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

[fredinno]

1 hour ago, PB666 said:

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. 

Well, Nuclear Electric is useful if you want to go to Ceres or beyond....

And wouldn't drop tanks and high-lifetime NTRs (made after extensive R&D) be cheaper?

[Spaceception]

On 2/1/2016 at 10:05 AM, Nibb31 said:

The ship is pretty realistic, if you take into account the magic propulsion. In reality, major part of such a vehicle would be tankage, even if it had SEP. Also, it would probably need more solar panels. The rotating section is unnecessary, but people have become accustomed to the idea that spaceships need a rotating section for artificial gravity.

The movie doesn't detail the actual mission architecture. It seems like the MAVs and maybe some cargo landers are prepositioned before the arrival of the crew, so we can assume that they use some other vehicle for cargo and MAV delivery. We also don't know what the crew uses for landing or whether that vehicle is prepositioned in orbit or if the Hermes carries it along.

Magic? The book said it was a Nuclear powered VASIMR thruster, (The solar panels were a backup just in case the nuclear reactor lost power ((So, you know, the crew wouldn't die)), And extra power for the thrusters ((The more power you give it, the faster you can go))) not magic, just a concept.

And the book also said they needed Artificial gravity, otherwise when they got to Mars they wouldn't be able to walk properly for the first week.

Edited February 5, 2016 by Spaceception

[sevenperforce]

A few points...

Reactors need not necessarily be heavy nor dangerous. They're not featherweight, granted, but we've come a long way since the late 70s. And Russia's strained history with them had more to do with putting them on satellites in unstable orbits with unreliable RCS that resulted in uncontrolled re-entry.

As I mentioned above, using a bimodal NTR that can provide both thrust and electrical power allows you to use a minute propellant flow as an active, expendable coolant which also provides a bit of additional impulse. Again, this also means you don't have to shut off the reactor at all, just decrease the reaction rate. The primary shielding can also be the neutron reflector, in order to improve the overall power-to-weight ratio, and the secondary shielding can do dual duty as a heat radiator. Recall that the ship needs a source of heat as well, so that's an additional sink.

I'm all about augmenting with solar power where possible, though scaling is an issue. Solar panels are very lightweight, but they occupy a great deal of space and must be orientable toward the sun, which increases weight as you scale up. 

The thrust of ion thrusters is limited by two things: power and surface area. So if you can manage a lot of surface area with high power, you can get decent thrust. I'd love to see 3D-printed panels with solar cells on one side and very small ion thrusters on the other, that could be folded up, launched all at once, and then folded out:

[sevenperforce]

4 hours ago, Halo_003 said:

On 2/4/2016 at 4:04 PM, sevenperforce said:

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?

As I mentioned before, trying to build an interplanetary transfer spacecraft capable of many missions to multiple worlds would probably be best realized if the hab was built as an autonomous craft on its own, docking with a larger frame to carry auxiliary fuel, cargo, and the ion engines. While propulsively-landed aerobraked capsules a la Dragon V2 would always remain the primary way to land on worlds with atmospheres, building a hab that is sturdy enough to break away from the rest of the spacecraft and make independent landings on worlds without atmospheres (Ceres, Luna, etc.) makes sense. Otherwise, you will be designing a different lander and ascent vehicle for every mission to a new world, which defeats the whole purpose.

And if it's that sturdy and carries its own high-thrust engines, then the possibility of occasional Martian landings emerges. Definitely worth considering, especially if the engines are nuclear thermal rockets with selectable propellant to allow multiple thrust/Isp configurations; that makes refuel and SSTO a breeze.

Of course, if we've already gone to those lengths, we might as well go a bit further and allow for propulsive Earth re-entries...certainly not routine, but for periodic repairs or in emergency situations.

Quote

wouldn't drop tanks and high-lifetime NTRs (made after extensive R&D) be cheaper?

Drop tanks and high-lifetime NTRs with selectable propellant (and optional air-augmentation) make even Earth SSTO routine. Probably couldn't carry much payload if you had to turn around and come back under power, but we've got to assume that fuel is going to be in orbit (since that would be required for an Earth Orbit Rendezvous assembly of the Hermes anyway).

Quote

the book also said they needed Artificial gravity, otherwise when they got to Mars they wouldn't be able to walk properly for the first week.

Spin'er up!

Edited February 5, 2016 by sevenperforce

[benjee10]

I feel like building a habitat unit designed for landings is needlessly complex and would make the whole machine a lot more massive than it needs to be. A habitat capable of sustaining multiple (at least 6 I would imagine) astronauts for very long periods of time as well as land on a planet and return to orbit would need to be simply gigantic and I can't imagine a sane way of launching it in one go as you suggest. Not to mention that the drive module would have to be even more gigantic to push/pull the whole thing around, propellant, landing legs, landing engines and all. We're talking about some huge machinery here; even on very low-G worlds, the inertia of a ship that size (I imagine something Skylab+landing section in size & shape based on your descriptions, correct me if I'm reading it wrong) is gonna be massive, and you're gonna need some hefty landing gear and a fairly large amount of propellant as well as a big old engine to slow that thing down and then get it off the surface again. Why take ALL the supplies of a multi-year mission down with you and then bring them back up again when you may only need 1/3 of them or less for the surface stay (depending on destination and utilisation of ISRU)?

Does it not make a little more sense to have a permanently space-based hab as well as a reusable (presumably single-stage) lander rated for Lunar missions and below? With that set up you can land on 90% of the notable objects in the solar system, only needing a larger lander for Mars and more unique places, at which point the practicality of single-stage landers goes out of the window (particularly if you're lugging your entire hab, complete with long-duration life support, around with you) so reusability stops being an issue. You can send a pop-up, disposable hab down with you for shorter missions and send something sturdier in advance for long duration ones. 

[Halo_003]

5 hours ago, sevenperforce said:

As I mentioned before, trying to build an interplanetary transfer spacecraft capable of many missions to multiple worlds would probably be best realized if the hab was built as an autonomous craft on its own, docking with a larger frame to carry auxiliary fuel, cargo, and the ion engines. While propulsively-landed aerobraked capsules a la Dragon V2 would always remain the primary way to land on worlds with atmospheres, building a hab that is sturdy enough to break away from the rest of the spacecraft and make independent landings on worlds without atmospheres (Ceres, Luna, etc.) makes sense. Otherwise, you will be designing a different lander and ascent vehicle for every mission to a new world, which defeats the whole purpose.

And if it's that sturdy and carries its own high-thrust engines, then the possibility of occasional Martian landings emerges. Definitely worth considering, especially if the engines are nuclear thermal rockets with selectable propellant to allow multiple thrust/Isp configurations; that makes refuel and SSTO a breeze.

Of course, if we've already gone to those lengths, we might as well go a bit further and allow for propulsive Earth re-entries...certainly not routine, but for periodic repairs or in emergency situations.

Ah gotcha, I thought what you were talking about deorbiting the entire ship to land on Earth between every mission. I didn't realize you were talking about landing it on other bodies, that makes a lot more sense.

 

Unless public perception changes significantly I don't think we will ever see NTRs used on/around Earth though because of the average person's fear of anything nuclear.

[sevenperforce]

2 hours ago, benjee10 said:

I feel like building a habitat unit designed for landings is needlessly complex and would make the whole machine a lot more massive than it needs to be. A habitat capable of sustaining multiple (at least 6 I would imagine) astronauts for very long periods of time as well as land on a planet and return to orbit would need to be simply gigantic and I can't imagine a sane way of launching it in one go as you suggest. Not to mention that the drive module would have to be even more gigantic to push/pull the whole thing around, propellant, landing legs, landing engines and all. We're talking about some huge machinery here; even on very low-G worlds, the inertia of a ship that size (I imagine something Skylab+landing section in size & shape based on your descriptions, correct me if I'm reading it wrong) is gonna be massive, and you're gonna need some hefty landing gear and a fairly large amount of propellant as well as a big old engine to slow that thing down and then get it off the surface again. Why take ALL the supplies of a multi-year mission down with you and then bring them back up again when you may only need 1/3 of them or less for the surface stay (depending on destination and utilisation of ISRU)?

Does it not make a little more sense to have a permanently space-based hab as well as a reusable (presumably single-stage) lander rated for Lunar missions and below? With that set up you can land on 90% of the notable objects in the solar system, only needing a larger lander for Mars and more unique places, at which point the practicality of single-stage landers goes out of the window (particularly if you're lugging your entire hab, complete with long-duration life support, around with you) so reusability stops being an issue. You can send a pop-up, disposable hab down with you for shorter missions and send something sturdier in advance for long duration ones. 

Lugging along a lander with its engine, plus some sort of ascent vehicle, plus a smaller hab feels like a waste to me when you're dealing with airless worlds. The space-hab that would descend is only going to be the hab itself, the selectable-thrust/isp engines, and descent/ascent fuel; the main drive and aux fuel tanks would stay in orbit. It only makes sense to use a separate lander if you can aerobrake with a capsule and land near an existing permanent hab.

17 minutes ago, Halo_003 said:

Unless public perception changes significantly I don't think we will ever see NTRs used on/around Earth though because of the average person's fear of anything nuclear.

Sadly, this is true. Even though an air-augmented LOX-afterburning selectable propellant nuclear thermal turborocket is probably the most realizeable SSTO we're going to see 

[fredinno]

 

 

3 hours ago, sevenperforce said:

Lugging along a lander with its engine, plus some sort of ascent vehicle, plus a smaller hab feels like a waste to me when you're dealing with airless worlds. The space-hab that would descend is only going to be the hab itself, the selectable-thrust/isp engines, and descent/ascent fuel; the main drive and aux fuel tanks would stay in orbit. It only makes sense to use a separate lander if you can aerobrake with a capsule and land near an existing permanent hab.

Sadly, this is true. Even though an air-augmented LOX-afterburning selectable propellant nuclear thermal turborocket is probably the most realizeable SSTO we're going to see 

No, because that option would make the reusable landers less useful and more expensive- a smaller lander only designed with a week of life support can be a lot smaller, be used as a "outpost" during long missions on rovers, crew and cargo transport to and fro a moon base, and short duration Missions. Longer duration HAB landers will likely lead to base modules, so there's not really any point to those being able to go back into orbit.

[sevenperforce]

8 hours ago, fredinno said:

 

 

No, because that option would make the reusable landers less useful and more expensive- a smaller lander only designed with a week of life support can be a lot smaller, be used as a "outpost" during long missions on rovers, crew and cargo transport to and fro a moon base, and short duration Missions. Longer duration HAB landers will likely lead to base modules, so there's not really any point to those being able to go back into orbit.

I think it all comes down to the mission profile. The thing is, if we're going to have a really wide range of possible missions (which is part of the point of having a single interplanetary transfer ship), then the hab is going to need to have a long-lasting power supply and autonomous maneuvering capabilities. And so if you're thinking of something like a landing on Luna or Ceres, does it really make sense to lug along the dead weight of a separate descent/ascent engine, separate fuel tanks, and a separate hab with separate life support systems...when you have a perfectly good hab with high-thrust engines and autonomous capabilities?

I would be inclined to argue that it only makes sense to use a separate hab and landing craft and ascent craft if you've already established a semipermanent presence, but don't yet have access to fuel on the surface. 

[fredinno]

3 hours ago, sevenperforce said:

I think it all comes down to the mission profile. The thing is, if we're going to have a really wide range of possible missions (which is part of the point of having a single interplanetary transfer ship), then the hab is going to need to have a long-lasting power supply and autonomous maneuvering capabilities. And so if you're thinking of something like a landing on Luna or Ceres, does it really make sense to lug along the dead weight of a separate descent/ascent engine, separate fuel tanks, and a separate hab with separate life support systems...when you have a perfectly good hab with high-thrust engines and autonomous capabilities?

I would be inclined to argue that it only makes sense to use a separate hab and landing craft and ascent craft if you've already established a semipermanent presence, but don't yet have access to fuel on the surface. 

Luna, using seperate HABs and Landers makes sense- but Ceres has almost no gravity- so you'd be better off just carrying landing legs on the front of your ship, and docking it directly to Ceres.

[Emperor of the Titan Squid]

Some things i want to explain: Hermes is VASIMIR powered,has 4 nuclear reactors an parks in LEO.Also, i heard a nasa guy say that if we want to avoid radiation,we should use nuclear thermal.Also, he said that a centrifuge is not practical, whithout significant R and D.

[sevenperforce]

On 2/6/2016 at 2:44 PM, fredinno said:

Luna, using seperate HABs and Landers makes sense- but Ceres has almost no gravity- so you'd be better off just carrying landing legs on the front of your ship, and docking it directly to Ceres.

Yet once you've got landing legs and engines capable of landing on Ceres, the only thing preventing you from doing the same thing with Luna is a bit more fuel. 

[fredinno]

On 2/6/2016 at 0:02 PM, Emperor of the Titan Squid said:

13 hours ago, sevenperforce said:

Yet once you've got landing legs and engines capable of landing on Ceres, the only thing preventing you from doing the same thing with Luna is a bit more fuel. 

 

Luna actually has gravity- so you'd need a lot more structural bearings and much stronger landing legs and landing systems. It's also very inefficient- compare the size of Apollo Direct Lunar landings to the actual LOR landings.

[sevenperforce]

3 hours ago, fredinno said:

Luna actually has gravity- so you'd need a lot more structural bearings and much stronger landing legs and landing systems. It's also very inefficient- compare the size of Apollo Direct Lunar landings to the actual LOR landings.

Well, Ceres has gravity too; just less.

Your landing legs will need to be stronger for sure, but since we're going for persistence, that's acceptable overengineering. And NTR can have their thrust/Isp adjusted by choosing heavier or lighter propellants, so they should be able to manage a propulsive landing.

[fredinno]

3 hours ago, sevenperforce said:

Well, Ceres has gravity too; just less.

Your landing legs will need to be stronger for sure, but since we're going for persistence, that's acceptable overengineering. And NTR can have their thrust/Isp adjusted by choosing heavier or lighter propellants, so they should be able to manage a propulsive landing.

As I said earlier, direct landings (no seperate landers) require a lot more fuel than using sepoerate landers if your body has a decent amount of gravity. Ceres barely has gravity at all. You might as well be saying Phobos has gravity- it does, but in practice, it might as well be 0G.

[sevenperforce]

2 hours ago, fredinno said:

As I said earlier, direct landings (no seperate landers) require a lot more fuel than using sepoerate landers if your body has a decent amount of gravity. Ceres barely has gravity at all. You might as well be saying Phobos has gravity- it does, but in practice, it might as well be 0G.

Well, I guess it comes down to the math at some point. Given a core hab large enough for interplanetary transfers with ntr engines, does it cost more to bring a separate lander with a separate hab and a separate engine and separate tankage sufficient for powered descent and ascent, or to just strengthen your landing legs and bring along extra fuel?

The other option, I suppose, is something like Interstellar's lander that docks into the hab and provides it with propulsion, etc.

[Kertech]

On 1 February 2016 at 6:12 PM, sevenperforce said:

 

To start with, "the world's space agencies" basically means the US and Russia, and maybe China. Nobody else has anywhere near the budget for this sort of thing. And since the US and Russia aren't really getting along right now...yuck. 

Isn't the current plan for a manned mars mission to have the transfer unit built by ESA

[fredinno]

5 hours ago, Kertech said:

Isn't the current plan for a manned mars mission to have the transfer unit built by ESA

As if we have a plan.

[Kertech]

Annoyingly true