Practical propulsion methods for manned interplanetary travel

[Top8]

Take a look at microwave sails - beam riders.

[SargeRho]

Was the NERVA capable of using 235 though? I thought it was just hot plutonium like an RTG.

No, NERVA uses an actual nuclear reactor very similar to the ones used in nuclear powerplants, not decaying plutonium.

[Roastduck]

Take a look at microwave sails - beam riders.

Still need a way to slow down at the destination, which will require a local source of thrust like a chemical rocket

[SargeRho]

Or aerocapture with a subsequent periapse raise, which doesn't require a whole lot of fuel from what I know.

[Nuke]

If you have the capability to produce enough antimatter to power vasimr, you don't need vasimr anymore

using antimatter in a power system would likely use significantly less antimatter than using it for propulsion. you wouldn't use vasimr in that case, you would have the power supply to use much better engines than vasmir. like mpd thruster arrays. but you are right if you can carry antimatter, you might as well go big.

[Soda Popinski]

As to NERVAs, the general population seems fine with the RTGs that went on Curiosity. I'm not sure if the layperson even knows the difference between an RTG and a fission based reactor (a friend of mine was worried about radioactive materials from their broken microwave).

It might be possible politically.

Wouldn't be needed for Mars, as standard chemical fuel could get us there with enough time. Pushing it for asteroids, and I would think the outer planets would definitely need something better than chemical.

[SargeRho]

Near Earth Asteroids are sometimes easier to reach than the Moon

[Simon Ross]

In a direct answer to the OP's challenge, it simply cannot be done with the chemical technology available to use today and the restrictions limiting the development of nuclear power sources in the immediate future.

And yes, it's a pretty depressing outlook from someone who was around to watch the original moon landing :-(

[Nuke]

either need more money or some technology that doesn't exist yet or a rapid change in people's opinions about nuclear. even then its iffy.

[Winter Man]

No, NERVA uses an actual nuclear reactor very similar to the ones used in nuclear powerplants, not decaying plutonium.

Then why not just pass xenon over a hot radioisotope? A mass issue perhaps? There's nothing wrong with launching RTGs (which are a fair deal more dangerous than NERVAs, but hippies getting in the way, etc.) so why not this?

[Ralathon]

Then why not just pass xenon over a hot radioisotope? A mass issue perhaps? There's nothing wrong with launching RTGs (which are a fair deal more dangerous than NERVAs, but hippies getting in the way, etc.) so why not this?

The problem is power generation. As the xenon (or other reaction mass) passes over your radioisotope it'll transfer heat. The xenon heats up and your fuel cools down. This is exactly what you want to happen. But the higher the temperature the better your Isp, so if the fuel doesn't generate enough heat to keep up with the heat stolen by the xenon your Isp will suffer.

RTG's don't generate enough heat to keep up at any reasonable thrust level. So this means you have to either use very low thrust burns (in which case you're better off with an electric engine) or your ISP will be horrible (As in, so horrible you might as well use chemical rockets).

Not to mention that the fuel for a RTG is a variety of plutonium that's rather rare. There's only a few dozen kg in the world, and nobody wants to make more of the stuff. You also can't switch an RTG off to conserve fuel and keep the reactor from overheating, so you need to attach some sort of cooling system that adds extra mass.

If you want a NERVA you simply need the high energy output of a nuclear reactor to make it work. Furthermore, it has to be a nuclear reactor running on weapons grade uranium to achieve the best efficiency. As you might imagine that's a bit scary to the politicians and the general public.

[Winter Man]

True, it is far simpler. A hybrid fusion/fission reactor might be best for this application then. Non-breakeven fusion reactor to produce a ton of neutrons (a Farnsworth or Polywell, why not) surrounded with a sub critical thorium (assuming it's all converted to U-233) shell so it can be controlled with the flick of a switch. Or turn of a dial, as it may be.

[crazyewok]

If you have a Mars colony by 2038 then why not build a Nuclear pulse ship there and use it? No environmental issues or political then. Though by that time we may very well have fussion drives anyway.

[crazyewok]

In a direct answer to the OP's challenge, it simply cannot be done with the chemical technology available to use today and the restrictions limiting the development of nuclear power sources in the immediate future.

And yes, it's a pretty depressing outlook from someone who was around to watch the original moon landing :-(

I agree politics has done a lot to slow space exploration and exploitation down.

Still Mars is doable with chems.

If we can get a base on the moon and/or mars it opens up the nuclear option a lot more as you cant pollute a already toxic landscape.

Unless the restictions you place are lifted though OP or Fusion drives come along I will say anything futher than mars is a no go for manned missions.

Edited January 28, 2014 by crazyewok

[Idobox]

Nuclear is the way to go.

NERVA is an obvious option, Orion is technically illegal, and salt-water rockets are a completely unproven technology, that can relatively easily be weaponized (one of the big question is how to make run rather than explode).

Nerva is, in my opinion, the only one of the three that has any chance of being developed by a western country in the near/mid future. I'm not sure about China, but I think their opinion wouldn't care too much if they developed salt-water, and other countries wouldn't have the balls to stop them. It might even help the west to overcome political limits (evil communists that aren't even white are building nuclear rockets, we need to build bigger ones).

I don't know much about fusion rockets, but anything based on fusors, polywell or lasers will have a tiny power to mass ratio, and will be more comparable to an ion engine than to a chemical rocket.

[crazyewok]

Nuclear is the way to go.

NERVA is an obvious option, Orion is technically illegal, and salt-water rockets are a completely unproven technology, that can relatively easily be weaponized (one of the big question is how to make run rather than explode).

Nerva is, in my opinion, the only one of the three that has any chance of being developed by a western country in the near/mid future. I'm not sure about China, but I think their opinion wouldn't care too much if they developed salt-water, and other countries wouldn't have the balls to stop them. It might even help the west to overcome political limits (evil communists that aren't even white are building nuclear rockets, we need to build bigger ones).

I don't know much about fusion rockets, but anything based on fusors, polywell or lasers will have a tiny power to mass ratio, and will be more comparable to an ion engine than to a chemical rocket.

NERVA would be of use to mars, though may be cheaper to just use chems but beyond mars?

Beyond mars you would have to look into building ships on moon or mars as you would have alot more leyway in what you can do, politcaly, enviromentaly and engineering wise.

[shynung]

Two years ago, on August 6, a 900kg rover the size of a car, named Curiosity, landed on the surface of Mars. This fact alone proves than we, as a civilization, are able to travel to Mars on current technology. However, to get there, it had to travel over a period of approximately 9 months (it was launched on November 26, the year before).

The problem of conducting manned missions over this sort of time distance is not technical, but logistical. How do we feed and take care of astronauts/cosmonauts, inside a (typically) cramped spacecraft, isolated from the rest of humanity by space and time? Even more, how do we ensure they do not kill themselves, or otherwise render themselves inert, before their mission is complete?

Regardless of our solutions, what are the ethics of conducting this sort of missions?

More advanced propulsion systems may drastically shorten the time taken for such missions, but they have their own problems. As mentioned by countless posts above (and in other threads), nuclear-powered systems are not politically viable solutions. This is a problem, because nuclear fuels are one of the densest forms of energy storage accessible to mankind. Any alternative electric propulsion systems that promises comparable performance have power requirements so high, you might as well bring nuclear-powered generators in the first place.

In short, we have the technological means to carry manned missions to Mars by 2025, with the catch that it will be a very long trip. Some people (like me) would respond by building spacecrafts the size of skyscrapers, carrying enough supplies for such a long trip, and back again. Only if political holds are released, are we able to build more powerful and exotic propulsion systems, up to and including the ever-infamous nuclear pulse propulsion. By then, the time distance to Mars would be measured by weeks (or even days), rather than months.

[Soda Popinski]

9 months is not that long for the trip. Look at folks on the ISS. A number of astronauts and cosmonauts have done that. I don't think you would need a skyscraper sized ship to do it (you don't need that much tonnage of supplies).

Something to consider, supply wise, is the fact they need to stay on Mars for about 500 days until the next Earth return window (assuming a return trip). That means supplies for the trip there, trip back, and 500 day mission.

NASA already has several design proposals for a Manned Mars mission using basically today's technology.

Here's the latest one from 2007, using the Orion CEV + Ares I, and Ares V launch vehicles. It involves 7 launches,

Quick background on NASA's Mars Design Reference 5

90 page PDF

380 page PDF

Edit: Put Atlas, when I meant Ares.

Edited February 5, 2014 by Soda Popinski

[kiwiak]

9 months is not that long for the trip. Look at folks on the ISS. A number of astronauts and cosmonauts have done that. I don't think you would need a skyscraper sized ship to do it (you don't need that much tonnage of supplies).

I think you got a point, but with deep space mission situation somewhat complicates.

Astronauts cant communicate with earth in real life, and know that if something goes wrong, they are goners.

If iss had some critical failure they all can just board soyuzs.

[Neotician]

MPD Thrusters.

They have a crazy high specific impulse and a potential thrust of up to 200N

Here's the result from a Russian prototype test:

Organization Power (kW) Current (kA) Speciï¬Âc Imp. (s) Efï¬Âciency (%)

NIITP 300–1000 6–15 3500–5000 40–60

As for the power source, LFTRs (Liquid-Flouride Thorium Reactors), They're more compact (and much more flexible in size), more efficient and safer than conventional nuclear reactors, they also produce (MUCH) less waste.

In short, we have the technological means to carry manned missions to Mars by 2025, with the catch that it will be a very long trip. Some people (like me) would respond by building spacecrafts the size of skyscrapers, carrying enough supplies for such a long trip, and back again. Only if political holds are released, are we able to build more powerful and exotic propulsion systems, up to and including the ever-infamous nuclear pulse propulsion. By then, the time distance to Mars would be measured by weeks (or even days), rather than months.

Well, getting to mars in say, 2 weeks, assuming going in a straight line and closest approach, you'd need to go about 48 km/s on average, that's (very) roughly 80 km/s of deltaV needed ( given that we're not startin from nor ending at 0 m/s velocities).

And i'm neglecting a lot of details increasing this number, like the fact that you'd have to go above the 48 km/s to compensate for time needed to accelerate and decelerate, gravity (although that works both ways) and more.

Given, i'm also neglecting factors reducing that number, like gravity(again), aero breaking, slingshot maneauvers etc.

Point being: I think a trip to mars lasting only days, is very far off

Edited January 28, 2014 by Neotician

[sgt_flyer]

The problem with nuclear reactors for powering spacecrafts would be the cooling systems in vacuum, it's very hard to dissipate thermal energy. (The case of NTRs is different, as you 'expel' this thermal energy in the fuel.)

[Soda Popinski]

Looking through NASA's 90 page Mars Design Reference 5, they propose either using Nuclear Thermal Rockets, or a combination of chemical rockets with aerobraking. I guess it would be a bad idea to have aerobraking with an NTR, in case of...bad things happening.

[Top8]

One thing that should be noted about those refrence desgins is they were very conservative. They are probably not indicative of what will happen with commercial space operations that are forced to innovate. Not to say they don't apply - much of the science will not change - but the technology will.

[crazyewok]

Looking through NASA's 90 page Mars Design Reference 5, they propose either using Nuclear Thermal Rockets, or a combination of chemical rockets with aerobraking. I guess it would be a bad idea to have aerobraking with an NTR, in case of...bad things happening.

There not much of a issue with areobraking over mars. Who cares if a Nuclear reactor or hell a million A bombs go off on mars its already a toxic radiation scorched place.

[mdatspace]

I weighed down nuclear propulsion with real political and environmental problems so it would not be the easiest way out. Anybody who has used an LV-N in KSP knows why NTR is a good idea for interplanetary travel.

As much as I like NERVA, I must say chemical propulsion is still our best friend.

Edited January 28, 2014 by mdatspace