Everyday Space Drives

[shynung]

2 hours ago, regex said:

PJMIF may be simpler than I think and gets good performance for a pinch system, and would be my preferred "high isp" fusion system given it doesn't require things like lithium liners.

The lithium liner on an MIF motor doubles as the propellant, and it's denser than stuff like liquid hydrogen. Combined with the high specific impulse of the motor itself, it might yield a favourable mass ratio compared to fission thermal motors.

Edited October 3, 2017 by shynung

[regex]

26 minutes ago, shynung said:

The lithium liner on an MIF motor doubles as the propellant, and it's denser than stuff like liquid hydrogen. Combined with the high specific impulse of the motor itself, it might yield a favourable mass ratio compared to fission thermal motors.

And it needs to be manufactured, and stored (and loaded into the engine) in a much more complex manner than a simple pressure vessel, which is why I argue against it as an "everyday" sort of drive. ISPs on ICF drives are generally really good though, I see no reason why they wouldn't be in use but not necessarily by smaller entities.

Edited October 3, 2017 by regex

[shynung]

@regex If propellant packaging and loading is an issue, it's possible to build a MIF motor like a firearm, and load the lithium as prepackaged magazines. When empty, the magazine can be detached from the motor, and a filled one swapped in, much like a firearm operator changing magazines.

[regex]

9 hours ago, shynung said:

@regex If propellant packaging and loading is an issue, it's possible to build a MIF motor like a firearm, and load the lithium as prepackaged magazines. When empty, the magazine can be detached from the motor, and a filled one swapped in, much like a firearm operator changing magazines.

It's hard to argue against that but I would still argue against the overall complexity of the system. For the everyday spacefarer I feel dirt simple is better. Of course, that all depends on what the infrastructure and culture look like...

[tater]

I would assume the ideal propellant/ox mixture would be hydrolox, assuming water is around.

That leave pretty large craft (big tanks), and boiloff as a substantial challenge. Alternately, I think the solar thermal version is pretty cool, perhaps the reflectors double as shields for the cryos. 

For envisioning private craft, though, the logistics are so very non-trivial. You don't just hop in and go someplace, you need to plan. You'd have a computer do this for you, obviously, but given very finite total dv for any craft, you'd have to plan launch windows accordingly. Any travel farther than something like a LEO O'Neill colony to the Moon and back would be like planning an Earth circumnavigation in the age of sail---except with zero chance of getting anything along the way---or harder than that.

[GreenWolf]

Throwing my two cents in, since I've been doing a lot of number crunching and general thinking on this subject lately for a book.

I doubt we'll ever see manned interplanetary spacecraft with a crew of less than half-dozen (maybe a little bit lower if you make some generous assumptions about AI and robotics), and that would be on non-bulk cargo carriers. (Caveat that I'm assuming something like Rick Robinson's Mission Control Model, where the crew are mostly there to oversee the automated subsystems of the ship and maybe make repairs.) Personal spacecraft will be rare or non-existent, especially for passengers/leisure/tourism, because transfer windows will constrain when you can use it (and thus make it easier and cheaper to just take a dedicated liner). Spacecraft will be more like trains than cars, and most of them will be owned by larger collective entities (like corporations and cooperatives) rather than individuals. Part of that is cost of owning and operating a deep space vehicle, part of it is the way in which spacecraft would be used, and part of it is regulatory, to a degree.

As for the design of such a spacecraft, that depends on what you want. If you're moving ice or metals, time isn't really a concern, so you'll be angling to maximize payload and specific impulse. For ice, you'll get popsicle-pushers -- basically a nuclear reactor on a stick, shoved into a chunk of ice carved off a comet, using some of the comet as remass. Your propellant is "dirty" (in that it's going to be a mixture of whatever the comet is made out of -- likely water, ammonia, and some carbon dioxide), so your specific impulse isn't great for an NTR, but you've got plenty of material available, and you can afford to take low-energy, long-transit transfers. For metals/minerals mined out of asteroids, you'll probably be using nuclear electric instead of nuclear thermal -- using a nuclear reactor to run an ion thruster or mass driver. Again, travel time isn't a concern, and since you don't have a readily available source of reaction mass, you'll be optimizing for efficiency. But that's fine, metals don't care.

For things carrying people -- be it passenger liners doing an Earth/Mars run, or non-bulk cargo carriers with a minimal crew -- you'll be wanting to strike a balance between travel time and delta-v. NTRs hit this balance pretty well, and depending on your propellant and core design, you can get exhaust velocities of as much as 8000 m/s. A lot of people like hydrogen, because of its low mass and ready availability, although I personally favor ammonia, at least on an Earth/Mars cycler. Your exhaust velocity isn't as good (only in the range of 5000 m/s), but your fuel is a lot denser and easier to handle (providing some impressive savings on tankage and structural mass), and the increased thrust lets you be more aggressive with your maneuvering. And ammonia, while less abundant than water (and thus, hydrogen), is still pretty easy to get.

The basic design of one of these spaceships would then look like a very long dumbbell, with the drive unit (with the reactor and a cluster of propellant tanks) at one end of a long structural truss, and the hab module at the other. Cargo and additional propellant tanks can be attached along the length of the truss. While not exactly modular, the drive section could be detached for servicing, or jettisoned in an emergency (although the hab module would then be left with limited or no independent maneuvering).

Edited October 3, 2017 by GreenWolf

[wumpus]

Just now, tater said:

I would assume the ideal propellant/ox mixture would be hydrolox, assuming water is around.

That leave pretty large craft (big tanks), and boiloff as a substantial challenge. Alternately, I think the solar thermal version is pretty cool, perhaps the reflectors double as shields for the cryos. 

For envisioning private craft, though, the logistics are so very non-trivial. You don't just hop in and go someplace, you need to plan. You'd have a computer do this for you, obviously, but given very finite total dv for any craft, you'd have to plan launch windows accordingly. Any travel farther than something like a LEO O'Neill colony to the Moon and back would be like planning an Earth circumnavigation in the age of sail---except with zero chance of getting anything along the way---or harder than that.

Hydrolox is great stuff, but I don't think you can realistically split hydrogen faster than boiloff, so some form of zero-boiloff will be needed.  I'd also expect you to purify/store the water, then only split it when a rocket is coming for  refueling (no realistic way of avoiding leakage).  I'd expect lunar Al/O combinations to happen first (unless we find an available comet or similar).

As far as "pinch fission" the problems include "building a 2km superconducting ring" and "expensive fuel isotopes".  I'm really wondering how this could be an advantage over a classical Orion (especially one based on H-bombs).

 

[tater]

Lunar AL hybrid rockets don't seem all that likely to me, but I'm not someone who thinks that you go to space to stay, then fall down a gravity well... guess I'm a belter, lol.

[regex]

3 hours ago, GreenWolf said:

Throwing my two cents in, since I've been doing a lot of number crunching and general thinking on this subject lately for a book.

I always try to convince myself that it's for fiction I might someday write but, at the end of the day, I just like to daydream about a future in space.

[tater]

http://dreamsofspace.blogspot.com

scroll down and look on the right for "Labels" Some of the 1950s and 1940s stuff is awesome.

Part of me wants a KSP total replacement as the early von Braun style progression.

[DAL59]

With 30-60 km/s of delta-vee, transits are still measured in months, but you can ignore launch windows for passenger ships.  A gas-core ship with a reasonable mass ratio of about 3 could achieve this.  Pulsed plasmoids would require a heavy generator and would be expensive, and low thrust(so it would take too long to spiral out of orbit), but might be useful for missions to the outer system.  Nuclear gas cores are pretty much the most efficient thing somewhat practical with modern tech(orion drives ignored.)  They are still very complex, due to heat management, keeping the uranium vortex contained inside the engine, ect.

This is the setting for a book I'm writing, so I'm a bit biased

 

[Bill Phil]

Maybe some kind of mini-mag orion? The fuel is not made of bombs (albeit high energy).

Probably something as simple as possible, though. I guess pressure fed chemical rockets...

Depends on the universe. And how easy/hard it is to get a license and a spaceship, as well as how difficult it is to fly stick...

[tater]

Solar thermal is actually competitive with NTRs. The one that was built and tested had an exhaust velocity of ~8000m/s. None of the drawbacks, either. If light levels are lower, you point multiple mirrors at one heat exchanger (so fewer engines running, but with higher velocity exhaust (better I_sp)).

 

 

[GreenWolf]

12 minutes ago, tater said:

Solar thermal is actually competitive with NTRs. The one that was built and tested had an exhaust velocity of ~8000m/s. None of the drawbacks, either. If light levels are lower, you point multiple mirrors at one heat exchanger (so fewer engines running, but with higher velocity exhaust (better I_sp)).

 

 

But it comes with its own drawbacks, such as thrust drop-off the further you get from the Sun, planetary occlusion (that's going to be a really big one if you intend to use it for planet-to-planet transfers), and a huge mirror/heat exchange apparatus that probably ends up massing more than a decently designed nuke + shadow shield.

Edited October 4, 2017 by GreenWolf

[tater]

1 minute ago, GreenWolf said:

But it comes with its own drawbacks (such as thrust drop-off the further you get from the Sun, planetary occlusion [that's going to be a really big one if you intend to use it for planet-to-plane transfers], and a huge mirror/heat exchange apparatus that probably ends up massing more than a decently designed nuke + shadow shield.

True, but it's worth consideration. I guess we'll know more when we fly an NTR and get more data on how well they function over the long haul (I know the Los Alamos guys thought they figured out the ablation issues). It's my understanding that the Marshall nuke people are pretty confident.

[MatterBeam]

8 hours ago, DAL59 said:

With 30-60 km/s of delta-vee, transits are still measured in months, but you can ignore launch windows for passenger ships.  A gas-core ship with a reasonable mass ratio of about 3 could achieve this.  Pulsed plasmoids would require a heavy generator and would be expensive, and low thrust(so it would take too long to spiral out of orbit), but might be useful for missions to the outer system.  Nuclear gas cores are pretty much the most efficient thing somewhat practical with modern tech(orion drives ignored.)  They are still very complex, due to heat management, keeping the uranium vortex contained inside the engine, ect.

This is the setting for a book I'm writing, so I'm a bit biased

 

Gas Core nuclear rockets are pretty awesome. At 2000s Isp, they outperform many electric rockets while providing good mount of thrust in a very high power density package - that doesn't need any radiators! If you give it radiators to absorb the waste heat absorbed by the pressure vessel, Isp in the order of 5000s or more is available. Some preliminary research has been done and physical models built to test some aspects of the reactor, such as handling the fuel particle vortex or how to extract power by bleeding off some of the plasma into an MHD chamber. Look at the 'Nuclear Gas Core rocket' papers on the Nasa Technical Reports Server, or at the RD-600 design.

However, I don't see this technology as being available to civilians. They won't benefit much from the high thrust and low mass when just loading up on more propellant is cheaper. There's also the fact that it consumes a lot of expensive fissionable fuels and it has 'explosive containment failure' as a failure mode. 

8 hours ago, tater said:

Solar thermal is actually competitive with NTRs. The one that was built and tested had an exhaust velocity of ~8000m/s. None of the drawbacks, either. If light levels are lower, you point multiple mirrors at one heat exchanger (so fewer engines running, but with higher velocity exhaust (better I_sp)).

Solar thermal rockets so far have focused on the very simple and robust heat exchanger design, where sunlight is used to heat a temperature-resistant metal like tungsten, then run propellant over the metal. 8km/s with hydrogen comes out to a temperature of 2565K, assuming complete dissociation. It is more likely that some hydrogen does not break down and the operating temperature is closer to 3000K. 
Advanced materials such as tantalum hafnium carbide can survive 4000K temperatures, pushing the maximum solar-thermal heat-exchanger design's exhaust velocity to 10km/s. 

I am working on a design that can achieve 12km/s using liquid droplets as the heat exchanger.

If your departure point is Earth, and you are heading to Mars, you'll have 100% of the thrust at the departure burn and 44% of the thrust at the insertion burn. Thankfully, your spaceship would have expended most of its propellant reserves near Earth so your effective acceleration still goes up.

8 hours ago, GreenWolf said:

But it comes with its own drawbacks, such as thrust drop-off the further you get from the Sun, planetary occlusion (that's going to be a really big one if you intend to use it for planet-to-planet transfers), and a huge mirror/heat exchange apparatus that probably ends up massing more than a decently designed nuke + shadow shield.

Huge mirrors that reflect solar wavelengths can be made very very lightweight. After all, the Sun is an easy target to track, so you don't need any of the adaptive optics that massively increase the kg/m^2 rating of the mirrors. 

In fact, if you really want to chase your mass savings, you can use the same designs and structures as solar sails, except that you're not bouncing away the sunlight where it came from but focusing it onto a collector on your spaceship. Solar sails can mass as low as 7g/m^2. A 5 kilogram sail of Mylar at 7g/m^2 can collect as much as a megawatt of solar power.

Edited October 4, 2017 by MatterBeam

[GreenWolf]

 

Huge mirrors that reflect solar wavelengths can be made very very lightweight. After all, the Sun is an easy target to track, so you don't need any of the adaptive optics that massively increase the kg/m^2 rating of the mirrors. 

In fact, if you really want to chase your mass savings, you can use the same designs and structures as solar sails, except that you're not bouncing away the sunlight where it came from but focusing it onto a collector on your spaceship. Solar sails can mass as low as 7g/m^2. A 5 kilogram sail of Mylar at 7g/m^2 can collect as much as a megawatt of solar power.

Mylar is a pretty terrible structural material though, and if you want your mirrors to hold up under acceleration, you'll need to reinforce them with stronger -- and heavier -- structural supports. Additionally, your mirrors will still be absorbing some thermal energy from the sun, and that has to be radiated away (separately and in addition to any waste heat generated by the ship's electrical systems) which will necessitate more mass in the form of radiators.

Also, that still doesn't address the planetary occlusion issue, which is a major problem if you want to insert into orbit -- you aren't going to be doing an aerocapture with a set of giant fragile wing mirrors attached, and orbital dynamics necessitates that the orbital insertion burn into a prograde orbit for a spacecraft traveling from Earth to Mars will happen on the night side of the planet. (You can, of course, get around this by not burning at closest approach, but this massively increases your delta-v requirements and forces you to do a lot of orbital maneuvering to get into the proper orbit -- and if any of that maneuvering requires you to perform a burn on the night side of the planet [which is almost a certainty] you're totally screwed.)

(Actually, it gets even worse than that, because I remember that the phase angle for Earth/Mars Hohmann transfers involves beginning your transfer burn on the night side of Earth too.)

Your solution at that point is to move the collector off the spacecraft and rely on beamed power/lasers, which is all well and good, but has its own issue in terms of infrastructure requirments and also doubling as an orbital superweapon.

Edit: Also, some figures for space nuclear reactors, just for reference. Most proposed/prototyped designs for spaceborne nuclear reactors (be it for propulsion or power) have a mass in the range of 0.5 to 1 metric ton for the core (unshielded). Adding a shadow shield adds another ton or so. 2 metric tons is not at all unreasonable for an engine that can get exhaust velocities of in excess of 8000 m/s and can function as a power plant in low-power mode. And doesn't go out whenever you get occluded by a planet.

Edited October 4, 2017 by GreenWolf

[MatterBeam]

 

Mylar is a pretty terrible structural material though, and if you want your mirrors to hold up under acceleration, you'll need to reinforce them with stronger -- and heavier -- structural supports. Additionally, your mirrors will still be absorbing some thermal energy from the sun, and that has to be radiated away (separately and in addition to any waste heat generated by the ship's electrical systems) which will necessitate more mass in the form of radiators.

Also, that still doesn't address the planetary occlusion issue, which is a major problem if you want to insert into orbit -- you aren't going to be doing an aerocapture with a set of giant fragile wing mirrors attached, and orbital dynamics necessitates that the orbital insertion burn into a prograde orbit for a spacecraft traveling from Earth to Mars will happen on the night side of the planet. (You can, of course, get around this by not burning at closest approach, but this massively increases your delta-v requirements and forces you to do a lot of orbital maneuvering to get into the proper orbit -- and if any of that maneuvering requires you to perform a burn on the night side of the planet [which is almost a certainty] you're totally screwed.)

(Actually, it gets even worse than that, because I remember that the phase angle for Earth/Mars Hohmann transfers involves beginning your transfer burn on the night side of Earth too.)

Your solution at that point is to move the collector off the spacecraft and rely on beamed power/lasers, which is all well and good, but has its own issue in terms of infrastructure requirments and also doubling as an orbital superweapon.

Edit: Also, some figures for space nuclear reactors, just for reference. Most proposed/prototyped designs for spaceborne nuclear reactors (be it for propulsion or power) have a mass in the range of 0.5 to 1 metric ton for the core (unshielded). Adding a shadow shield adds another ton or so. 2 metric tons is not at all unreasonable for an engine that can get exhaust velocities of in excess of 8000 m/s and can function as a power plant in low-power mode. And doesn't go out whenever you get occluded by a planet.

Well, the solar thermal rockets we are discussing probably accelerate at milligee rates, so structural support is of little concern. Being so thin, the Mylar can be held up by a combination of solar light pressure and centripetal force. Myar absorbs between 5 and 0.1% of the sunlight it receives, depending on the manufacturing quality. Even at 5%, it is absorbing  65W/m^2. If the anti-solar face is moderately black, with an emissivity of 0.8, it will reach equilibrium temperature when radiating at 194K. That is very cool!

Your perform burns at the periapsis until you break orbit, to make the most of the oberth effect. Make sure it is on the sunlit side! If you can't burn on the sunny side, raise your orbit first. This will increase your orbital period and how long you can stay in the sun. Going from LEO to GEO takes about 4km/s. Going from GEO to Mars costs another 2km/s. You can burn the 2km/s with continuous sunlight available.

Nuclear thermal technology is great, I agree. However, a lot of 'solutions' that are great on paper run head-on to political, social or economical problems. 

The OP requested a propulsion technology which you or might might have access to in the future. I am certain governments will not care if you're touting around large sheets of metal to focus sunlight, but might be bothered by the tens of kilograms of refined uranium you'll get with any solid core nuclear engine. 

[GreenWolf]

 

Well, the solar thermal rockets we are discussing probably accelerate at milligee rates, [...]

Then they aren't comparable to or competitive with NTRs (which was the initial context they were brought up in) since one of the biggest advantages of an NTR is that it gets significant thrust and specific impulse. Also, nothing with humans onboard is going to do Earth/Mars transfers at milligee accelerations, because it would take literal years.

 

Your perform burns at the periapsis until you break orbit, to make the most of the oberth effect. Make sure it is on the sunlit side! If you can't burn on the sunny side, raise your orbit first. This will increase your orbital period and how long you can stay in the sun. Going from LEO to GEO takes about 4km/s. Going from GEO to Mars costs another 2km/s. You can burn the 2km/s with continuous sunlight available.

Except that's not how orbital mechanics works. That 2 km/s figure is for a Hohmann transfer, which requires you to burn at a specific phase/ejection angle. But if you're burning on the sunward side of Earth, your ejection angle will be almost 180 degrees away from where it should be, which will require you to perform expensive correction maneuvers in deep space, which massively increases you transit time and delta-v requirements, effectively losing any propellant savings you might get from having a higher exhaust velocity.

You'll run into a similar problem at the other end when you reach Mars, essentially having to match velocities with the planet before you get into its SOI and then gradually getting closer, so that you can actually perform your insertion burn once you get into the SOI and not just fly off into space again. The long spiralling trajectory that the Dawn probe took out to Ceres is a good example of what your actual transfer would look like.

 

I am certain governments will not care if you're touting around large sheets of metal to focus sunlight, but might be bothered by the tens of kilograms of refined uranium you'll get with any solid core nuclear engine. 

There already exist a number of civilian sea-faring ships that are powered by nuclear reactors, and most nuclear power plants are operated by civilian contractors or companies. Considering the ease of tracking things in space, and the difficultly of launching things, it'd be pretty easy to regulate nuclear reactors in space.

Edited October 4, 2017 by GreenWolf

[DAL59]

A solar sail could be used for missions to the outer solar system.  The craft could slow its orbit so it falls towards the sun for the next few months, and then use the intense light and oberth effect to shoot it out into the outer system.  

 

Considering the ease of tracking things in space

TANSIS- There ain't no stealth in space.  

[tomf]

19 hours ago, GreenWolf said:

phase/ejection angle. But if you're burning on the sunward side of Earth, your ejection angle will be almost 180 degrees away from where it should be, which will require you to perform expensive correction maneuvers in deep space, which massively increases you transit time and delta-v requirements, effectively losing any propellant savings you might get from having a higher exhaust velocity.

You'll run into a similar problem at the other end when you reach Mars

There already exist a number of civilian sea-faring ships that are powered by nuclear reactors, and most nuclear power plants are operated by civilian contractors or companies. Considering the ease of tracking things in space, and the difficultly of launching things, it'd be pretty easy to regulate nuclear reactors in space.

Out at GEO the earths shadow only subtends an angle of 16.8 degrees. by splitting your burn either side of the darkness you can get pretty close. Of course from out there you aren't getting the hohmann effect but a high enough ISP could make up for that. I recon you will need 24.km/s more than for a direct  transfer from low orbit, the ejection burn is 1500 less from geo but you need 2500 to circularise at GEO.

Also when you are capturing at a superior planet for a prograde orbit you are going to be capturing on the sunny side. At the ap of your transfer you are going slower than your target so falling into it's SOI from in front so the prograde orbit is going to have it's pe a little after the noon-point on the planet.

I don't know about the performance of solar thermal propulsion but Dawn ,that had to do slow spirals, had about 80 micro-gs of acceleration. If MatterBeam is looking at milligs that is a pretty significant difference.

Edited October 5, 2017 by tomf
I did the maths

[MatterBeam]

21 hours ago, GreenWolf said:

Then they aren't comparable to or competitive with NTRs (which was the initial context they were brought up in) since one of the biggest advantages of an NTR is that it gets significant thrust and specific impulse. Also, nothing with humans onboard is going to do Earth/Mars transfers at milligee accelerations, because it would take literal years.

Except that's not how orbital mechanics works. That 2 km/s figure is for a Hohmann transfer, which requires you to burn at a specific phase/ejection angle. But if you're burning on the sunward side of Earth, your ejection angle will be almost 180 degrees away from where it should be, which will require you to perform expensive correction maneuvers in deep space, which massively increases you transit time and delta-v requirements, effectively losing any propellant savings you might get from having a higher exhaust velocity.

You'll run into a similar problem at the other end when you reach Mars, essentially having to match velocities with the planet before you get into its SOI and then gradually getting closer, so that you can actually perform your insertion burn once you get into the SOI and not just fly off into space again. The long spiralling trajectory that the Dawn probe took out to Ceres is a good example of what your actual transfer would look like.

There already exist a number of civilian sea-faring ships that are powered by nuclear reactors, and most nuclear power plants are operated by civilian contractors or companies. Considering the ease of tracking things in space, and the difficultly of launching things, it'd be pretty easy to regulate nuclear reactors in space.

A more useful measure is power to weight ratio. W/kg. Solar thermal rockets can achieve higher W/kg ratings that solid core nuclear rockets. Since space systems in the forseeable future will be restricted mostly by how much mass you can put into orbit, you'll be comparing mass budgets for the solar thermal and NTR propulsion systems. If we use W/kg, we can directly compare the two propulsion systems. This Solar Moth concept, designed using 1979's materials and technology, achieves 167kW/kg. Modern materials, such as using ultra-thin Mylar instead of aluminium sheets, or carbon fibre struts and THC heat exchanger instead of tungsten, might improve this figure by quite a bit. Using this figure, a Solar Moth of 10 tons would produce anything between 378kN@900s to 1702kN@200s. At the highest specific impulse, it can push 3856 tons at 0.01g. That's ten milligees. 

A bit more maths on the acceleration time of a 3856 tons craft with a 0.01g initial acceleration:
3856 tons contains 10 tons propulsion. If we assume a mass ratio of 4, deltaV is 12.2km/s. Let's assume 6.1km/s is spent on the departure burn. Propellant mass is 2892 tons and dry mass is 964 tons. Propulsion fraction is 1%, which is an absolutely tiny engine on a very big rocket. Anyways, mass ratio for a deltaV of 6.1km/s is  1.995, meaning 959 tons of propellant. In other words the initial burn consumes 1932tons of propellant, and the average mass during departure is 2890tons. 

Average departure during acceleration is 0.13m/s^2 (0.0133g). The 6.1kms burn is completed over the course of 13 hours. A lot, but it's just over half of one GEO orbital period.

If we have a propulsion fraction of 10%, which is much more reasonable, then the craft will produce 3780kN and accelerate at 1.3m/s, completing the burn in just under an hour and a half! Not bad!

Now, all we have to do is to find figures for nuclear thermal rockets of similar kW/kg ratings. Running all the maths again will give you very similar results, as the maximum Isp of the NTR is pretty much the same as for the solar thermal rocket. Since we are using kW/kg, is is scalable to craft of any size. If sending 964 tons to Mars seems unreasonable, you can just reduce it by a factor 100 (9.64 tons) and get the same results. 

Be careful however of citing reports on NTRs from the 60s, as that was before any physical testing was performed. The Reusable Nuclear Shuttle designs suggested power to weight ratios of about 212kW/kg before radiation shielding is considered, so we're actually not far from the performance of solar thermal rockets.

The departure burn can be done on the night side of Earth, because Earth's shadow at that altitude only covers a tiny portion of the orbit. if you enter a GEO orbit other than over the Equator, you might not intersect Earth's shadow at all. 

 

[wumpus]

On 10/4/2017 at 1:19 PM, GreenWolf said:

Mylar is a pretty terrible structural material though, and if you want your mirrors to hold up under acceleration, you'll need to reinforce them with stronger -- and heavier -- structural supports. Additionally, your mirrors will still be absorbing some thermal energy from the sun, and that has to be radiated away (separately and in addition to any waste heat generated by the ship's electrical systems) which will necessitate more mass in the form of radiators.

The level of acceleration is going to be tiny (although far more than slows down the ISS), so it isn't likely to be an issue at all for mylar mirrors.  Presumably you will need similar (possibly dyed black) mylar radiators facing away from the sun as radiators.  No idea how well they conduct heat (is mylar used as ESD packaging material?  That would put it as "low, but not quite an insulator").

Don't underestimate just how little mass you can get away with for a space-assembled structure.  Stretching out the acceleration is always an option (Dawn used micro-gees of acceleration).

- Note: if this is too slow for you, you can always use the solar sails to deliver chemical fuel/oxidizer.  Use micro-g acceleration for cargo and high power for humans.

[regex]

23 hours ago, MatterBeam said:

The OP requested a propulsion technology which you or might might have access to in the future.

Well, it was more about your thoughts on the matter since I have my own "future canon". I like hearing competing ideas or reasons for things ending up the way they might, it not only helps me refine my own daydreaming but expands my knowledge. I personally think there are countless ways that humanity might expand into the greater solar system, as well as countless ways they might not, but there are usually very good reasons why we will likely never see certain scenarios (AKA libertarians or anarchists or "self-made men" in space). Hence this thread about "everyday" space drives. Whether you postulate a future where the individual can stick an electric drive and a few solar panels behind an old Bigelow and go for a ride or whether you believe all that would be strictly regulated, I think there's some good discussion material there. As evidenced by the thread.

Edited October 5, 2017 by regex

[ChrisSpace]

I should point out something (sorry for taking so long btw)

 

If you have a "rocketpunk" world where individuals can own personal spacecraft, I imagine you probably already have plenty of infrastructure (Mass Drivers, Launch Loops, Sky Hooks, Space Elevators, Orbital Rings etc etc) to launch almost anything into space without the need for any thrust from the vehicle itself. So the maximum TWR for such a vessel wouldn't need to be above around 1 (although reducing inefficiencies from the gravity of Mars/Venus would require the TWR to be higher). As for Dv, refer to this map:

'

If there are propellant stations in LEO, LLO, LMO and the L-points (which there would be), a Dv of a bit under 5km/s is enough to get anywhere. So stuff like Ion drives and Plasma thrusters wouldn't really be necessary. A solid-core NTR using Methane propellant would only need a mass ratio of about 2.21.

Edited November 9, 2017 by ChrisSpace