Yet another Mars architecture

RobertDyck · 2015-04-29 12:29:13

Actually, the first version of Mars Direct was developed in late 1989 and early 1990. The SP-100 nuclear reactor was just under development at that time. Dr. Zubrin believed an 85 kW reactor could be made with corresponding lower mass. His original 1990 version included that. The team developing SP-100 completed their work in 1992. They found that reducing power from 100 kW to 85 kW did not reduce mass at all. That was a disappointment. So Dr. Zubrin had to alter his plans to use the 100 kW reactor.

When I posted "updating Mars Direct", I started by suggesting the SAFE-400 nuclear reactor. Also 100 kW electric, but lower mass. SAFE-400 is much newer, development completed in 2007. Can't blame Dr. Zubrin because SAFE-400 wasn't even conceived in 1989/1990. Development of Safe Affordable Fission Engine (SAFE) began in 1995, at Los Alamos National Laboratory and the Marshall Space Flight Center. First there was SAFE-30, then SAFE-300, then SAFE-400. Getting larger and more efficient with each improvement.

louis · 2015-04-29 13:45:46

I agree Rob - I think the MCT may be a few phases down the line...you're not going to immediately land 100 colonists at a location without highly developed ISRU facilities are you? And there would be little pointing in using the huge MCT to take a small team. So I think something like the Red Dragon is much more likely to be the first phase.

RobS wrote:

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 13:52:28

Bigger than Red Dragon. Musk will want to land a capsule of humans on Mars with a system for launching it back to orbit, I suspect. So it'll need maybe a 25-tonne launch stage (2 tonnes of structure, plus either a tonne or two of hydrogen or 5 tonnes of methane). It'll also need maybe 10 tones of consumables and solar panels, for safety.

kbd512 · 2015-04-29 16:18:21

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)

A SAFE-400 reactor weighs 512kg and provides 100kWe and presumably provides waste heat that can be used to keep metallic components of the rover at more reasonable temperatures than Mars' frigid environment would otherwise allow. A 100hp electric motor would presumably provide all the speed required for your rover, would it not?

Shielding would have to be increased since the astronauts may spend a fair amount of time in close proximity to this thing, but it does solve a lot of other problems with power requirements for redundant ECLSS and reserve power for charging batteries and dashes to rescue injured or wayward astronauts.

If there was a way to halve that weight and reactor volume, presumably using better nuclear fuels that requires far less fissile material to achieve criticality and a far lighter moderator, you'd really be on to something.

Just a thought.

BTW, Rob, how much IMLEO for your plan?

louis · 2015-04-29 17:54:21

My suggestion for powering a Rover would be a solar array suspended above the roof of the vehicle (maybe 4x4 metres and a solar array "train" (about 10 metres by 6) that would be dragged behind the vehicle, together with solar panel cladding on the sides of the vehicle. . There would of course be ultra-lightweight solar panels. For the train I would suggest the PV panelling be set on a very lightweight framework of nylon rods or some other light material. Where the rods intersect there could be fixed hardy inflated plastic balls that would allow the train to bob over the rock strewn ground. This should avoid problems with snagging.

All those solar arrays should generate about 15 Kws average through the core daylight hours which should be enough to power the vehicle.

On board would be a 30 KwH battery. When the vehicle doesn't need the direct power from the solar arrays, they would replenish the battery.

RobS wrote:

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.

RobertDyck · 2015-04-29 19:12:40

kbd512 wrote:

If there was a way to halve that weight and reactor volume, presumably using better nuclear fuels that requires far less fissile material to achieve criticality and a far lighter moderator, you'd really be on to something.

Well, U-233 has smaller critical mass than U-235. Almost none in nature, you have to make it from thorium in a breeder cycle. Plutonium Pu-239 has even lower critical mass. That's obviously made in a breeder reactor. And then there's our good friend Americium Am-242m. It doesn't have as much energy density as plutonium; I think it's about as good as uranium, but has the lowest critical mass of all. The catch is it's only produced as a byproduct of making plutonium, so it's expensive. And any new reactor design adds delay and expense to the project. See GW Johnson's comments about a new engine for aircraft development, or new launch vehicle for a space exploration program.

SAFE-400 already uses light moderator, and rotating neutron reflectors for control and to reduce critical mass. And they really improved the energy conversion system. SP-100 produced 2,000 kWt to produce 100 kWe, while SAFE-400 produces 400 kWt to produce 100 kWe. That's primarily how they reduced mass.

I looked for a light truck as comparison. I found this one: alkè ATX210E. It's small, little bigger than a golf cart, but rated for roads. Can carry 635kg and tow 2,000kg, draws 8kW under normal conditions, 14kW max, with energy recovery brakes. There are lots of electric vehicles today, this is just one I found with a quick Google. You would need one with pressure hull, life support, able to pressurize/depressurize without destroying the hull (Apollo LM was only rated for 5 cycles), able to handle rough off-road conditions, reasonable speed, and designed for Mars gravity, temperature, and dust.

kbd512 wrote:

BTW, Rob, how much IMLEO for your plan?

My plan is described in detail in this thread. Detailed description on page 2, post #37. Click here
Updated launch requirements listed here
And another configuration focusing on stuff Congress favours listed here

That's a total of 4 options. Each describes 1st, 2nd, and 3rd missions, with subsequent missions looking like the 3rd. That was to build a base with lots of backups. GW Johnson raised a concern whether a second mission would happen if the first didn't complete a base, so each option could skip the 2nd mission, going straight to the 3rd. That means build the base with the 1st mission, then all subsequent missions go to that base.

SpaceNut · 2015-04-29 19:36:39

Landing the RV habitat for that follow up missions is a must at that point to increase the science and exploration from the base. The scouting missions need to look for insitu materials to make use of for further base expansion.

RobertDyck · 2015-04-29 20:58:43

I was thinking of a smaller pressurized habitat. Basically enough for all 4 astronauts in spacesuits to sit in seats. Think of a minivan, not an RV.

Initially I tried to reduce mass to the minimum. That plan didn't include a pressurized rover at all, just an unpressurized rover with the habitat. As Robert Zubrin said, in case the hab lands too far from the ERV (or MAV). Instead have an open, unpressurized rover similar to a lunar rover but designed for 4 astronauts. And carry a pressure tent. Just a tent. I came up with a simple tent design for one astronaut, that used the spacesuit PLSS for life support. A 2 meter diameter dome tent, with aluminized polymer walls, and a polymer window with flap to cover the window. The floor would be an integrated air mattress; obviously round for a dome tent. With multiple layers of polymer within the air mattress, and a light fan (about the size of a computer case fan) to circulate air within the air mattress. This fan would be controlled by a thermostat. A tent on Mars would lose more heat to the ground than atmosphere, so the fan is for temperature control. Carry a bottle of pressurized oxygen to inflate the tent. The tent would only pressurize to suit pressure. And connect the PLSS to the rover to recharge the battery, and continuous power when sleeping at night. CO2 sorbent in PLSS for EMU on ISS is no longer lithium hydroxide; now they use silver oxide sheet metal. That can be regenerated simply by baking CO2 out in an electric oven. But I have a paper from the mid-90s, from the NASA technical support server, about silver oxide granules regenerated with a microwave oven. The granules have more surface area per unit mass of silver oxide, so it reduces mass. And the granules are compatible with microwave regeneration, which uses less power than an electric oven. (duh!) So include a portable regeneration oven on the rover; about the size of a toaster oven. That means losing CO2, but this is Mars, the hab can always harvest more CO2 from Mars atmosphere. The current EMU has activated charcoal integrated in the same cartridge, it's called "contaminant control cartridge". Activated charcoal removes bad smalls, and it can be thermally regenerated as well. So the whole cartridge is regenerated in an oven.

By the way, "activated charcoal" is just charcoal foam. It has very thin walls to the bubbles of the foam to maximize surface area. Bad smells bond to the surface.

How about tenting instead of RV-ing?

RobS · 2015-04-29 21:21:59

Robert, why not make the "tent" an inflatable pressurized rover. If the floor rests on the chassis, it won't be in contact with the ground. The design would be a bit different; you'd need big windows in front for driving.

Terraformer · 2015-04-30 04:42:32

How much does the radiation protection for the reactors mass, though? At a base you can cover it in regolith before starting it up, but that's not really an option for a rover.

A betavoltaic system using Sr-90, on the other hand, doesn't produce gamma radiation, only beta, so you should be able to use quite light shielding. The power density of the fuel (?) itself is about 0.47 kW/kg; of course, when packaged with it's shielding and energy conversion systems, that will be quite a bit lower. But We could probably get 50 W/kg out of it? Such batteries are in development at the moment, and it's a lot less politically troublesome. Strontium-90, being a common byproduct of nuclear reactors, is also relatively cheap (especially compared with plutonium). There is absolutely no need to mess around with nuclear reactors.

louis · 2015-04-30 06:24:16

RobS wrote:

Bigger than Red Dragon. Musk will want to land a capsule of humans on Mars with a system for launching it back to orbit, I suspect. So it'll need maybe a 25-tonne launch stage (2 tonnes of structure, plus either a tonne or two of hydrogen or 5 tonnes of methane). It'll also need maybe 10 tones of consumables and solar panels, for safety.

Why couldn't you land the ascent vehicle separately?

RobS · 2015-04-30 07:07:53

You could, but that may create a 1 in 1000 chance of "loss of mission" (i.e., something lands too far away and everyone dies).

louis · 2015-04-30 13:19:03

RobS wrote:

You could, but that may create a 1 in 1000 chance of "loss of mission" (i.e., something lands too far away and everyone dies).

Not if you land it first. The descent vehicle only follows if you have established the ascender has landed safely and in good condition.

RobS · 2015-04-30 13:29:23

Then you have to worry about the landing vehicle with the people landing in the wrong place, though if the ascent vehicle for the next mission is coming along shortly, you could always direct it to the lander.

kbd512 · 2015-04-30 13:35:45

RobertDyck wrote:

I was thinking of a smaller pressurized habitat. Basically enough for all 4 astronauts in spacesuits to sit in seats. Think of a minivan, not an RV.
Initially I tried to reduce mass to the minimum. That plan didn't include a pressurized rover at all, just an unpressurized rover with the habitat. As Robert Zubrin said, in case the hab lands too far from the ERV (or MAV). Instead have an open, unpressurized rover similar to a lunar rover but designed for 4 astronauts. And carry a pressure tent. Just a tent. I came up with a simple tent design for one astronaut, that used the spacesuit PLSS for life support. A 2 meter diameter dome tent, with aluminized polymer walls, and a polymer window with flap to cover the window. The floor would be an integrated air mattress; obviously round for a dome tent. With multiple layers of polymer within the air mattress, and a light fan (about the size of a computer case fan) to circulate air within the air mattress. This fan would be controlled by a thermostat. A tent on Mars would lose more heat to the ground than atmosphere, so the fan is for temperature control. Carry a bottle of pressurized oxygen to inflate the tent. The tent would only pressurize to suit pressure. And connect the PLSS to the rover to recharge the battery, and continuous power when sleeping at night. CO2 sorbent in PLSS for EMU on ISS is no longer lithium hydroxide; now they use silver oxide sheet metal. That can be regenerated simply by baking CO2 out in an electric oven. But I have a paper from the mid-90s, from the NASA technical support server, about silver oxide granules regenerated with a microwave oven. The granules have more surface area per unit mass of silver oxide, so it reduces mass. And the granules are compatible with microwave regeneration, which uses less power than an electric oven. (duh!) So include a portable regeneration oven on the rover; about the size of a toaster oven. That means losing CO2, but this is Mars, the hab can always harvest more CO2 from Mars atmosphere. The current EMU has activated charcoal integrated in the same cartridge, it's called "contaminant control cartridge". Activated charcoal removes bad smalls, and it can be thermally regenerated as well. So the whole cartridge is regenerated in an oven.
By the way, "activated charcoal" is just charcoal foam. It has very thin walls to the bubbles of the foam to maximize surface area. Bad smells bond to the surface.
How about tenting instead of RV-ing?

Rob,

I still think a much larger and heavier rover, like the MTVL concept I put forth, is required. All the minimum mass surface mobility options seem to have a lot of ways in which they could fail. I totally blanked on the fact that Martian dust storms can just about kill solar panel output. A reactor like SAFE-400 may be required for my rover concept, even though I was trying to avoid placing a reactor anywhere near the astronauts. I didn't budget for the extra mass of the reactor, either. The F9H + SEP tug + MTVL + EDL hardware solution I spec'd were designed to fulfill requirements for habitation by two astronauts for approximately 250 days and the propulsion and descent solution was designed to land ~17t on Mars using hardware that's already in the development pipeline. The mass of my solution is right at the limit of what can reasonably be done, though, and doesn't leave much room at all for mass increases.

I don't know what the right answer is, but I prefer solutions that don't test the limits of human endurance or technological innovation. Actually living in an inflatable tent or a rover the size of a minivan is a lot different than briefly using one for temporary shelter or transportation.

RobertDyck · 2015-04-30 16:30:21

For the tent, I recommend aluminized PCTFE, sold by Honeywell under brand name Clarus. Aluminized polymer is a long mature technology. Aluminized mylar is used for multilayer insulation for spacesuits. I want to use PCTFE because it doesn't become brittle until 100°C colder than the coldest point ever recorded on Mars, highly UV resistant, the most impermeable to moisture of any known polymer, and any polymer more impermeable to oxygen cannot withstand the cold of a Mars night. This is the same stuff I want to use for an inflatable greenhouse. Aluminized to contain radiant heat. The air mattress would have multiple layers suspended within, with air gap between each layer. Windows on Earth use that to create multiple panes for heat insulation. Only the outside and inside panes have to be glass, the middle layers can be stretched plastic film. Similar idea to multilayer insulation, but of course it works a lot better in vacuum than Mars atmospheric pressure, or air mattress pressure. I talked with some engineers about heat control, this air mattress with built-in multilayer insulation ensures you don't get too cold. But engineers also raise the possibility of getting too hot, just from body heat. That's why I suggest an air channel from the bottom most layer of the air mattress to the top most layer. With nothing but a fan the size of a computer case fan to circulate air through that channel, and a thermostat to control the fan. A thermostat controlling a fan isn't exactly cutting edge. You may want to add a flocked top for astronauts to crawl/sit/lie on, so they don't slide around. The same surface as a camping air mattress.

louis · 2015-04-30 18:26:24

No you don't. We've been over that. With transponders on the surface the chances of failure to land in the correct place are as near zero as anyone could want.

You could equally argue that if you put the ascent vehicle with the descent vehicle you risk damage to both in a non-fatal accident, such that people could be left stranded without a functioning ascent vehicle (whereas with separate landings if there is some damage to the descent vehicle, you still have your ascent vehicle ready to go).

RobS wrote:

Then you have to worry about the landing vehicle with the people landing in the wrong place, though if the ascent vehicle for the next mission is coming along shortly, you could always direct it to the lander.

GW Johnson · 2015-05-01 08:50:39

The ascent vehicle/descent vehicle safety issue can be avoided, but not at minimum mass. I think it is wrong to let minimum-mass as a constraint drive you into a corner with no outlet.

It is fairly easy to show that a single stage chemical rocket vehicle capable of a two-way trip to the surface and back from LMO is quite possible. Its only drawback is low payload fraction, meaning many tons of fueled vehicle must be sent to Mars to land a handful of tons on the surface, and still ascend. There is no way around that.

But the advantage is that this vehicle could be refueled either on the surface or in LMO, and thus be reusable. It could make many such trips, very important for constructing some sort of base on that first mission. This becomes very attractive indeed, if in-situ propellant production proves practical at massive production rates.

The safety issue also goes away if you bring more than one of these landers to Mars with you. All it needs is a temporary-occupancy abort capsule for its command cabin, in which the entire crew rides. The other vehicle(s) can be rescue birds if descent or ascent abort becomes necessary. That capsule looks an awful lot like Red Dragon in its characteristics, actually.

The only problem with all of this is the mass of payloads that must be sent to Mars. Those are large enough to require assembly in LEO, not direct shots to Mars. No way around that, either.

But as launch prices have fallen from the $25,000-30,000/lb we had with shuttle to the $2500/lb we have with Atlas-5 and Falcon-9, and to the $1000/lb we will soon have with Falcon-Heavy, this is not an objection that would rule out this kind of mission architecture.

Only the high costs associated with SLS would preclude this approach as unaffordable.

So far, the only real downside I see to this approach is the need to do LEO assembly of those landers. Their diameters are typically around 10 m, too large to put together on the ground and ride up. The real choice is (1) do we need an SLS to launch these landers (a minority of the tonnage to send)? or (2) do we learn how to provide the assembly bay and spacesuits necessary to do on-orbit assembly of this type?

We have about 5-10 more years before we need to freeze the design for the Mars mission for the 2030-2035 time frame. Which do you believe would be easier to do in 5-10 years: (1) get SLS flying, or (2) learn how to do assembly in space and develop a supple spacesuit?

GW

Last edited by GW Johnson (2015-05-01 08:53:36)

louis · 2015-05-01 09:29:00

Couldn't agree more GW, it's all doable now with current technology. What's missing is political will. Musk may well subsitute entrepreneurial will for that. We will see.

GW Johnson wrote:

The ascent vehicle/descent vehicle safety issue can be avoided, but not at minimum mass. I think it is wrong to let minimum-mass as a constraint drive you into a corner with no outlet.
It is fairly easy to show that a single stage chemical rocket vehicle capable of a two-way trip to the surface and back from LMO is quite possible. Its only drawback is low payload fraction, meaning many tons of fueled vehicle must be sent to Mars to land a handful of tons on the surface, and still ascend. There is no way around that.
But the advantage is that this vehicle could be refueled either on the surface or in LMO, and thus be reusable. It could make many such trips, very important for constructing some sort of base on that first mission. This becomes very attractive indeed, if in-situ propellant production proves practical at massive production rates.
The safety issue also goes away if you bring more than one of these landers to Mars with you. All it needs is a temporary-occupancy abort capsule for its command cabin, in which the entire crew rides. The other vehicle(s) can be rescue birds if descent or ascent abort becomes necessary. That capsule looks an awful lot like Red Dragon in its characteristics, actually.
The only problem with all of this is the mass of payloads that must be sent to Mars. Those are large enough to require assembly in LEO, not direct shots to Mars. No way around that, either.
But as launch prices have fallen from the $25,000-30,000/lb we had with shuttle to the $2500/lb we have with Atlas-5 and Falcon-9, and to the $1000/lb we will soon have with Falcon-Heavy, this is not an objection that would rule out this kind of mission architecture.
Only the high costs associated with SLS would preclude this approach as unaffordable.
So far, the only real downside I see to this approach is the need to do LEO assembly of those landers. Their diameters are typically around 10 m, too large to put together on the ground and ride up. The real choice is (1) do we need an SLS to launch these landers (a minority of the tonnage to send)? or (2) do we learn how to provide the assembly bay and spacesuits necessary to do on-orbit assembly of this type?
We have about 5-10 more years before we need to freeze the design for the Mars mission for the 2030-2035 time frame. Which do you believe would be easier to do in 5-10 years: (1) get SLS flying, or (2) learn how to do assembly in space and develop a supple spacesuit?
GW

RobertDyck · 2015-05-01 14:10:46

We can do on-orbit assembly at ISS. No assembly bay required. Just don't drop any tools. Assembly crew for ISS itself have already proven they can do it. A true HLLV is really nice to have, for example you can launch a ERV or MAV by direct launch from KSC to the surface of Mars. No stop in LEO, no stop in Mars orbit, no orbital assembly. Just direct launch and direct atmospheric entry, like Curiosity et al. But we can make do with F9H. One reason I keep talking about SLS is just that Congress likes SLS.

SpaceNut · 2015-05-02 07:12:35

I could not agree more with

louis wrote:

GW, it's all doable now with current technology. What's missing is political will. Musk may well subsitute entrepreneurial will for that. We will see.

Also a deep set of pockets to fund the mission.

I think that we are all on the same page with a mars mission in that we need to trim the Mass to the least without effecting mission out come. With the landing Mass on Mars and the launched Mass to Earth orbit must be well managed within a cost parameter as well as launch vehicle count.

As far as SLS let it have its place in large single launched items but keep it to just the one so as to keep a mission affordable.

The concept vehicle RobertDyck is outlining for traveling around the surface with a partial or full tent or inflateable does make for a lighter landed Mass for that item and if we do the same for other large items we can get the total Mass for earth launch into something less in size than the SLS.

kbd512 · 2015-05-02 09:12:08

GW Johnson wrote:

The ascent vehicle/descent vehicle safety issue can be avoided, but not at minimum mass. I think it is wrong to let minimum-mass as a constraint drive you into a corner with no outlet.

If you have real surface mobility, which is only provided by a substantial pressurized rover, you can land up to a 100km off course and not suffer the consequences. Where do you draw the line and say, "No matter what your equipment does, this is your task and if you can't accomplish it, we can't help you." Obviously we have to repeatedly test the equipment under realistic operating conditions ahead of time. That can only be accomplished by sending the mission hardware to Mars.

GW Johnson wrote:

It is fairly easy to show that a single stage chemical rocket vehicle capable of a two-way trip to the surface and back from LMO is quite possible. Its only drawback is low payload fraction, meaning many tons of fueled vehicle must be sent to Mars to land a handful of tons on the surface, and still ascend. There is no way around that.

We certainly can do that if we deem it necessary, but is that really the best way to design ascent/descent systems?

GW Johnson wrote:

But the advantage is that this vehicle could be refueled either on the surface or in LMO, and thus be reusable. It could make many such trips, very important for constructing some sort of base on that first mission. This becomes very attractive indeed, if in-situ propellant production proves practical at massive production rates.

I don't think ISPP is nice to have. I think it's a hard requirement. Can we design vehicles that are reusable after reentry? We certainly could, but is that the best use of limited available funding? How much would it cost to design a reliable reusable lander versus the cost of another comparatively inexpensive F9H flight? If F9H achieves first stage reusability, will it be less expensive to launch the F9H again?

GW Johnson wrote:

The safety issue also goes away if you bring more than one of these landers to Mars with you. All it needs is a temporary-occupancy abort capsule for its command cabin, in which the entire crew rides. The other vehicle(s) can be rescue birds if descent or ascent abort becomes necessary. That capsule looks an awful lot like Red Dragon in its characteristics, actually.

Safety issues don't exist, except between peoples' ears. On the other hand, failures in complicated systems like propulsively landed reentry capsules are very real. The frequency of the failures can be mitigated by thorough testing in as many different conditions as is feasible and not making any particular piece of hardware more complicated than it absolutely has to be to achieve design objectives.

The problem I see with the combination descent/ascent vehicles is that this type of EDL solution attempts to account for such a wide range of potential problems that the end result is fairly complicated. I think it's unrealistic to think a thorough testing program can be conducted with one or perhaps two descents/ascents at Mars.

GW Johnson wrote:

The only problem with all of this is the mass of payloads that must be sent to Mars. Those are large enough to require assembly in LEO, not direct shots to Mars. No way around that, either.

The number one problem we currently contend with is the incredible cost of getting to LEO. If launch costs were between a quarter and half of what F9H promises to deliver, we'd have no problems sending the more massive combination descent/ascent vehicles to Mars. Heck, a lot of problems are solved if launch costs drop to between $250/kg to $500/kg.

GW Johnson wrote:

But as launch prices have fallen from the $25,000-30,000/lb we had with shuttle to the $2500/lb we have with Atlas-5 and Falcon-9, and to the $1000/lb we will soon have with Falcon-Heavy, this is not an objection that would rule out this kind of mission architecture.
Only the high costs associated with SLS would preclude this approach as unaffordable.

SLS was clearly designed to maintain STS status quo, with respect to launch costs and complexity. There are some payloads, due to dimensions or mass, that would best be lofted with SLS. As long as we have SLS, we may as well use it where it makes other aspects of space exploration, like orbital assembly, less complicated.

GW Johnson wrote:

So far, the only real downside I see to this approach is the need to do LEO assembly of those landers. Their diameters are typically around 10 m, too large to put together on the ground and ride up. The real choice is (1) do we need an SLS to launch these landers (a minority of the tonnage to send)? or (2) do we learn how to provide the assembly bay and spacesuits necessary to do on-orbit assembly of this type?

We're already assembling the MTV at ISS, so why not assemble the landers there, too? If there wasn't such a fixation on landing the astronauts together, this wouldn't be a problem. If everyone involved can't get past that fixation, then ISS assembly is the solution.

GW Johnson wrote:

We have about 5-10 more years before we need to freeze the design for the Mars mission for the 2030-2035 time frame. Which do you believe would be easier to do in 5-10 years: (1) get SLS flying, or (2) learn how to do assembly in space and develop a supple spacesuit?
GW

We don't have to learn how to do assembly in space. We have decades of experience with that. SLS will fly, the only question is whether or not the solution will be retained for those rare instances where using SLS solves so many other problems that the cost penalty for flying it is worth it. SLS has plausible uses for real space exploration, even if it is prohibitively expensive to actually use it, whereas Orion has no plausible uses for anything other than astronaut transfer to the moon.

If SLS Block IB can be developed in any reasonable amount of time, I think a SLS upper stage derived MTV (Skylab II) should be flown to ISS for testing. Although Skylab II is not a minimum mass and volume ITV design, such as the ISS derived hardware DSH is, it makes the transit to and from Mars a lot more comfortable for the astronauts.

We do need a MCP suit for EVA's on Mars. The Z-2 suit will suffice for exoatmospheric EVA's. Development programs for both of those suits could be completed in five years with sufficient funding.

Last edited by kbd512 (2015-05-02 09:13:19)

RobS · 2015-05-02 09:57:11

The sort of reusable lander/MAV I imagine would be similar to the dragon capsule complete with its current trunk, but the trunk would continue the slanted sides of the capsule and engines similar to the super dracos would be in the trunk. The trunk would contain methane and oxygen tanks and the "super dracos" would use that combination instead. The trunk would need to be large enough to store about 25 to 30 tonnes of propellant. The engines would fire off-axis from the sides of the trunk both for powered landing and for ascent to Mars orbit. Firing the engines off axis would produce a lower effective ISP than a true vertically firing engine, but would allow the engines to survive entry into the Martian atmosphere at orbital speeds.

The way you would test this combination is on Earth. The delta-v of the "capsule" and its trunk mounted engines would be about 4 km/sec (the velocity necessary for achieving Martian orbit). You'd use the trunk as the capsule's second stage for reaching facilities in low Earth orbit. In low Earth orbit, you could refuel the trunk's tanks and the trunk's delta-v would be sufficient to put the capsule and trunk in a high lunar orbit or transport it to L1. Refueling it again there, and it could land on the lunar surface. Another refueling and it could return to low Earth orbit. So the capsule/trunk combo could be used extensively, tested extensively, and a lot of experience could be acquired before it was ever sent to Mars. If the methane/oxygen engines were firing at a 75 degree angle (rather than 90 degrees or straight along the vehicle's axis) the effective ISP for methane/oxygen would be about the same as kerosene/oxygen when firing on-axis. If we can make methane on the moon (the cold traps probably have CO2 as well) we'd have an effectve reusable lunar transportation system as well as a reusable Mars transportation system.

kbd512 · 2015-05-02 11:37:42

Put another way, under normal operating conditions, the crew is aboard the descent/ascent or descent and ascent vehicles for less than 24 hours. The minutes spent during descent and ascent are critical to the outcome of the mission, but should we devote a major portion of available funding to those few minutes because of what might happen afterwards or account for what might happen afterwards with the design of the rest of the mission hardware?

If there's contention about what our general landing accuracy on Mars is or contingency scenarios that have to be accounted for, should we design a propulsively landed solution that necessarily has significant mass and complexity, or should we develop surface transportation solutions that can retrieve our astronauts if they land off target? What solution has the lowest level of technical complexity, small single man capsules with pressured rovers to retrieve astronauts who land off course or a multi-person propulsively landed capsule that lands the entire crew in one operation? What problems or contingency scenarios does landing the entire crew at the same time solve?

Obviously the propulsively landed solution is capable of some degree of course correction to land as near to the target as feasible, but what happens if even that solution lands a little off target? For example, let's say that a propulsively landed multi-person capsule lands 10km from where intended. Does that mean our astronauts have to carry the oxygen and water to walk back to the habitat module? I think you still need a rover of some kind to retrieve the astronauts. You don't need a pressurized rover and perhaps don't even need a rover that carries the astronauts, but you still need a small robotic rover to carry oxygen and water.

For any real surface exploration effort, a pressurized rover that provides some measure of shielding against SPE's is a requirement. Supposedly, we're going to Mars to explore. A substantial pressurized rover is therefore a requirement if we're to accomplish that stated mission objective.

Establishment of a base on Mars is a far future goal that would require some level of cooperation between the various space agencies. It would be really nice to have, but it's not required for surface exploration and shouldn't stand in the way of a surface exploration mission.

RobS · 2015-05-02 13:16:46

I suppose we can come up with a half dozen scenarios--however unlikely--where the crew might need more than 24 to 48 hours in the ascent or descent vehicle before docking or rescue. So I am very worried about the idea of a minimum-mass vehicle.

I think Zubrin calculated that with a hydrogen-oxygen third stage, a Falcon Heavy could launch 13 tonnes on a Mars trajectory and presumably can land about 8 or 10 tonnes on the surface. That means two Falcon Heavy launches can land about twice that much; one would launch a trans-Mars injection stage and the other would launch a smaller stage and the payload. So even with a non-reusable Heavy, $200 million will get into LEO 18 to 20 tonnes destined for the Martian surface. So we certainly can get a pressurized rover and a good-sized hab on the surface. For that reason, I think we can afford a decent sized rover, a reasonable sized pressurized rover, and a reasonable sized ascent/descent vehicle.

But I may be wrong.

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