New Mars Forums

Hi GW

It's ever a pleasure to hear you.

That's your opinion, but not a fact. Please don't confuse opinion with fact. The only real fact is that we still don't know.
It may be or may be not. So if you don't want to risk, you have to add a new centrifugal module to the ISS and spend another 10-15 years studying the effects of 3.9 G plus weighted suits on human body, before performing your mission. But in the end, probably the orbit-to-orbit modular spinning ship will cost less.

Well, I think we can do better, but however 22 tons are acceptable to have in return one MWe of continuous power.
I'd rather perform the BFR mission in a Zubrin-like fashion, sending first an unmanned BFS with LH2 and a nuclear reactor on an unmanned rover, that is lowered on the ground (a reactor on a rover is easier to deploy unmanned than a big solar array, but if you know a good way to do the latter is OK the same). The second manned BFS will go to Mars only when the refueling of the first is complete, so we will not bet the life of the crew on the success of the propellant production.
Having an unmanned ship, which takes all the risks, we don't need a backup reactor and can spare mass for other stuff.

Los Alamos one-megawatt-heat-pipe nuclear reactor has a weight of only 493 kg, and it's very sturdy and reliable having very few moving parts.
You can dig a 20-meter-deep hole 500 m far from the landing site, tow the reactor there with the rover and do the start-up when you are back.
Producing 1100 tons of propellant is not a piece of cake and you need a robust power source able to work continuously day and night. Take in mind that martian sand-storms cannot topple a lander like in the movie "The Martian", but they can last for a month obscuring the sun, drastically reducing the power supply to your life support and the ISPP-device. That's why you need one nuclear reactor, or rather two of them - one working and one for back-up - for your first mission.

On Mars surface boil off would be much more than in space, so you probably need a good cryocooler, which imply a nuclear reactor, but you surely need a megawatt range nuclear reactor if you want to produce 1100 ton of propellant. So the cryocooler might be the best solution.

We still don't now what is the right genetic pattern. In the next fifty years we can also have space-compliant GM astronauts, who will be microgravity and cosmic-ray resistant. The problem is only about WHEN do you want to do your Mars mission.
If you want to do it in 2070, it may be OK. But if you want to do it in 2030-35, you have to realize that we still don't know many issues, so a correct precautionary principe might be to assume the worst-case scenario as true and to use the state of the art of the current technology to cope with it (read spinning ship).
It's a question of ethic: if you want to spare the money of a spinning ship, you have to delay your mission to 2050-60 and use these years to gain a better knowledge of the many microgravity diseases and how to cope with them.

I never said there is no water: I think that there is water and probably we will find even some life form. I only said that you cannot bet on it until you have the certainty that water is really there. So for the first mission it's wiser to bring hydrogen from Earth, like also Zubrin had planned for his Mars Direct.

If you want to bring hydrogen from Earth, you have also to consider the boil-off losses. So you have to bring more hydrogen than you have calculated and this inevitably reduces the amount of payload you can deliver on Mars surface. Sure you can project and develop a new zero-boil-off cryocooler able to work either in the void or on Mars surface, but this takes years of R&D, and if it breaks down you surely lost the crew (take in mind that even Robert Zubrin didn't bet the lives of the astronauts on ISRU: in fact, he proposed to send an unmanned rocket and send the crewed one only WHEN and IF the refueling had been completed.)

I'm not so sure that BFR will be the final solution for space exploration.

The rocket equation says NO.
You cannot do a direct entry on the moon, using the atmospheric drag to kill the delta-V, like on Mars, and you have to do all the maneuvers propulsively.
The outward journey from LEO to Moon surface takes 5.9 km/s of delta-V and the inward journey from Moon surface to TEI takes 2.8 km/s. You enter from TEI in the Earth atmosphere, using the drag to slow down, but then you needs almost 1 km/s for a safe propulsive landing, so the total delta-V of the mission is 9.7 km/s.
The BFR has an empty mass of 85 ton, brings 1100 tons of LOX-LCH4, and its rockets have a vacuum exhaust velocity of 3.675 km/s. So without payload, the total delta-V of the BFR is almost 9.68 km/s (I've not take count that the sea-level exhaust velocity is less than the vacuum one).
So even going to the Moon without payload, the BFR will likely run out propellant during the final landing. You can mitigate this issue by refueling in an elliptical orbit, to spare 1.275 km/s for the TMI and have only 4.675 km/s of outward journey, but even in this case, you can land on the Moon almost 85 tons (that's also very good.)
Take in mind that an elliptic rendez-vous is difficult to perform and was never tested, but it's not impossible. So the BFR can go to the Moon, but is surely not optimized for this kind of mission, where a ship-plus-lander architecture would be better suited.

According to NASA, the mean bone loss in microgravity is almost 1.5%/month, and you have from 8.5 to 6 months for the outward journey 8.5 months for the inward journey plus 18 months on Mars surface at 3.9 gee, where the bone loss might something between 0%/month in the better case scenario and 1.5%/month in the worst case scenario: it's unethical to bet on the skin of the astronauts, so, in absence of data, we are forced to consider the worst case scenario as true (I don't want to talk about the risk of kidney stones due high level of calcium in urine).
You can mitigate the bone loss using biphosphonate drugs, but then you have to deal with the frozen-bone syndrome, where bone calcium is almost normal, but bones broke suddenly because micro-fracture don't heal and bone trabeculae are not well oriented to cope with the loads.
I remember that Italian astronaut Samantha Cristoforetti, after spending 200 days on ISS, needed to take the shower sitting on a chair because she was unable to do it standing up. So she said in an interview.
Are you sure you want an astronaut in so poor physical condition to explore an alien planet?

You cannot see from orbit if a terrain is good for landing and even an unmanned rover cannot predict if one leg of the BFR, in time, will ted to sink more than the others.

You have to process a huge amount of regolith with the right stuff: but to develop it you needs a perfect knowledge of the amount of water that the regolith has in the place you will land. So the issue is the same: you need a perfect knowledge of what you will find in your landing place.

The problem is that what we know now is not enough to bet human lives on it.
So if we really want to make a Mars mission in the next 10-15 years, we have to do it only with the technology we have now, and with all the propellant and the stuff we bring from Earth, and avoiding the risk we just know. So an orbit-to-orbit spinning ship using storable hypergolic propellant (NTO-MMH) and propellant depot sent in mars orbit with electric propulsion together with Mars landers.
In the next 20 years we can upgrade our orbit-to-orbit spaceship with solid core nuclear thermal propulsion and zero-boil-off cryocoolers for LH2, then with gas-core NTR.

I referred both the issues: the reentry is the more lethal, but even with an orbit-orbit ship I prefer artificial gravity because I would like to have an healthy crew to explore an unknown planet, rather than a crew of almost seek zombie-walking people with osteoporosis, anemia, impaired vision and muscular hypotrophy.

You can find a way to pair two ships and spinning them, then separate and de-spin them before entry. But my preference lies with a NTR orbit-to-orbit spaceship, able to spin during coasting, that do all propulsive maneuvers. If we find a way to develop some kind of mini-magnetosphere that may shield even from GCR - there are many interesting researches on the issue - we will have a ship that can bring us almost every were in the solar system, from Mercury to the moons of Jupiter.

In a previous post GW has answered this topic far better than I could do.

Anyway, Musk is far from going to Mars, because it will pass many years before he will have a reliable ISRU-device, able to produce 1100 metric tons of LOX-LCH4 (I still don't know if the SpaceX guys have just started the R & D for the ISRU).
Probably, in waiting for the ISRU-device, the BFR will be used to go to the Moon - it has enough delta-V to do it, after refueling in elliptical orbit - where the transfer time is too short to have microgravity health problems. It can deliver 20-30 tons of stuff on the Moon surface every mission, that can be used to build a Moon-base.

So, to recap, IMHO, SpaceX Mars Mission has three main weak-points:

1) the lack of artificial gravity
2) the tail-landing which needs a perfect knowledge of the stability of the terrain
3) the ISRU which needs a prefect knowledge of the alleged buried glaciers in equatorial latitudes

Primum non nocere.
As a physician, I perfectly agree with you on the necessity to spin the ship to not damage the astronauts: whatever it costs, is far less than the cost of a dead or at least severely crippled crew.

Joining two ships tail-to-tail and spinning them needs to redesign the power system: either spinning the two ships on the ecliptic plane and mounting the solar panels on a counter-rotating platform, or using a nuclear reactor (the latter IMHO is wiser, given they need to produce a huge amount of propellant for coming back.)

Just out of curiosity, but how to protect the landing legs from being damaged by the rocket blast during take off from mars?

But we perfectly know how to make artificial gravity by spinning, so what's the problem?
It's only just a bit more expensive, but certainly less expensive than a dead crew.

Thanks GW. You are a very good teacher.

So, just out of curiosity, which is a good area ratio for the rocket nozzle of a lander that has to take off from Titan where the surface atmospheric pressure is almost 2 Bar?