Yet another Mars architecture

kbd512 · 2015-04-27 16:40:58

Louis,

Reusing Red Dragon for subsequent landings is not possible. It's wishful thinking. No spacecraft ever built has been reused after reentry. The months of refurbishment work to re-certify the orbiters for flight after each reentry in enormous specialized terrestrial facilities that employ hundreds of workers doesn't count.

You either land a really small and light one man capsule for a human or you land as big a cargo package as your lift vehicle and EDL solution allow with a single launch. That'd be between 15t and 17t for F9H if you use SEP to send the payload to Mars. The key to survival on the Martian surface is mobility and complete redundancy. If you land far off course in Red Dragon or any other EDL solution that does not include a fully fueled ascent vehicle and you're not mobile, you'll probably be a permanent resident. If you design a mobile mission architecture that's required for any real surface exploration effort, a side benefit is that you can screw up landings something fierce and live to tell the tale. You still have to land within a hundred kilometers or so of a waiting surface rover, but landing 20km away from a habitat module doesn't equate to loss of crew and loss of mission.

Put another way, what typically happens here on Earth when you miss the runway? The same thing will happen on Mars. Some things you just have to get right. EDL on Mars is just one of those things.

Littering the landing area with supplies won't help, either. You can't throw supplies all over the landing area and hope you land near them unless money is not a problem. If money is not a problem, you can afford to land the most massive multi-person lander your heart desires, complete with a fully fueled ascent vehicle and habitat module. Money is a major problem. That's why scattering extremely expensive payloads all over the Martian surface isn't an option and neither is designing massive EDL vehicles.

Last edited by kbd512 (2015-04-27 16:41:29)

louis · 2015-04-27 17:54:59

I wasn't arguing Red Dragon could be reused.

Why on Earth do you think we would be incapable of landing accurately in the desired zone if we have transponders on the surface? How many aircraft completely miss airports? Landing with retro rockets has none of the difficulty associated with landing a plane on a runway. It has its own challenges - but remember all the Apollo landings (and ascents) were successful - and they took place nearly 50 years ago!

It's not a question of "scattering" supplies. Once the first robot is down and has laid transponders, all the landings will be accurate. Remember we have cheap cars on Earth that can park themselves. No reason why in a multi-billion dollar project we can't get robot landers into the desired zone. A robot tow vehicle can ensure that all the supplies are towed accurately to collection point which will be within 150 metres of the lander zone.

I'd get a mapping robot down there first so it can map the whole area, lay transponders and paint marker stones white. The map data can then be sent back to Earth and used to aid guidance of the pre-landers.

The mapping robot would probably be something like a one tonne robot vehicle with capability for: laying equipment (e.g. transponders and PV panels; pushing rocks and small boulders out of the way; and maybe digging.

kbd512 wrote:

Louis,
Reusing Red Dragon for subsequent landings is not possible. It's wishful thinking. No spacecraft ever built has been reused after reentry. The months of refurbishment work to re-certify the orbiters for flight after each reentry in enormous specialized terrestrial facilities that employ hundreds of workers doesn't count.
You either land a really small and light one man capsule for a human or you land as big a cargo package as your lift vehicle and EDL solution allow with a single launch. That'd be between 15t and 17t for F9H if you use SEP to send the payload to Mars. The key to survival on the Martian surface is mobility and complete redundancy. If you land far off course in Red Dragon or any other EDL solution that does not include a fully fueled ascent vehicle and you're not mobile, you'll probably be a permanent resident. If you design a mobile mission architecture that's required for any real surface exploration effort, a side benefit is that you can screw up landings something fierce and live to tell the tale. You still have to land within a hundred kilometers or so of a waiting surface rover, but landing 20km away from a habitat module doesn't equate to loss of crew and loss of mission.
Put another way, what typically happens here on Earth when you miss the runway? The same thing will happen on Mars. Some things you just have to get right. EDL on Mars is just one of those things.
Littering the landing area with supplies won't help, either. You can't throw supplies all over the landing area and hope you land near them unless money is not a problem. If money is not a problem, you can afford to land the most massive multi-person lander your heart desires, complete with a fully fueled ascent vehicle and habitat module. Money is a major problem. That's why scattering extremely expensive payloads all over the Martian surface isn't an option and neither is designing massive EDL vehicles.

RobertDyck · 2015-04-27 18:53:56

Rendezvous is something NASA has down. Apollo 12 landed close enough to Surveyor 3 that astronauts walked over, removed some parts, and returned them back to Earth. Those parts were studied to see what long term exposure to the lunar environment did to them. If Apollo could do that with 1969 technology, then how reliable is modern technology?

When I talked to Robert Zubrin about Mars orbit rendezvous, he didn't like it. He thought it was risky. Especially a mission plan that requires both surface rendezvous (landing close to something pre-landed) and orbit rendezvous. But Dragon and Cygnus do it all the time, completely automated/unmanned. And Russia did it with Progress, Europe with ATV, Japan with HTV. And NASA completed several manned rendezvous/docking: Apollo 11-17 (13 docked but didn't rendezvous), Apollo 10 (LM didn't land but did everything else), Apollo 9 docked in LEO. Gemini VI-A rendezvous with Gemini VII in December 1965. And all those Shuttle flights to Mir and ISS. I think orbit rendezvous and docking is mature now.

louis · 2015-04-28 03:41:50

RobertDyck wrote:

Rendezvous is something NASA has down. Apollo 12 landed close enough to Surveyor 3 that astronauts walked over, removed some parts, and returned them back to Earth. Those parts were studied to see what long term exposure to the lunar environment did to them. If Apollo could do that with 1969 technology, then how reliable is modern technology?
When I talked to Robert Zubrin about Mars orbit rendezvous, he didn't like it. He thought it was risky. Especially a mission plan that requires both surface rendezvous (landing close to something pre-landed) and orbit rendezvous. But Dragon and Cygnus do it all the time, completely automated/unmanned. And Russia did it with Progress, Europe with ATV, Japan with HTV. And NASA completed several manned rendezvous/docking: Apollo 11-17 (13 docked but didn't rendezvous), Apollo 10 (LM didn't land but did everything else), Apollo 9 docked in LEO. Gemini VI-A rendezvous with Gemini VII in December 1965. And all those Shuttle flights to Mir and ISS. I think orbit rendezvous and docking is mature now.

I agree Robert. If we have GPS, and can have cars that park themselves, cars that drive themselves, I simply don't believe we can't have accurate rendezvous and docking in a Mars Mission context.

kbd512 · 2015-04-28 06:03:59

Louis,

I've never thought that landing accuracy was a particularly challenging aspect of the mission. JPL is trying to call a snow day on manned Mars landings due to their aerocapture entry accuracy, which is still less than 10km. No manned mission needs to use aerocapture. Any manned EDL should start in LMO.

Until your explanation of what you wanted to do, I thought you'd suggested littering the landing area with supplies to ensure that the astronauts would have consumables once they'd landed. We already have high precision surface maps. We need to test a transponder system or GPS that permits two or more vehicles to rendezvous on the surface.

If you want to set up a base of operations as soon as you get there, then you can have the robot or robots start deploying structures for a base. Understand that as soon as you establish a base, you're never going very far from that base.

Pushing rocks and debris out of the landing area is an excellent idea but that could take a long time, if it's even possible, without real earth moving equipment.

louis · 2015-04-28 10:36:40

The Mars Rover expeditions have shown there are many places on Mars where there are large plains scattered with rocks of manageable size.

The pre-landings could serve several purposes:

1. Laying guidance transponders.

2. Clearing a safe landing zone.

3. Manufacturing rocket fuel for an ascent vehicle (or maybe on a first mission, storing such fuel).

4. Providing long term consumables for a two year stay.

5. Providing mining robots to mine for water, iron ore or other resources.

6. Establishing a surface hab for the first colonists to transfer into.

7. Providing an automated farm hab to grow food on the surface.

8. Mapping of the surface - down to 10s of cms.

kbd512 wrote:

Louis,
I've never thought that landing accuracy was a particularly challenging aspect of the mission. JPL is trying to call a snow day on manned Mars landings due to their aerocapture entry accuracy, which is still less than 10km. No manned mission needs to use aerocapture. Any manned EDL should start in LMO.
Until your explanation of what you wanted to do, I thought you'd suggested littering the landing area with supplies to ensure that the astronauts would have consumables once they'd landed. We already have high precision surface maps. We need to test a transponder system or GPS that permits two or more vehicles to rendezvous on the surface.
If you want to set up a base of operations as soon as you get there, then you can have the robot or robots start deploying structures for a base. Understand that as soon as you establish a base, you're never going very far from that base.
Pushing rocks and debris out of the landing area is an excellent idea but that could take a long time, if it's even possible, without real earth moving equipment.

kbd512 · 2015-04-28 11:04:51

Russel wrote:

Reliable lightweight aeroentry is vital to landing large masses on Mars. And I'm not suggesting it be developed later rather than sooner. Seriously, I'm not. It just doesn't matter as far as the crew goes. It will always be safer to land the crew separately (And indeed that is what you're suggesting if you have the kind of mission where the crew arrive 2 weeks later).

Wow! I was beginning to think, based on all the other responses I've received in other threads, that everyone else was in favor of killing the entire crew if something goes wrong with human EDL on Mars.

Russel wrote:

I will differ on one issue and that's I think you don't need to land anything on Mars larger than about 15 tonnes. Practically anything you can think of can be built up from that scale. I see Mars drive as a response to the "big lander problem" but I believe they're going a bit too far in the other direction. In any case they don't really make the landing problem go away either.

A 15t landed payload corresponds to what a single F9H flight could deliver to Mars using a SEP tug and ADEPT. If NASA wants to give a SEP tug "something to do", perhaps they could land a mobile habitat on the surface of Mars.

Russel wrote:

You're defending Mars direct. Fortunately I'm not

Mars Direct looks a lot like flags and footprints to me because the proposed HLLV costs so much to develop and operate and the landed payloads are so massive. Mars Semi-Direct looks like a more plausible, and certainly more affordable, way to get humans to Mars.

Russel wrote:

What I do see though is a lot of frustration people have with NASA coming up with ridiculously large mission mass. And thus a lot of architectures that are attacking the problem from particular directions. Mars direct is largely a response to the NASA overkill. I kinda like it in some ways. Its got a certain minimalism. But, for me the solution ultimately lies in something a bit more considered, a bit more conservative, but still not wasteful either.

Yes, lots of us are frustrated that NASA says it wants to conduct a meaningful surface exploration campaign of Mars, but the only proposed mission architectures from NASA and most everyone else are essentially establishing a base, which de-facto means all operations would be conducted at or near the base. Establishing a base of operations on Mars may require much larger landed payloads and it may be a worthy long term exploration or colonization goal, but it's guaranteed to be so that we simply won't go to Mars to begin with.

Russel wrote:

However we get to Mars, its going to take a decade or two. And that's time enough to for everyone to sit around and bang heads together and not get too wedded to their particular approach.

If it takes more than ten years, it's probably not going to happen. Each successive political administration has their own personal proclivities and NASA, Congress, and the President are all deeply afflicted with NIH syndrome.

Russel wrote:

NASA isn't the only body that theoretically could do this. I think ESA has the resources to do this eventually. Japan, China could all play the part.

I agree. No one else has an EDL tech program to land anywhere but Earth. However, ESA, Russia, and China could eventually complete successful tele-robotic surface exploration campaigns from manned platforms orbiting Mars.

Russel wrote:

The thing about the US is that if you've got a great architecture, whose going to build it. If its NASA then you've got to get through its strange meld of hard core rationality, plain old fashioned bureaucracy, and US style corporate welfare. Not that the ESA doesn't have its own politics at play.

NASA has to develop and manufacture the human habitability and EDL solutions in house. There's no current commercial interest in the systems required for long duration space flight or landing on another planet. NASA can contract with SpaceX for launch services and crew transfer services.

Russel wrote:

Point is, even the most spartan Mars architectures are not pocket money. Eventually the big space agencies will have to be won over. And if that's the case they're going to be won over with with architectural proposals that don't just save launch costs, but also simplify development effort and most of all don't make them look like they're taking unnecessary risk with their crew.

Easier said than done. NASA constantly and literally tries to reinvent the wheel and if someone already invented something that works really well, they don't seem to show much interest in it. The one big exception are the JPL guys. They're constantly trying to figure out what they can do with what they have and what they know.

Russel wrote:

And that's about as far into politics as I dare venture. I'm interested in this as a problem worth solving that hasn't been solved well by anyone, yet.

For me, politics is a lot like watching paint dry. That said, it may be necessary to get into the game to get the kind of support required for a project like this one.

Russel wrote:

We do have time to get the technology right, to scrutinize what we're doing in unprecedented detail and do it safely, and I might add, in style.

One would think that. There doesn't appear to be any coherent program at NASA or any other government sponsored space exploration agency with that objective.

kbd512 · 2015-04-28 11:12:21

louis wrote:

The Mars Rover expeditions have shown there are many places on Mars where there are large plains scattered with rocks of manageable size.
The pre-landings could serve several purposes:

Yep.

louis wrote:

1. Laying guidance transponders.

This certainly helps improve landing accuracy and needs to be seriously pursued.

louis wrote:

2. Clearing a safe landing zone.

This requires serious earth moving equipment. Perhaps a nuclear powered robotic Bobcat could be used?

louis wrote:

3. Manufacturing rocket fuel for an ascent vehicle (or maybe on a first mission, storing such fuel).

This solves landed payload mass requirements and needs to be seriously pursued.

louis wrote:

4. Providing long term consumables for a two year stay.

This is definitely required for exploration.

louis wrote:

5. Providing mining robots to mine for water, iron ore or other resources.

This is a long term permanent human habitability goal that's not required for surface exploration.

louis wrote:

6. Establishing a surface hab for the first colonists to transfer into.

This is a long term permanent human habitability goal that's not required for surface exploration.

louis wrote:

7. Providing an automated farm hab to grow food on the surface.

This is a long term permanent human habitability goal that's not required for surface exploration.

louis wrote:

8. Mapping of the surface - down to 10s of cms.

I believe we've already done that, but I'll have to go back and check with the resolutions were.

RobertDyck · 2015-04-28 11:45:24

kbd512 wrote:

based on all the other responses I've received in other threads, that everyone else was in favor of killing the entire crew if something goes wrong with human EDL on Mars.

Your mission plan is not the only one. Claiming every other plan constitutes "killing the crew" is hyperbole and bullshit.

My plan includes pre-landing a laboratory. And include a pressurized rover with that lab, with recycling life support. Crew lands with the habitat. Normally, the lab will be connected to the hab; the hab will provide all life support. But if something goes wrong with the hab, then the pressurized rover will be connected to the lab, which is the backup habitat. If crew have to use the lab as hab, then expect laboratory equipment will have to be removed in favour of living space. So using the lab as the hab means loss of mission, but not loss of crew.

kbd512 · 2015-04-28 13:37:48

RobertDyck wrote:

Your mission plan is not the only one.

Good to know.

RobertDyck wrote:

Claiming every other plan constitutes "killing the crew" is hyperbole and bullshit.

If you put the entire crew in one capsule, which probably would not have been tested on Mars more than once due to the mission hardware and launch costs, and that capsule has a problem that inhibits a successful landing, what else would you call that?

If NASA had a budget 25% greater than what it currently receives, I'd be all for a multi-person lander. There'd be more than enough funding to devote to EDL hardware and launch costs associated with testing. Funding has already been allocated where it has and current commitments have survived two administrations from opposing political parties. The next administration is not likely to kill Orion or SLS, given how far along those projects are in development.

RobertDyck wrote:

My plan includes pre-landing a laboratory. And include a pressurized rover with that lab, with recycling life support. Crew lands with the habitat. Normally, the lab will be connected to the hab; the hab will provide all life support. But if something goes wrong with the hab, then the pressurized rover will be connected to the lab, which is the backup habitat. If crew have to use the lab as hab, then expect laboratory equipment will have to be removed in favour of living space. So using the lab as the hab means loss of mission, but not loss of crew.

Landing the crew in the habitat module is certainly an option, but that also means the entire EDL solution has all the attendant baggage that a man rated system carries with it. In other words, it will take NASA an inordinate amount of time, cost beaucoup bucks, and be heavier than absolutely necessary. How many crew members do you intend to send and what's the target mass of the solution?

What's your plan for transferring the crew to the lander? Do you favor an orbital rendezvous at Mars or does the lander + habitat + rover have to be connected to the MTV? What will your IMLEO be for the MTV + lander + habitat + rover?

If something goes wrong with the primary habitat module, do you have a method worked out to transfer consumables from the primary habitat module? In other words, does the rover or habitat module have an airlock that can accommodate a cargo container or two and a suited crew member? How many crew members can the rover's ECLSS handle, long-term, does it have ECLSS redundancy, and how big is that rover?

GW Johnson · 2015-04-28 14:31:25

Looking at post #426 above, I see no reason why a vehicle with an entry heat shield cannot be used more than once. Mars hypersonic entry heat protection is easier than here at Earth, simply because the velocities are lower.

And it's already been done.

The Gemini capsule used in the one-and-only unmanned flight test of "manned orbiting laboratory" (MOL) in 1969 was a re-used capsule that had already flow early in the Gemini program. They actually cut a hole through the used heat shield to represent the hatch and doorway of the Gemini-B configuration needed for MOL. It worked fine.

Dragon already flies with a PICA-X shield capable of free-returning from Mars. That means at lunar return speeds, you could probably fly it twice. And from LEO, perhaps dozens of times.

No telling how many times a Dragon heat shield could fly from LMO. Many, for sure.

Heat protection for Mars EDL isn't the issue. It's using "whatever" to decelerate further in that near-vacuum-of-an-atmosphere, once you come out of the hypersonics. That's where the problems are.

As for landing accuracy, if you are "sitting" on propulsive jets, you have no excuse to miss a transponder-marked "x". Hanging from a chute, you have a lot less control. No different than here, except for that thin "air". Using roll-controlled lift on any heat-shield capsule gets you decent accuracy coming out of hypersonics.

GW

Last edited by GW Johnson (2015-04-28 14:32:25)

RobertDyck · 2015-04-28 14:37:57

Pre-land the Mars Ascent Vehicle. The MAV would use ISPP, and carry astronauts and samples to Mars orbit. The MAV is inspired by Mars Direct and Semi-Direct.
Pre-land a laboratory with pressurized rover. The rover would have recycling life support, allowing for extended excursions from base.

The crew would ride in the Interplanetary Transit Vehicle: ITV. The ITV would be assembled in LEO, using ISS as construction shack. Add TMI propulsion stage, add the lander. This whole assembly would travel to Mars as one unit. Aerocapture into high Mars orbit. Then crew would descend in the lander. Yes, the lander would be habitat, with recycling life support.

For return, the MAV would act as the TEI stage. This allows for ISPP for the entire return to Earth. The ITV would aerocapture in to Earth orbit, then aerobrake down to LEO, rendezvous with ISS and dock. The ITV would use a single Dragon as escape pod, in case aerocapture fails. If not used, it would stay attached for the next mission. Since the ITV will dock with ISS, any vehicle capable of returning crew from ISS can be used. If Dragon is to be used, then the "escape pod" may as well be used for crew return. In this case the second crew would ride Dragon up to ISS, then dock that Dragon to the ITV as their escape for return.

A couple additional backup modes. Notice the ITV has food and life support for transit from Earth to Mars, and back. The lander/hab has food for the surface stay. But the lander will be attached during capture into Mars orbit. This means if a free return to Earth is necessary, all that food will be available for the trip. And life support in the lander/hab would be available as backup for the ITV.

Mars Direct has life support on the hab/lander, and ERV. My plan has life support on the hab/lander, ITV, and rover. So the rover is an additional layer of backup. Is that too much? Could recycling life support be designed more compact than currently on ISS? Compact enough for a minivan-size rover?

I propose 4 crew.

kbd512 · 2015-04-28 16:15:23

Rob,

The concept sounds great, but how much IMLEO?

Why are you going to aerocapture the ITV with the lander attached? Why not just capture first and then rendezvous with the lander? What type of thermal management requirements would the ITV and/or lander have to attempt what you're asking? Maybe GW can provide an indication of what type of thermal flux this thing is going to be subjected to when it aerocaptures. Is the lander's heat shield so large that it acts as the brake for the ITV and lander stack?

The ECLSS NASA is currently working on is significantly more compact, capable, and reliable than what's aboard ISS right now. It's going to be at least several more years before that's ready for prime time, maybe even five or six years with current funding. I'd prioritize that project, but I don't run NASA.

RobertDyck · 2015-04-28 22:13:14

I was just doing some further research. The guys who designed the SAFE-400 nuclear reactor, designed a smaller one: HOMER-15. It has 15 kWt producing 3 kWe, with reactor system mass 214kg. The rover would require 2 of these, or one twice the size. In the discussion "Light weight nuclear reactor, updating Mars Direct", I quoted figures for ISS life support. For 4 astronauts that worked out to 1.8 kW. I found a website selling an electric light truck that consumes 8 kW normal, 15 kW max. And a Chinese electric light garbage truck, motor rated for 6.3 kW. That's for Earth. Could a pressurized rover the size of a minivan, operating in Mars 38% gravity, trundle along at 50 km/h (30 mph) on 4.2 kW electricity, plus 1.8 kW for life support?

Curiosity rover may be much bigger than Spirit or Opportunity, but Curiosity's nuclear power system generates 125 watts beginning of life, 100 watts end of life. This vehicle would have 6 kW. That's 60 times as much power.

RobertDyck · 2015-04-28 22:30:02

kbd512 wrote:

Why are you going to aerocapture the ITV with the lander attached? Why not just capture first and then rendezvous with the lander? What type of thermal management requirements would the ITV and/or lander have to attempt what you're asking? Maybe GW can provide an indication of what type of thermal flux this thing is going to be subjected to when it aerocaptures. Is the lander's heat shield so large that it acts as the brake for the ITV and lander stack?

The ITV would aerocapture into Mars orbit, and aerocapture into Earth orbit. And reusable. So the heat shield has to be non-ablative. And the lander requires a separate heat shield. Whatever the lander uses, won't leave the surface of Mars. So this means 2 heat shields. But aerocapture isn't as tough, doesn't produce as much heating, as direct entry. You're suggesting separating the lander, have it use its own heat shield for aerocapture, then rendezvous & dock in Mars orbit? Then transfer crew for landing? That would reduce the load on the ITV heat shield, but that means another rendezvous. Robert Zubrin has already complained that my mission plan requires both surface rendezvous with the ascent vehicle, and orbital rendezvous with the interplanetary vehicle. If Mars aerocapture fails, you want all the food in the lander with you for the free return to Earth. Aerocapture is probably the riskiest part of the entire mission, because the atmosphere keeps moving due to changes in heating from solar wind.

Terraformer · 2015-04-29 04:41:54

3 kWe, for 214 kg? Would it not be better to investigate the use of strontium nuclear batteries? I'm sure they could achieve much better power density than that, and without the issues that typically arise when talking about nuclear reactors IN SPACE?

louis · 2015-04-29 05:11:14

I like the sound of all that, Robert. However, the ascent vehicle could use methane manufactured on Mars.

I would go with a double mission - two crews of three. They could probably allgo on one ITV.

RobertDyck wrote:

Pre-land the Mars Ascent Vehicle. The MAV would use ISPP, and carry astronauts and samples to Mars orbit. The MAV is inspired by Mars Direct and Semi-Direct.
Pre-land a laboratory with pressurized rover. The rover would have recycling life support, allowing for extended excursions from base.
The crew would ride in the Interplanetary Transit Vehicle: ITV. The ITV would be assembled in LEO, using ISS as construction shack. Add TMI propulsion stage, add the lander. This whole assembly would travel to Mars as one unit. Aerocapture into high Mars orbit. Then crew would descend in the lander. Yes, the lander would be habitat, with recycling life support.
For return, the MAV would act as the TEI stage. This allows for ISPP for the entire return to Earth. The ITV would aerocapture in to Earth orbit, then aerobrake down to LEO, rendezvous with ISS and dock. The ITV would use a single Dragon as escape pod, in case aerocapture fails. If not used, it would stay attached for the next mission. Since the ITV will dock with ISS, any vehicle capable of returning crew from ISS can be used. If Dragon is to be used, then the "escape pod" may as well be used for crew return. In this case the second crew would ride Dragon up to ISS, then dock that Dragon to the ITV as their escape for return.
A couple additional backup modes. Notice the ITV has food and life support for transit from Earth to Mars, and back. The lander/hab has food for the surface stay. But the lander will be attached during capture into Mars orbit. This means if a free return to Earth is necessary, all that food will be available for the trip. And life support in the lander/hab would be available as backup for the ITV.
Mars Direct has life support on the hab/lander, and ERV. My plan has life support on the hab/lander, ITV, and rover. So the rover is an additional layer of backup. Is that too much? Could recycling life support be designed more compact than currently on ISS? Compact enough for a minivan-size rover?
I propose 4 crew.

louis · 2015-04-29 05:13:13

Given we have been successfully operating solar powered vehicles on the surface, way beyond expected lifetime, I would suggest that we run with that. Electric motors are very simple.

RobertDyck wrote:

I was just doing some further research. The guys who designed the SAFE-400 nuclear reactor, designed a smaller one: HOMER-15. It has 15 kWt producing 3 kWe, with reactor system mass 214kg. The rover would require 2 of these, or one twice the size. In the discussion "Light weight nuclear reactor, updating Mars Direct", I quoted figures for ISS life support. For 4 astronauts that worked out to 1.8 kW. I found a website selling an electric light truck that consumes 8 kW normal, 15 kW max. And a Chinese electric light garbage truck, motor rated for 6.3 kW. That's for Earth. Could a pressurized rover the size of a minivan, operating in Mars 38% gravity, trundle along at 50 km/h (30 mph) on 4.2 kW electricity, plus 1.8 kW for life support?
Curiosity rover may be much bigger than Spirit or Opportunity, but Curiosity's nuclear power system generates 125 watts beginning of life, 100 watts end of life. This vehicle would have 6 kW. That's 60 times as much power.

louis · 2015-04-29 07:48:40

Interesting speculation on Space X plans for a Mars Mission:

https://www.youtube.com/watch?v=wGAvLSnQZN0

RobertDyck · 2015-04-29 07:52:38

Terraformer wrote:

3 kWe, for 214 kg? Would it not be better to investigate the use of strontium nuclear batteries? I'm sure they could achieve much better power density than that, and without the issues that typically arise when talking about nuclear reactors IN SPACE?

Ok. I looked at the nuclear battery on Curiosity. The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) generates 125 watts Beginning-Of-Life (BOL), 100 watts End-Of-Life (EOL), contains 4.8kg of plutonium, and total mass 45kg. Energy density is much lower than HOMER-15. Producing 6 kWe (EOL) would require 60 of them, totalling 2,700 kg. A pair of HOMER-15 reactors would total 428 kg. If you know something better, I'm all ears. (Or eyes on the forum)

RobertDyck · 2015-04-29 08:35:01

louis wrote:

Given we have been successfully operating solar powered vehicles on the surface, way beyond expected lifetime, I would suggest that we run with that. Electric motors are very simple.

The best currently available are NeXt Triple Junction (XTJ) solar cells. A couple companies make them, space rated cells typically come from one of two American companies. I'll quote from Spectrolab, owned by Boeing for a few years now.

They convert sunlight to electricity with high efficiency. Improved Triple Junction (ITJ) converts 26.8% BOL, 22.3% EOL, @ load voltage, min. average efficiency @ maximum power. Ultra Triple Junction (UTJ) converts 28.3% BOL. NeXt Triple Junction (XTJ) converts 29.3% BOL @ load, 29.5% BOL @ max power. Their specifications sheets don't list EOL for UTJ or XTJ, so I have to assume the ratio of BOL to EOL holds. That's an assumption, but all I have available. That means EOL will produce 22.3 / 26.8 = 83.2% power EOL. All of these numbers are at 28°C, I don't have temperature performance.

Panels include cells, interconnections, and cover glass. XTJ panels with area > 2.5 m^2 @ 28°C BOL, produce 366 W/m^2. With 3 mil ceria doped coverslide, they mass 1.76 kg/m^2. With 6 mil ceria doped coverslide, they mass 2.06 kg/m^2.

Note this is in Earth orbit. Mars receives 47% illumination because it's farther from the Sun. I don't know how much the atmosphere further attenuates illumination, so let's ignore that. Assume power generated on Mars will be 47%.

Generating 6 kW EOL will require 6000 / 366 / 0.832 / 0.47 = 41.9 m^2. If we use the thin coverslide, that works out to 41.9 * 1.76 = 73.776 kg. Then add support structure. And adjust for dust contamination. And adjust for temperature.

And the rover will only be able to operate during daylight. The point of recycling life support is to last more than 2 weeks. If it will only operate for less than 2 weeks from base, then we could use bottled oxygen and a regenerable sorbent. Sorbent regeneration would still take some power, but at least no electrolysis unit. The rover could refill the oxygen bottle at base. But if the rover doesn't have recycling life support, then it can't be life support backup for the lab. Trade-offs. But you will at least require life support at night. That means generating surplus power to charge batteries, then run life support from batteries. If it's used for extended sojourns from base, then you want to drive at night. That means more power from batteries. So double solar panel area, and mass. And add mass for batteries.

Robert Zubrin's Mars Direct used solar for the hab. It would fold up solar panels for entry/descent/landing. Then those same solar panels would be deployed on the surface with cables to the hab. It would land within walking distance of the ERV, which would have a nuclear reactor. So two different power sources. I like that mix, let's stick to it. But the rover?

Last edited by RobertDyck (2015-04-29 08:41:40)

RobS · 2015-04-29 08:37:33

The Youtube video offers useful numbers and facts that are hard to find. But I am not as confident as he is. Space X has backtracked about the size of the Raptor engine, for example. Space X does not yet have a Mars plan; they are searching for one and have ideas, but not a plan. There is also the question of phase 1, phase 2, etc.; it may be that Musk wants to send 100 colonists at a time in a vehicle of the size the video suggests, but that may prove to be phase 3 or phase 4. If phase 1 is more of the sort of scale that NASA would imagine (and therefore could support) we may be talking about something with 4-6 crew launched by 3 Falcon Heavies or 2 upgraded Heavies, closer to the size of Mars Direct. It makes sense to start with something of that scale; one could get government support for it and one could start with a small crew who would scout out outpost sites, scout for water, etc. Besides, it is easier to start with smaller scale technology demonstrators, and cheaper to start with rockets for which there is existing demand. No one in the world wants a launcher than can put 1,000 tonnes into LEO, and the cost of developing it would be immense even for Space X. So it seems to me they must be thinking incrementally. That is also what they did with Falcon 1, 5, 9, and Heavy.

RobS · 2015-04-29 11:22:05

Returning to your question about powering the rover, Bob, I have a different suggestion. I doubt you could do very much with a vehicle powered by 4 kw (or 6 kw without running life support). It would be slow, and that would be risky if you go out several hundred kilometers and someone has a medical emergency. It would also be very underpowered if you wanted to run a crane or push rocks or fill a trailer with dirt.

So I suggest you leave the reactors at base, where their radiation can be mitigated with dirt shielding anyway (I'm not sure you want two reactors on board your rover; what's the radiation risk, anyway?). I'd haul several hundred kilograms of liquid methane and oxygen along instead, which could even provide some shielding against cosmic radiation if they were on the roof. I'd run them through a small internal combustion engine (basically, a natural gas electric generator like the kind you can buy for your house or a small business) and power your six wheels with six independent electric motors. I don't know the mass, but it should be easy to look up natural gas electric generators. it's a well developed technology. Let us say we had two 30-kilowatt electric generators, one for normal use, one for backup if the first failed, and both for unusual situations (fast dashes back to base, powering a crane or a bulldozer, etc.). I bet they'd mass somewhere around 30 kg each. If methane-oxygen fuel cells were available--they're being developed--they'd be even better, because of their higher efficiency.

Other advantages of this arrangement: you'd have plenty of oxygen and water for the crew (you'd capture the water from the engine exhaust). If you brought along 100 kg/50 square meters of solar panels, you'd be able to power your life support system in an emergency. Double that mass (200 kg/100 square meters) and you'd produce 100 kilowatt-hours of power every day. If you had a small sabatier reactor and electrolysis system on board, you could manufacture methane and oxygen from Martian CO2 and stored water and that would allow limited, slow movement if you ran out of methane and oxygen (100 kw-hrs of batteries would do the same thing).

Back at the base, the reactors would continually put out power that could be used to make methane and oxygen, which would be stored up for the next expedition.

P.S.: You noted in your posting that 42 square meters of solar panels would put out 6 kilowatts of power, but that's when the sun is overhead. Since the relationship between the circumference of a sphere and its diameter is diameter times pi, that means a stationary (unsteerable) solar panel would put out about a third that much per square meter over a 24-hour period (almost 2 continuous kw). It's easier to think in terms of kilowatt-hours. 42 square meters would put out about 46 kilowatt-hours of power per day (in orbit facing the sun it'd produce 144 kilowatt-hours; divide that by pi). If 42 square meters masses 84 kg (roughly) and produces 46 kwhr (let's round it down to 42 kwhr to keep things even), than means you are getting roughly 1 kwhr per square meter and per 2 kg of solar panels. This is useful because Zubrin wanted a 100 kge (2400 kwhr) reactor. With solar panels, you'd need 2400 square meters and 4,800 kg of panels. That's about the mass of the reactor he was proposing (which was larger than necessary) . Of course, in dust storm season you'd get about 1/6 as much power, I think. But this gives us a good idea of the capacity of solar energy to power a Mars base.

GW Johnson · 2015-04-29 11:38:28

One-way shots of small things to Mars are fairly easy with the modest rockets currently flying.

Even unmanned, getting something back from Mars is quite difficult. It's the exponential behavior of delta-vee and mass ratio that is the killer.

Worse yet with men because (1) it also must be 2-way, and (2) because of all the extra mass associated with life support, radiation protection, and prevention of microgravity disease. There's sort of 3 disparate design arenas there.

For one shot delivery of smaller items, you can get pretty good landing accuracy once the first item is on the ground with a transponder. It'll be better accuracy from LMO than for direct entry trajectories. That's just inherent. If you don't get blown around too badly while hanging on a chute, your CEP could be 100's of m instead of few-to-several km.

If you do get the small CEP, you have to start worrying about proximity damage effects from rocket blast and thrown rocks. There's ways to handle it, though. But damage if you neglect it.

That's what the Red Dragon idea seems to be all about: one-way delivery of relatively small stuff (what Dragon can carry inside). The one thing I've seen about Red Dragon and sample return was a tiny rocket with gram-sized payload to return the sample. This would be carried inside the Dragon, and somehow put outside for launch. Between fading memory and the unclarity of a concept, I don't know any details.

For bigger stuff, like 1-3 guys in a lander of some kind, plus supplies and equipment for some sort of surface stay, the Mars EDL difficulty starts to "bite" you. It's not really about heat protection during hypersonics (although the solutions get into that), it's about getting from local Mach 3 down to low subsonic before you smack the ground. That's complicated by factor-2+ variation in surface "air" density around Mars, too.

If any of the various inflatable/flexible/extendible heat shield ideas can be turned into well-characterized technologies fully ready-to-apply, that's one way to raise the M3 altitude by drastically lowering ballistic coefficient. Another way is simple supersonic retropropulsion, starting as you get close to M3, right after the big deceleration and heat pulse. Or both. Get that altitude up, then a series of chutes becomes feasible, even in large sizes. It's merely a matter of time-to-impact. Longer is better.

I rather doubt we'll get to a chute technology allowing reliable landings of big payloads at under-20 mph speeds on Mars, the "air" is just too thin and too variable in density. But there's nothing wrong with turning some rockets on, in the last seconds, to cushion the landing. That is relatively proven and ready-to-apply, at least for some militaries, although apparently not any of the space folk.

Spacex is beginning to prove out the supersonic retropropulsion rather nicely with its stage descents, and Blue Origin is going to add to that, I think.

Spacex's SuperDraco thrusters will prove out chuteless landings fairly soon here on Earth with Dragon v2. Ithink Blue Origin wants to rocket-land its stages.

This stuff will be made ready rather soon, at least in Spacex's case, because they're supposed to start flying astronauts with it in about 2 years. That may well beat the readiness date for the new heat shield technologies. Who knows?

Point is, we should be ready to freeze big Mars lander designs in about 4-5 years based around one or more of these technologies. Why screw around with flyby missions, when by the time you could go, you'll have the means to land?

I just thought I'd ask the silly question.

GW

Terraformer · 2015-04-29 11:54:37

RobertDyck wrote:

Terraformer wrote:
3 kWe, for 214 kg? Would it not be better to investigate the use of strontium nuclear batteries? I'm sure they could achieve much better power density than that, and without the issues that typically arise when talking about nuclear reactors IN SPACE?
Ok. I looked at the nuclear battery on Curiosity. The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) generates 125 watts Beginning-Of-Life (BOL), 100 watts End-Of-Life (EOL), contains 4.8kg of plutonium, and total mass 45kg. Energy density is much lower than HOMER-15. Producing 6 kWe (EOL) would require 60 of them, totalling 2,700 kg. A pair of HOMER-15 reactors would total 428 kg. If you know something better, I'm all ears. (Or eyes on the forum)

I didn't say anything about using plutonium. Or indeed thermoelectrics. Strontium is relatively cheap, puts out more power, and betavoltaics are being developed at the moment. See my post in life support. I expect that we could get at least 50 W/kg from them.

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