. The whole project would need to be slimmed down. 54,000 tons is something like 100 times the weight of the International Space Station. The launch costs alone would be several trillion dollars at today's prices. Modern nanotechnology should be able to trim the weight of this 1970's proposal and it seems at least reachable in the near future, given enough financing. Antimatter fuel would be more efficient but I don't think anyone has solved the problem of converting the gamma rays produced by antimatter annihilation into usable rocket thrust.

How about using a particle accelerator to produce thrust? Deliver the energy for the accelerator using a solar powered laser or maser as in the NASA concept I mentioned above. The particles would either come from an onboard tank or collected from space if that was feasible. I believe a small particle accelerator would be the most efficient possible rocket. I'm not thinking the Large Hadron Collider, something more like the small linear accelerators used for cancer therapy.

Giant parabolic reflectors aimed at nearby bioplanets would be a sort of infrastructure for interstellar travel and communication, sort of like railroads, highways, and fiber optic cables on Earth.
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Dale Smith
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Re: Question re: starships

Post by Dale Smith »

I ran some calculations that may (or may not) be of interest.

Imagine a spacecraft traveling at 1/10th the speed of light (0.1 c or 30,000 km/sec or 108,000,000 km/hour). At that rate it would take about 40 years to reach Proxima Centauri, the nearest star. If one is going to be cooped up on a craft for that long it is going to have to be bigger than the International Space Station, which has a mass of about 400 tons. Let us say our craft weighs 1000 tons (much too small to allow for the agriculture necessary to sustain the crew, radiation shields, propulsion system and power plant, etc. but it is a nice round number).

At this speed the Lorentz transformation to correct for relativity is only 1.01. In other words, using Newtonian physics to simplify the calculations only introduces a 1% error.

To visualize the distance scale of the stopping process, let us choose something familiar. Imagine the craft is approaching our own solar system and the goal is to park the craft in orbit around earth (orbital velocity = 20,000 mph or 32,000 km/hour). If we wait until the craft is only 1 astronomical unit (AU or 93,000,000 miles) from earth, the deceleration will take almost 3 hours and the occupants will be subjected to a continuous 308 G’s which would kill everyone on board and probably rip the craft apart. I wish I hadn’t dozed off during the episode where Scotty explained how the Enterprise’s inertial dampers work. The engines would have to produce about 4.5 x 10^14 Watts of energy continuously during this period. This assumes the energy is somehow converted into braking with 100% efficiency (method still to be determined).

If deceleration is started at Jupiter’s orbit the numbers are 75 G's, 11.5 hours and 1 x 10^14 Watts.

Pluto’s orbit (≈40 AU from the sun) - 7.7 G's, 4.6 days 1 x 10^13 Watts. Fighter pilots with G-suits can withstand this G force for brief periods without blacking out, but I suspect that 4.6 days of weighing 1500 lbs and having blood forced to one end of the body would cause injury or death.

110 AU (twice as far as the outer edge of the Kuiper belt) - 2.8 G's, 12.6 days and 4 x 10^12 Watts

250 AU - 1.2 G's, 29 days and 1.8 x 10^12 Watts (about 600 times the rate of energy production by a large nuclear power plant or about 1500 times the 1.21 gigawatts of Doc Brown’s DeLorean – assuming a jigawatt is the same as a gigawatt). We have finally reached a point where we won’t kill or damage the crew. We are also far outside the zone where there are any planets whose atmosphere we could use for aerobraking – which would not work anyway because it would require deceleration within at most a few thousand miles of atmosphere, not hundreds of AU. At this distance the solar intensity is only 0.0016% of what we are used to here on earth at 1 AU. That is not much light intensity with which to generate all that power photovoltaicly.

134,073 au (half the distance to Proxima Centauri) – 0.0023 G, 42.13 years, 3.38 gigawatts. Put another way, this is the last half of the scenario referred to by Dick Jacobson in which a “spacecraft would accelerate during the first half of the journey and decelerate during the second half.” (assuming velocity at midpoint is 0.1c).

At 0.1 c the craft would be moving about 3,000 times faster than orbital velocity. One of the space shuttles once returned with a cracked windshield. It was estimated that it was caused by collision with a particle about the size of a sand grain. 3,000 times the velocity translates to 9,000,000 times the collision energy. Unfortunately, I also missed Scotty’s talk on navigational deflectors. Particulate types of ionizing radiation (e.g. α particles, ß particles, neutron radiation) consist of atomic scale particles moving at or near relativistic velocities (e.g. α particles move at ≥ 0.05 c). A craft moving at 0.1 c would perceive all atoms in its path as ionizing radiation. Whether macroscopic dust particles or individual atoms, the attempt to capture any significant mass en route for use as fuel would result in one heck of a sandblasting.
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Dave Venne
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Re: Question re: starships

Post by Dave Venne »

Interesting calculations, Dale --

They illustrate an advantage of some sort of low-thrust propulsion that is used for the entire trip, with a turn-over point halfway between origin and destination.

I did read an "explanation" of inertial dampers long ago: some sort of widget creates a counteracting gravitational field. If your ship lurches ahead, the field pulls you forward so you're not smushed against the rear wall. (Probably related to the widget that lets everyone walk around as if in a 1 G field.) I'd hate to see the results if that sort of system got a little out of sync with the accelerations.
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Dick Jacobson
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Re: Question re: starships

Post by Dick Jacobson »

Interesting numbers, Dale! For me, it shows how utterly unrealistic is the idea of sending manned spacecraft to the stars. Unless someone overturns the laws of physics, it's beyond my imagination how anything like Star Trek could occur in the foreseeable future.

If, a few centuries from now, there are millions of people living in giant space colonies scattered around the Solar System, it is conceivable that some of the colonies could vote to emigrate from the Solar System if they have no need of its energy and material resources. Decades ago I read a novel by Heinlein, "Orphans in the Sky" (if I remember right), that was based on such an interstellar Ark.

While manned interstellar travel seems unrealistic, I think the idea of sending robot spacecraft to the stars is very definitely possible. I've been doing a little more study on my idea of using a particle accelerator as a rocket. The Large Hadron Collider is a ridiculously poor design for a rocket, but it is possible to calculate the thrust of the LHC based on the energy (7 trillion electron volts per proton in each of the two beams if it reaches its design goal), and the beam current (0.582 amperes). Using the formula for the thrust of an ion thruster, the thrust of the LHC beam comes out to exactly 50 pounds. In other words, this is the force you would feel if you held a target in the beam and it absorbed all the protons. The LHC will accelerate protons to 99.999999 percent of the speed of light. The rocket equation tells us that any type of rocket will reach its exhaust velocity if the propellant mass is about 1.7 times the spacecraft mass, a modest figure. So if we put the LHC in space, gave it unlimited energy from a laser, and an adequate tank of protons, it would eventually accelerate itself to 99.999999 percent of the speed of light. I haven't tried to calculate what "eventually" means, likely millions of years.

I think it would be possible to build an accelerator with thousands, maybe millions of parallel beams that would make a practical interstellar drive. The spacecraft would look like a giant umbrella. The shaft would be the accelerator. The canopy would reflect the laser light onto the accelerator's photovoltaic converter. If you wanted to decelerate, you might have two concentric accelerators, and jettision the outer one when the acceleration phase was completed. The inner accelerator (or "decelerator" if you prefer) would send its particles in the opposite direction.

The British Interplanetary Society is practically begging for someone, anyone, to come up with a more practical starship design. In the meantime they have started Project Icarus, which will try to downsize Project Daedalus into something that could actually be built. I've read a little about this at the Web site http://www.centauri-dreams.org/ which contains superb writing about current astronomical topics in addition to speculation about starships.
30-inch homemade Newtonian with periscope
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6-inch Russian Maksutov-Newtonian on Vixen equatorial mount
Too many small scopes and binoculars to mention
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