Mars manned exploration missions - logic behind a Hermes style vehicle?

[DerekL1963]

3 hours ago, Temstar said:

Notice how the Hermes on DA by francisdrakex had these funny looking radiators arranged in a triangular pattern? That's so they all remain in the radiation shadow cast by the shadow shield.

You see that in the design of the JimO probe as well...   If the radiators extend beyond the shadow, they can reflect radiation from the reactor back into the shadowed region.

[Nibb31]

10 hours ago, fredinno said:

Well, the rotating section kind of serves the purpose of mitigating the effects of Zero-G.

You don't say !

It certainly serves the purpose of avoiding using special effects to simulate zero-g when filming the crew.

In real life, I doubt we will ever see rotating sections on spacecraft. They wlll either deal with zero-g through medical methods or spin up the entire spaceship using a barbecue roll or a tether.

[fredinno]

28 minutes ago, Nibb31 said:

You don't say !

It certainly serves the purpose of avoiding using special effects to simulate zero-g when filming the crew.

In real life, I doubt we will ever see rotating sections on spacecraft. They wlll either deal with zero-g through medical methods or spin up the entire spaceship using a barbecue roll or a tether.

Why not? There was the Nautilus-X proposal showing you can make these things small and inflatable, reducing launch mass and size considerably. You'd likely need one to test artif-G on a reasearch station (something NASA has deleted from the ISS, sadly) and it's likely easier than having to deal with the structural modifications one would have to make to make a ship spinnable, which also increase mass. Not to mention you'd have to carry a lot of fuel to de-spin for each mid-course burn.

Edited February 2, 2016 by fredinno

[lugge]

13 hours ago, benjee10 said:

Well, I didn't watch the movie but I read the book.

Andy Weir did a good job, I only had a few moments thinking "not possible in RL".

There were also mentioned some mission details, like how they descended from Hermes to Mars (can't remember, but imho using an one-way-lander). I also cannot remember what the RTG was good for, but after they used it right at mission begin, they buried it far away from the base. However, Mark recovered it.

The ascending module and the supplies were indeed brought by unmanned missions prior to the crewed missions. Thats why Mark has an ascending module waiting for him a few thousand kilometers away.

Andy Weir did good research. He also provided actual numbers. You can read how Mark calculated the numbers of calories he gains from potatoes and the amount of water he gets in his "lab", burning fuel.

I didn't check the numbers but they seemed OK.

 

[Nibb31]

7 minutes ago, fredinno said:

Why not? 

Because reasons.

Nautilus-X was a low-badget paper study without any actual engineering. It was as close to a real interplanetary spacecraft as the early Apollo designs looked like the actual Apollo, or the early Shuttle concepts ressembled the real Shuttle. 

[magnemoe]

7 hours ago, SargeRho said:

A VASIMR mission to mars would probably follow a brachistochrone trajectory, but the Hermes uses Xenon Ion Engines, not VASIMR engines.

I think this kind of design would have made more sense for the Hermes, though: http://www.deviantart.com/art/Hermes-Rotate-564814706

I think this is more like they described it in the book. Hollywood wanted an more sexy spaceship. 
Putting the engine at center and radial was smart as you can burn while rotate. 
Also why is he burning prograde at mars? Going home I guess. 

[_Augustus_]

Why is everyone saying it parks in LEO? It parks at EML2, this saves fuel. And why else would Orion be needed to get to it?

[fredinno]

55 minutes ago, _Augustus_ said:

Why is everyone saying it parks in LEO? It parks at EML2, this saves fuel. And why else would Orion be needed to get to it?

if it does it in LEO, you can save fuel via the oberth effect.

[benjee10]

It almost certainly parks in EML2. You would wind up losing a lot more fuel than you'd gain by berth with such a low thrust engine. It'd take months spiralling out of Earth's gravity well, months which (if crew were on board) would likely cut the mission pretty short due to radiation exposure from spending so much time in the Van Allen Belts. Why waste fuel dropping into a low orbit on return and then having to claw your way out of it every time?
Hermes was likely assembled in LEO, since sending all those spaceship chunks one at a time to EML2 would be a ridiculous waste of fuel, then spiralled out to EML2 where the first crew rendezvoused with it in Orion. 

[fredinno]

13 minutes ago, benjee10 said:

It almost certainly parks in EML2. You would wind up losing a lot more fuel than you'd gain by berth with such a low thrust engine. It'd take months spiralling out of Earth's gravity well, months which (if crew were on board) would likely cut the mission pretty short due to radiation exposure from spending so much time in the Van Allen Belts. Why waste fuel dropping into a low orbit on return and then having to claw your way out of it every time?
Hermes was likely assembled in LEO, since sending all those spaceship chunks one at a time to EML2 would be a ridiculous waste of fuel, then spiralled out to EML2 where the first crew rendezvoused with it in Orion. 

Well, the crew would need to get there in Cis-lunar, due to the Van Allen, but otherwise... everything else is better off starting in LEO.

[Halo_003]

So does it make sense then to over engineer and spend extra on it in order to future proof it for more missions? My thought was if you do it right from the first time and make it upgradeable as possible, then you ultimately save money over the long run versus building a specialized ship for each and every mission. Sort of a spend $200 billion on it now, and save $400 billion over the next 10 years kind of deal.

 

The ship in the movie actually lined up well with what I expected it to look like based on the book, except I expected more radiators and less solar panels lol.

[fredinno]

4 hours ago, Halo_003 said:

So does it make sense then to over engineer and spend extra on it in order to future proof it for more missions? My thought was if you do it right from the first time and make it upgradeable as possible, then you ultimately save money over the long run versus building a specialized ship for each and every mission. Sort of a spend $200 billion on it now, and save $400 billion over the next 10 years kind of deal.

It probably will, unless you do an Apollo-style thing with only a few landings, which is sadly the most likely scenario. Then it only makes sense to reuse the propulsion system, maybe for a reusable GEO satellite tug.

Edited February 3, 2016 by fredinno

[sevenperforce]

5 hours ago, fredinno said:

10 hours ago, Halo_003 said:

So does it make sense then to over engineer and spend extra on it in order to future proof it for more missions? My thought was if you do it right from the first time and make it upgradeable as possible, then you ultimately save money over the long run versus building a specialized ship for each and every mission. Sort of a spend $200 billion on it now, and save $400 billion over the next 10 years kind of deal.

It probably will, unless you do an Apollo-style thing with only a few landings, which is sadly the most likely scenario. Then it only makes sense to reuse the propulsion system, maybe for a reusable GEO satellite tug.

If the political climate improves by the time we're setting out to do this, then building an overengineered interplanetary transport capable of going to Mars or to Ceres or to Venus would probably be the best way of ensuring that we actually go to Mars, to Ceres, and to Venus. Once they've invested heavily it doesn't make sense not to keep going. 

You don't want to over-overengineer, though, because then your cost goes too high and they're loath to build a second or a third or a fourth one. You kinda want a fleet of these things if you're going to be able to sustain a manned Martian base. So you want to design a craft which is robust enough to last for a long time and be adapted for a lot of different missions (i.e., more spaceship than spacecraft), but is still cheap enough that we could conceivably build several of them.

To that end, I'd suggest a combined hab/command center with some autonomous function, probably assembled on Earth and lifted to orbit by a single super-heavy launch. Lots of open space to keep your crew from going stir-crazy and to allow for expansion/adaptation. It would need to have internal airlocks, of course, to prevent catastrophic depressurization in the event of a hull breach, but you'd still want to try and manage a fairly open design. It would need its own engines (ideally VASMIR, but alternatively some sort of hybrid engine that can use either ion prop or chemical prop through the same nozzle) and power supply (fold-out or body-fixed solar panels with a decent battery system) so that it could still limp along if a meteoroid impact or other problem took out the main propulsive engines. Probably want to throw on an electrolysis system for converting water to fuel using solar power if you needed to.

For the sake of expanded options in future missions, you'd want it to be able to be used as an emergency lander with low-gravity worlds without atmospheres, since those are the worlds where an aerobraking capsule is useless. So...Luna, Ceres, etc. ought to be realizable. 

This spaceship would mount to a sturdy frame which would be used for interplanetary transfers. The frame would include a spartan pressurized command capsule and feature attachment points for engines, cargo, fuel, and power supplies. The frame ought to be strong enough that it could be used for brachistrochrone transfers once our engine technology advances. This allows for modular replacement/upgrading without modification to the main spaceship, and the frame could be left in orbit if an emergency landing was needed.

Edited February 3, 2016 by sevenperforce

[fredinno]

3 hours ago, sevenperforce said:

If the political climate improves by the time we're setting out to do this, then building an overengineered interplanetary transport capable of going to Mars or to Ceres or to Venus would probably be the best way of ensuring that we actually go to Mars, to Ceres, and to Venus. Once they've invested heavily it doesn't make sense not to keep going. 

You don't want to over-overengineer, though, because then your cost goes too high and they're loath to build a second or a third or a fourth one. You kinda want a fleet of these things if you're going to be able to sustain a manned Martian base. So you want to design a craft which is robust enough to last for a long time and be adapted for a lot of different missions (i.e., more spaceship than spacecraft), but is still cheap enough that we could conceivably build several of them.

You're telling us to overengineer or not overengineer?

3 hours ago, sevenperforce said:

alternatively some sort of hybrid engine that can use either ion prop or chemical prop through the same nozzle)

You can just use 2 different sets of engines, and save billions trying to merge the two.

3 hours ago, sevenperforce said:

Probably want to throw on an electrolysis system for converting water to fuel using solar power if you needed to.

Then you need a 3rd set of engines. Ion + Storable basically fills all necessary niches pretty well.

3 hours ago, sevenperforce said:

For the sake of expanded options in future missions, you'd want it to be able to be used as an emergency lander with low-gravity worlds without atmospheres, since those are the worlds where an aerobraking capsule is useless. So...Luna, Ceres, etc. ought to be realizable. 

Ceres, probably, you could land a mothership on it with only modifications being landing legs.

Luna? No, gravity is too high.

[sevenperforce]

4 minutes ago, fredinno said:

You're telling us to overengineer or not overengineer?

Oh, it should definitely be overengineered, but just not so much that it would be cost-prohibitive to build another one, or another two, or another five. Balance between a sturdy, adaptable design and extremely high costs....

5 minutes ago, fredinno said:

You can just use 2 different sets of engines, and save billions trying to merge the two.

Engines are heavy, so you can save more in the long run. Plus, if you're really going to be able to keep this thing running for a long time, you'd want it to be able to run on whatever fuel you can find. Nuclear thermal propulsion with options for various propellant types is probably ideal...maybe with ion engines as well?

Design factor should probably be something like "build the largest ship you can launch on a Falcon Heavy that would be capable of making a propulsive landing on the Moon with nothing but water in its fuel tanks".

[fredinno]

8 minutes ago, sevenperforce said:

Oh, it should definitely be overengineered, but just not so much that it would be cost-prohibitive to build another one, or another two, or another five. Balance between a sturdy, adaptable design and extremely high costs....

Engines are heavy, so you can save more in the long run. Plus, if you're really going to be able to keep this thing running for a long time, you'd want it to be able to run on whatever fuel you can find. Nuclear thermal propulsion with options for various propellant types is probably ideal...maybe with ion engines as well?

Design factor should probably be something like "build the largest ship you can launch on a Falcon Heavy that would be capable of making a propulsive landing on the Moon with nothing but water in its fuel tanks".

FH is kind of bad for this stuff due to it's low-thrust 2nd stage and small payload fairing.

[sevenperforce]

2 hours ago, fredinno said:

FH is kind of bad for this stuff due to it's low-thrust 2nd stage and small payload fairing.

No fairing required; the hab wouldn't be able to survive Earth re-entry but it ought to be sturdy enough for naked MaxQ. Falcon Heavy might not be the specific launch; just saying that's a good preliminary mass range.

It would be cool to see some possible mockups...

EDIT:

I can see the appeal of a Firefly-style vehicle: command center and living space above a cargo bay, with fuel tanks and high-Isp engines in the rear and two winglets with high-thrust rotatable engines.

Edited February 3, 2016 by sevenperforce
Adding discussion.

[fredinno]

5 hours ago, sevenperforce said:

No fairing required; the hab wouldn't be able to survive Earth re-entry but it ought to be sturdy enough for naked MaxQ. Falcon Heavy might not be the specific launch; just saying that's a good preliminary mass range.

It would be cool to see some possible mockups...

EDIT:

I can see the appeal of a Firefly-style vehicle: command center and living space above a cargo bay, with fuel tanks and high-Isp engines in the rear and two winglets with high-thrust rotatable engines.

Kind of bad to make the HAB launch w/o a faring, as the problem with having too small of a fairing is mainly that the stage diameter to fairing ratio is too high towards the latter side, and increases drag to the point where the rocket becomes unstable.

[sevenperforce]

Which is why you'd probably need a bigger rocket.

Just in the general lift range of FH.

[sevenperforce]

Update, given the pros and cons of various form factors....

I'm not sure how ambitious we want to be.

If you've got access to cheap propellant in orbit, then it becomes possible to re-enter Earth's atmosphere without lots of heavy, failure-prone heat shielding, simply by burning off a large proportion of your dV and then aerobraking at a more modest speed down to terminal velocity, and using a final burst to land propulsively. So while this certainly wouldn't be the typical landing mode, something built to last as long as we're thinking could probably be built to survive an emergency Earth return.

A massive internal cargo bay isn't too terribly important, because the interplanetary transfer frame can hold cargo with relative simplicity, but you need a little pressurizeable cargo space. With the right form factor, you could even allow for centrifugal gravity if you absolutely HAD to have it for a particular mission.

I confess I really like a slimmed-down Firefly form factor. The thrust of ion engines depends primarily on surface area, so if you have two high-thrust low-Isp engines on winglets for maneuvering and a large rear region with your high-Isp engines, that allows for a bit more thrust from them. Can ion engines be 3D-printed?

 

[PB666]

On 2/1/2016 at 0:24 PM, benjee10 said:

The ship is actually nuclear powered so if anything it has too many solar panels. I'm not an expert on electric propulsion by any means but I think that the amount of fuel is about right for a ship this size, the amount of power and the length of the trip. Someone would have to do some fact checking there though. Overall, however, it's surprisingly realistic overall considering it's a hollywood creation. The only things that really bug me are the offset module at the front (why? what could you possibly gain by having the front of the ship lower than the back that could justify the stability and CoT problems it'd cause?) and the fact that it has far more habitable space than would be needed. The rotating gravity drum is a bit questionable too. The design looks plausible but I'm not sure how realistic maintaining a rotating pressurised joint in vacuum for so long would be. The curved truss would be a nightmare to launch too. 

In terms of the idea of the ship - it's absolutely a good idea. Why build a new transfer vehicle every time you want to go somewhere when you could have a slightly bigger, more expensive ship which can make the trip several times? I can see a similar system being adopted if early Mars missions use NTRs or less efficient propulsion systems which require tanks to be jettisoned or can't be refurbished, with the propulsion module being discarded on return to Earth and being refitted with a new one between trips so that at least some of the ship is reused. 

I dont what Nuclear system they claim is part of the fictitious vessel, but any nuclear system so far designed lacks adequate power and would require cooling massively more than the drives could accelerate.

[fredinno]

4 hours ago, sevenperforce said:

Update, given the pros and cons of various form factors....

I'm not sure how ambitious we want to be.

If you've got access to cheap propellant in orbit, then it becomes possible to re-enter Earth's atmosphere without lots of heavy, failure-prone heat shielding, simply by burning off a large proportion of your dV and then aerobraking at a more modest speed down to terminal velocity, and using a final burst to land propulsively. So while this certainly wouldn't be the typical landing mode, something built to last as long as we're thinking could probably be built to survive an emergency Earth return.

A massive internal cargo bay isn't too terribly important, because the interplanetary transfer frame can hold cargo with relative simplicity, but you need a little pressurizeable cargo space. With the right form factor, you could even allow for centrifugal gravity if you absolutely HAD to have it for a particular mission.

I confess I really like a slimmed-down Firefly form factor. The thrust of ion engines depends primarily on surface area, so if you have two high-thrust low-Isp engines on winglets for maneuvering and a large rear region with your high-Isp engines, that allows for a bit more thrust from them. Can ion engines be 3D-printed?

 

No idea, but if you use Ion engines, 

1. You can't really refuel them without returning back to Earth, at least for the forseeable future

2. They have such a high ISP, and you need so much powerto operate them (a power source which is likely to beexpensive to replace), you really want to reuse these things as much as possible unless absolutely necessary (which isn't too often, especially since IONs have been proven to last for extremely long thrust durations, and VASMIR is supposed to go for even longer.

[sevenperforce]

The "return to Earth" issue isn't that big of a deal, given that these are intended as long-duration reusable spaceships which would definitely need periodic resupply launches. In any case, some experimental ion thrusters can use hydrogen or even water as propellant, so that's promising.

15 hours ago, PB666 said:

I dont what Nuclear system they claim is part of the fictitious vessel, but any nuclear system so far designed lacks adequate power and would require cooling massively more than the drives could accelerate.

We've had working nuclear fission reactors in space for quite a while now. Seebeck-effect thermocouples on radioisotope thermoelectric generators are terribly inefficient, but thermionic generators approach efficiencies of 20% when operated at a high enough temperature, while a good alkali-metal thermoelectric converter can reach efficiencies of 41% in operation. So waste heat, while challenging, won't be a serious problem.

For our extended-persistence interplanetary transfer spaceship, it would probably make sense to give it a pair of fission reactors which could be operated either as nuclear thermal rockets (using inert propellant) or as power generators (to power the craft and its ion engines). Such a design has the advantage of allowing active cooling of the reactors by a slow trickle of propellant if you need to run at high power, and it prevents startup/shutdown cycles on the reactor. This is called a bimodal NTR.

Another really nice thing about a nuclear thermal rocket is that you can have essentially selectable thrust and Isp by varying your propellant type. For example, a spacecraft powered by a liquid-hydrogen NTR may not have enough thrust to get off the ground...but if you add external drop tanks, you can inject higher-density propellant to boost your thrust tremendously, allowing for highly efficient staging with just a single engine cluster. Engines are expensive; tanks are cheap. When the higher-density propellant is LOX, this is called a LOX-augmented NTR, or LANTR; when combined with a power generation mode as described above, it becomes a trimodal NTR.

With the right form factor you could even leave the door open for my favorite tech, atmospheric thrust augmentation.

Ion engines have such a low T/W ratio that I'm not sure it makes sense to put them on the primary ship. Probably better to depend on the hydrogen cycle NTR for moving between orbits and dock with the external frame at EML2 or wherever else you want.

[PB666]

30 minutes ago, sevenperforce said:

The "return to Earth" issue isn't that big of a deal, given that these are intended as long-duration reusable spaceships which would definitely need periodic resupply launches. In any case, some experimental ion thrusters can use hydrogen or even water as propellant, so that's promising.

We've had working nuclear fission reactors in space for quite a while now. Seebeck-effect thermocouples on radioisotope thermoelectric generators are terribly inefficient, but thermionic generators approach efficiencies of 20% when operated at a high enough temperature, while a good alkali-metal thermoelectric converter can reach efficiencies of 41% in operation. So waste heat, while challenging, won't be a serious problem.

For our extended-persistence interplanetary transfer spaceship, it would probably make sense to give it a pair of fission reactors which could be operated either as nuclear thermal rockets (using inert propellant) or as power generators (to power the craft and its ion engines). Such a design has the advantage of allowing active cooling of the reactors by a slow trickle of propellant if you need to run at high power, and it prevents startup/shutdown cycles on the reactor. This is called a bimodal NTR.

Another really nice thing about a nuclear thermal rocket is that you can have essentially selectable thrust and Isp by varying your propellant type. For example, a spacecraft powered by a liquid-hydrogen NTR may not have enough thrust to get off the ground...but if you add external drop tanks, you can inject higher-density propellant to boost your thrust tremendously, allowing for highly efficient staging with just a single engine cluster. Engines are expensive; tanks are cheap. When the higher-density propellant is LOX, this is called a LOX-augmented NTR, or LANTR; when combined with a power generation mode as described above, it becomes a trimodal NTR.

With the right form factor you could even leave the door open for my favorite tech, atmospheric thrust augmentation.

Ion engines have such a low T/W ratio that I'm not sure it makes sense to put them on the primary ship. Probably better to depend on the hydrogen cycle NTR for moving between orbits and dock with the external frame at EML2 or wherever else you want.

Then you haven't been following the discussion, 70 kW of power per newton of ion drive thrust. So if you are using an electric drive, you are basically dealing with the need to produce alot of power, although the drives are around 90% efficient if you take that as 1 part, then 1.111 parts is the absolute efficitive energy efficiency and therefore even at 41% there are 2.71 parts per N or 189 kW of waste heat. Say you want 100N that 18.9 MW of waste heat. In addition these reactors are heavy and dangerous, the russians have already lost a couple, one exploded. So you add weight for reactor, weight for heat transfer, after about a 0.0001g of ac the weight of added supply super exceeds the benefit you get from energy begins to start saturating. At least with solar I got it up to .00025g

Nuclear as a power source for ion drives are no-go from the start, forget it, it will not work. Nuclear thermal rockets are a better choice; HOWEVER, they have never been tested after being started and then being reinitiated after 6 months. Once the fuel is activitated you cannot stop it from producing tons of heat, and that would eventually degrade the engine. So you would have to carry return trip engines once you get to your destination and jettison the first engine. Nuclear thermal could be used on the burn out of the solar system. ION drives have been tested and operate for years and years in space, and the newer models have less of a problem with electrode degradation than the older models.

Solar could be used in conjunction with nuclear thermal using rf generators and/or lasers to directionally accelerate the fuel even faster. It could potentially use less nuclear fuel but run at a much lower thrust.

ION drives have ISP now 2.5 to 4 times that of NTR. This is particularly advantages because leaving earth LEO and returning takes 19,000 dV. Thats alot for a single stage system and the NTR with the same amount of fuel is only going to get about 12,000 or so dV, though some of the new designs promise higher ISP. What that means is for NTR to complete the trip you need more than one stage. 

I am assuming that the lander is shipped to lower mars orbit by a robotic ion-drive ship and the landers weight is not apart of the mission. Adding a lander to the NTR ship weight basically makes the mission impossible. My test bed system, I have really focused on keeping the acc above 0.0002g which is satisfactory for producing the dAlt/t that would suffice to break and control exit into earth mars space in a reasonable timeframe, it takes 39 days (much less if the ION drive could get a fuel dock and boost from LEO with its iondrive assist. Add that to the <170 days to transfer. Just getting a ship to 2 earths radius (13,000 km) is useful because the sunblock time is much lower orbital velocity is 30% slower meaning that the time spent close to optimal burn point is higher, less burn wasted in a closely packed spiral. 

[Halo_003]

20 hours ago, sevenperforce said:

Update, given the pros and cons of various form factors....

I'm not sure how ambitious we want to be.

If you've got access to cheap propellant in orbit, then it becomes possible to re-enter Earth's atmosphere without lots of heavy, failure-prone heat shielding, simply by burning off a large proportion of your dV and then aerobraking at a more modest speed down to terminal velocity, and using a final burst to land propulsively. So while this certainly wouldn't be the typical landing mode, something built to last as long as we're thinking could probably be built to survive an emergency Earth return.

Why would you want the ship to reenter at Earth? Something like the HERMES wouldn't reenter, it would stay in orbit when not transiting. Or are you referring to something like the Orion capsule here?