What propulsion system should we use for Mars exploration?

[Albert VDS]

10 hours ago, Spaceception said:

We all know chemical propulsion isn't viable for our first Mars missions, too slow, too inefficient, and too primitive, so what propulsion system should we use?

You don't need to go fast to get to Mars, because that means you are favoring propellant over supplies and you can't get a free-return trajectory.
Which every you choose you want that free-return trajectory.
 

[Streetwind]

NERVAs are definitely a bad choice. I've seen the latest Mars Design Reference Architecture - the tank dry masses for zero boiloff hydrogen storage kills it hard. The only reason it is even considered is because it can be done in fewer launches, saving money... but, you'd have to spend more than you save through that for restarting the the NERVA program. So basically, NTRs are a dud.

What I'm really curious about is the fusion driven rocket. It kind of hinges on solar panel technology scaling up as predicted over the next ten years, but then again, it does have the unique property of not varying its specific impulse with input power like other electric engines would. The only thing that varies with input power is thrust, while Isp remains constant at about 3,000s (projected). I just wonder how much thrust per kW input power it can actually develop. I sure hope it's higher than the roughly 0.05 N/kW that modern electric engines can get at that Isp... Because if it doesn't significantly exceed that, then it's more or less a dead-end technology.

[HebaruSan]

4 hours ago, mikegarrison said:

Why not put Crazy Drive -- I mean, Pulsed Nuclear Detonation -- on the list?

This may be a dumb question, but can anyone speak to the technical feasibility of a role for single-burn nuclear pulse engines? I.e., send it high enough that no one will care, then drop one H-bomb shaped charge out the back and detonate it to get the dV to Mars all in one burst, and then drop the pusher plate. Does the pusher plate have to be too heavy for that to be worth it?

[Temstar]

2 hours ago, HebaruSan said:

This may be a dumb question, but can anyone speak to the technical feasibility of a role for single-burn nuclear pulse engines? I.e., send it high enough that no one will care, then drop one H-bomb shaped charge out the back and detonate it to get the dV to Mars all in one burst, and then drop the pusher plate. Does the pusher plate have to be too heavy for that to be worth it?

The maximum a ship can take is about 10m/s per detonation. Above that the ship will break apart.

Not to mention the people inside will be jelly.

[davidy12]

5 hours ago, Albert VDS said:

You don't need to go fast to get to Mars, because that means you are favoring propellant over supplies and you can't get a free-return trajectory.
Which every you choose you want that free-return trajectory.
 

I agree. People argue, if something goes wrong than they'd slowly die, well than what are you going to do, even if you had VASMIR or NERVA rockets, it won't make a difference. I still think it should be either NERVAs or Ion Propulsion to try and get there. To double the payload, not get there faster. (IF ONLY I COULD REPLICATE IT IN KSP)

4 hours ago, Streetwind said:

NERVAs are definitely a bad choice. I've seen the latest Mars Design Reference Architecture - the tank dry masses for zero boiloff hydrogen storage kills it hard. The only reason it is even considered is because it can be done in fewer launches, saving money... but, you'd have to spend more than you save through that for restarting the the NERVA program. So basically, NTRs are a dud.

What I'm really curious about is the fusion driven rocket. It kind of hinges on solar panel technology scaling up as predicted over the next ten years, but then again, it does have the unique property of not varying its specific impulse with input power like other electric engines would. The only thing that varies with input power is thrust, while Isp remains constant at about 3,000s (projected). I just wonder how much thrust per kW input power it can actually develop. I sure hope it's higher than the roughly 0.05 N/kW that modern electric engines can get at that Isp... Because if it doesn't significantly exceed that, then it's more or less a dead-end technology.

Isn't there a way to keep the LH2 cold (with insulation) to try and prevent it from boiling off? If that's not the case, than use Ion propulsion.

[mikegarrison]

For an Orion-type pulsed nuclear detonation ship, huge mass is actually an advantage. It damps out the peak accelerations from the detonations.

And dropping the pusher plate wouldn't make much sense. You have already accelerated it, so it's coming with you anyway whether it is attached or not. And what are you going to do when you get there? You are going to need to slow down again.

Also, if you keep that pusher plate between you and the sun, it will go a long way to shielding you from flares.

One of the big problems, though, is if we are not willing to do a direct-from-ground nuclear takeoff then it's going to be very, very painful getting that huge pusher mass into orbit.

[Spaceception]

Everyone who wants me to put the "Orion" system on the poll, or is talking about it, please stop. While I do think the Orion is awesome, I don't think we'll use it unless it's our last resort, hopefully, I'll be wrong someday, and we'll use on a "normal" mission, but for now it's not happening, there are much safer ways of doing the same thing.

[Streetwind]

40 minutes ago, davidy12 said:

Isn't there a way to keep the LH2 cold (with insulation) to try and prevent it from boiling off? If that's not the case, than use Ion propulsion.

Yes. It's called active cryocooling. It requires that you lug around coolant, cryopumps, radiators, and solar panels to run it all for the entire trip. Additionally, you need to prevent hydrogen from simply drifting through tank walls, an activity which hydrogen greatly enjoys doing, and thus requires a significant effort to prevent. And all that combined makes LH2 tankage so heavy that a NERVA-style engine struggles to exceed the dV of a classic hypergolic storable propellant rocket.

(For an idea of how that feels, try using a LV-N in KSP with only LF/Ox tanks, while also leaving roughly a third of the oxidizer still in the tank. Now the LV-N will perform roughly in the manner a real NERVA would perform with actively cryocooled zero-boiloff-tanks. Now compare that to simply re-adding the oxidizer and mounting a Terrier instead...)

The only reason why it would still be advantageous to use a NERVA-style engine is because the total vessel weight is lower, meaning you need less launches to lift everything into orbit, which saves money. That is literally all the advantage there is. Unfortunately though, you would need large sums of money to actually develop such an engine, so... welp.

Hypergolics > NERVA to Mars.

(Of course, it has been theorized that fully matured nuclear thermal rockets could significantly exceed the performance of the NERVA project. In such a case, a NTR would make sense for Mars trips. But it would take even more of an investment in time and money than to simply finish the half-complete NERVA.)

Edited January 21, 2016 by Streetwind

[magnemoe]

16 hours ago, SargeRho said:

Well, the MSNW Fusion Rocket solves the problem of not being able to produce power by ignoring it, using an external energy source, instead of the fusion reaction, to power itself. We've had working fusion reactors for decades, just none that produce any energy. Several components of that particular engine have already been tested.
 

Z-pinch might also work for this. Engine is diferent but both are pulsed and require power from solar or an reactor. 
Both are easier to ground test for long burn than nerva. 

[wumpus]

Pros/Cons of various stuff.

Ion drives (most electrics here):

Pro - great ISP, will deliver anything with minimal extra fuel schlepped up Earth's gravity well.  Bigger Pro: actually exists and used twice in space.

Cons - slow.  Do not expect them to deliver humans (delivering chemical/hybrid rockets to deliver humans is another story).  Will not escape Mars's gravity well.  Might have trouble with Mars capture.  Uses Xenon (but since you would need a more powerful system anyway, it should be possible to use argon instead (argon is less efficient, but more common than CO2 on Earth).

Other electrics:

Pro - great ISP.  Much faster, might even deliver humans.

Cons - don't exist yet.  Delivery of humans probably requires solar arrays significantly larger than ISS has, and even larger cooling arrays.

 

Chemical

Pro - actually exists.  If combined with electrical can overcome most of electrical's problems (this may require hybrid solid/liquid rockets for long-term storage in outer space).   Note that current plans include mining the oxygen (the heavy part) from the Mars atmosphere for Mars liftoff (H2 delivery will be hairy.  Any to-be-designed zero-boiloff system *will*not*work* for the H2 sent into re-entry for liftoff.  You will have a tiny launch window from sending the stuff down to launching the return home rocket, and nearly all the H2 will be boiled off in the process.  Using RP1 might make more sense by the time the whole system is designed).

Cons - massive to the point of failure.  Note that storing hydrogen for the return trip may be unfeasible (nobody has yet designed a working zero-boiloff system).

Nuclear:

Pro: great ISP

Cons: Political boogy-man makes this a non-starter (media reports of protests over RTGs exist, NERVAs would be much worse).  H2 issues also apply (see chemical).

[fredinno]

2 hours ago, wumpus said:

(nobody has yet designed a working zero-boiloff system).

You could build a JWST-derived sunshade to have a zero boiloff system.

[Elthy]

Is there data about the mass of modern solar power arrays in space? Like how much power can you pack in e.g. 100t?

[Ackoli]

22 hours ago, Spaceception said:

We all know chemical propulsion isn't viable for our first Mars missions, too slow, too inefficient, and too primitive, so what propulsion system should we use?

I think most people have voted for chemical propulsion

5 hours ago, wumpus said:

 

Cons: Political boogy-man makes this a non-starter (media reports of protests over RTGs exist, NERVAs would be much worse).  H2 issues also apply (see chemical).

I don't think people know that nuclear reactors and RTG's don't actually pollute, unless in the (very) rare case of a nuclear meltdown.

[tater]

Most people are not terribly smart, frankly.

As I said someplace above, solar thermal is also an interesting option, though in the near term, I'd assume chemical makes more sense.

[Nothalogh]

On 1/20/2016 at 6:41 PM, Kaos said:

I do not consider chemical propulsion too inefficient. Some month can be waited and the technology exists. The other methods are only a bit faster, if at all. But rely on stuff that does not exists.

We could build a fleet of Orions in 10 years from the word go. We'd of course have to disregard the lilac scented feelings of all the handwringers, though

[shynung]

On 1/21/2016 at 10:47 PM, Streetwind said:

Hypergolics > NERVA to Mars.

While this might be true to some respect, I'd say to Hell with the hypergolics. That thing's so nasty on the anatomy, it should've been limited to unmanned ships only. If it ever goes into the cabin, we'd have another ASTP disaster on our hands. Definitely something not desirable to happen on a multi-month trip to Mars.

If chemicals are our best bet, methane/LOX is my best bet. I_sp close to LH₂/LOX, but with none of the disadvantages of the latter; CH₄ is dense, and it will stay put in the tank. Still cryogenic, but much less so than LH₂. Plus, Mars has plenty of CO₂ in the atmosphere, so ISRU is possible.

Edited January 26, 2016 by shynung

[Alias72]

Another possibility is electric propulsion nuclear generation. Basically a submarine class reactor powering an ion-type array. The power output would partially compensate for the low thrust of current Ion engines (brute force). A constant burn trajectory allows for lower thrust options. Finally the thrust section could be re-usable. A series of missions could use the same reactor, thruster section.

[shynung]

That's actually not a bad idea.

The US Ohio-class nuclear ballistic submarines have a single S8G nuclear reactor, which produces 220 MWth and weighs 2750 tons. Assuming the same power ratio applies to a 10 ton reactor, we get an 800 kWth reactor. Assuming a 50% thermo-electric conversion efficiency, that's 400 kWe, just enough to run 2 of Ad Astra's VX-200 VASIMR thrusters.

Now, the question is whether we get such a reactor ready within a few decades. While I'm optimistic from the tech side, I doubt that the general public can simply agree to the idea. At least, I hope they're indifferent.

[Temstar]

1 hour ago, shynung said:

Assuming the same power ratio applies to a 10 ton reactor, we get an 800 kWth reactor

We can probably do better than that since marine reactors are designed to be compact with not much emphasis on weight. Space reactor need to be lightweight first and foremost. One easy way to get this is to use a shadow shield instead of shielding all the way around as with a marine reactor.

Edited January 26, 2016 by Temstar

[Elthy]

You cant compare submarine reactors to those in space. Submarines have unlimited supply of cooling-material to condense the steam form the reactors, im not sure how you would do that (in those scales) in space. You would need gigant raiators, maybe to the point where you could simply use solar power instead.

[magnemoe]

On 21.1.2016 at 11:28 PM, Ackoli said:

On 21.1.2016 at 6:08 PM, wumpus said:

Cons: Political boogy-man makes this a non-starter (media reports of protests over RTGs exist, NERVAs would be much worse).  H2 issues also apply (see chemical).

 

Someone will always protest against anything. Media loves this even if its just an small group of nuts.
Its totally irrelevant as long as its not large groups or they have good political connections. 

That being said, NERVA has issues with testing as the exhaust can have radioactive fragments and you need to test it multiple times for the full burn. 
Something like vasmir with an nuclear reactor is easier as you can test each part individually. 

[Andem]

Chemical. the most expensive to get int orbit, but undoubtedly the logical solution. they have enough power to make a direct mars transfer burn so you aren't wasting fuel on escaping earths' SOI.

[Sanic]

Chemicals FTW!

Heh heh... unsymmetrical dimethylhydrazine.

Or boring A50 and NTO.

Edited January 26, 2016 by Sanic

[shynung]

7 hours ago, Elthy said:

You cant compare submarine reactors to those in space. Submarines have unlimited supply of cooling-material to condense the steam form the reactors, im not sure how you would do that (in those scales) in space. You would need gigant raiators, maybe to the point where you could simply use solar power instead.

Right. Radiator mass need to be considered, too. Forgot to account that. 

Except for nuclear thermal rockets. These don't need radiators, because the heat is used directly on the propellant. It runs like an open cooling system, with the propellant doing double duty as the coolant.

[Albert VDS]

2 hours ago, Andem said:

Chemical. the most expensive to get int orbit, but undoubtedly the logical solution. they have enough power to make a direct mars transfer burn so you aren't wasting fuel on escaping earths' SOI.

Sure it can put the spacecraft into an escape trajectory faster than anything else, but it doesn't matter if you do it in a few minutes, hours or even days.
Anything more efficient than chemical rockets, that can put the craft in to an escape trajectory with the same travel time, is a better choice.
Mainly because it would mean more payload can be used for supplies(food, spareparts, etc).