What would it take to do "Mars Direct," let alone MCT?

[Pipcard]

I love most of the ideas behind Robert Zubrin's Mars Direct. Some parts make sense, such as Sabatier reaction ISRU to reduce the propellant that needs to be carried from Earth (a lot of people speculate that MCT will use the same kind of technology); they just need to be proven with precursor missions first. But it gets regarded as a plan that underestimates the mass necessary for a manned Mars mission.

It tries to assume a very simple architecture with two 30-tonne payloads on the Martian surface, one that is most likely too simple. They are supposed to be directly injected into a Mars-bound trajectory by two 120 (or 140?)-tonne-to-LEO super-heavy launch vehicles. One is an Earth Return Vehicle for the Mars-Earth transfer, the other is a Habitat for the Earth-Mars transfer and Mars stay. To avoid the complications of Mars orbit rendezvous, the ERV produces propellant and launches directly back to Earth. Zubrin estimates a mass of 7 tonnes for the crew cabin; that is considered by many to be too small for a spacecraft that four people will ride in for six months. The Hab has a mass of 25-30 tonnes, is that enough for the ~180 day transfer and ~500 day stay?

So, how big would Mars Direct really need to be, in terms of mass and/or interior volume? Would more cargo payloads need to be pre-positioned on the Martian surface, or in Martian orbit, before the first crew arrives? Would the mission profile have to be more like Mars Semi-Direct, with a Mars Ascent Vehicle that launches to an Earth Return Vehicle waiting in Mars orbit? This being a mission of only four people, compared to the several dozen that Elon Musk claims that the MCT will carry once it is fully operational.

[Pipcard]

For the ERV crew cabin, would a 15-tonne (about ISS module-sized) be sufficient? Of course, this would require a larger lander, unless the ERV cabin was left in orbit (so that it would not have to come all the way to the bottom of Mars's gravity well), and only the ISRU/small ascent cabin was put on the surface (similar to Mars Semi-Direct). The ISRU plant could also produce the fuel necessary for a stage to push the ERV cabin back to Earth.

However, that introduces a "mission-critical" orbital rendezvous that takes place far away from Earth. Mars Direct also intends for the ERV to serve as extra habitation space, and being on the surface lets the astronauts check on all of the cabin's systems before launching.

Let's also say that the Hab and ERV don't need to be sent directly to Mars, if there was refueling (with little-to-no assembly) in Earth orbit.

[Robotbeat]

Human beings can travel for months with about 5m^3 per passenger (including lavatory, hospital, etc). That was typical steerage class accommodations at around 1900. 1700s and 1800s trips to Australia we're more cramped and took as long as MCT will take to get to Mars (~100 days).

The thing to remember about MCT is it assumes some pre-landed infrastructure (such as power and ISRU) and the later colonization flights assume very, very extensive ground infrastructure. That makes an enormous difference. If you're flying to Hawaii in a full 737-900ER, you don't expect to pack a car (and refueling station) in your checked baggage, that would come via car transporter (and fueling station would already be established).

[Robotbeat]

Apollo did a very good job at high efficiency. The Apollo LM ascent module weighed just 2150kg without propellants, had a crew cabin volume of 6.7m^3 and habitable volume of 4.5m3 (but including mass for water, coolant, engines, 1960s computer, etc).

A crew of 4 should be able to live in a module with a mass of 10 tons for several months using modern techniques.

[Pipcard]

MCT will take to get to Mars (~100 days).

6 months (180 days) is sensible for a all-chemical mission (assuming MCT will use methane/LOX). 100 days would require some radical new propulsion system, or result in a lot more propellant required and a lot less useful payload transported to Mars. Arriving at Mars would also require either a riskier aerocapture/aerobraking manuever in the thin atmosphere, or even more propellant to enter Mars orbit.

Or maybe it's only for the passengers and 100 days of supplies, and cargo will take a longer path? Still, it will require a riskier/more expensive capture maneuver. Zubrin also claims that the 180-day path offers a free-return trajectory option.

[gbaikie]

MCT will take to get to Mars (~100 days).

6 months (180 days) is sensible for a all-chemical mission (assuming MCT will use methane/LOX). 100 days would require some radical new propulsion system, or result in a lot more propellant required and a lot less useful payload transported to Mars. Arriving at Mars would also require either a riskier aerocapture/aerobraking manuever in the thin atmosphere, or even more propellant to enter Mars orbit.

Or maybe it's only for the passengers and 100 days of supplies, and cargo will take a longer path? Still, it will require a riskier/more expensive capture maneuver. Zubrin also claims that the 180-day path offers a free-return trajectory option.

I think NASA should send it's crew to Mars in less than 3 months- 90 days.
Probably to lower costs one send passenger on longer transit times- + 120 days
To get to Mars in less than 3 months requires non hohmann transfers and one would need to use chemical
rockets [as they have more thrust and need something around 1 gee of thrust for about 5 to 10 mins- which only possible with chemical at this time].
120 days also requires non hohmann transfers and probably chemical, though about 1/2 of mass of propellent. So say, 30 to 40% cheaper per seat.

[Oli]

Problem of Mars Direct is that you have to launch a 20t+ habitat plus an Earth reentry vehicle (Orion is 21t incl. fuel, you may get away with less) from Mars to Earth, that is roughly 7km/s. Meaning you have to land a huge rocket on Mars.

Of course you can design some spartan mission with astronauts living like rats in a cage, but you have to convince NASA first, good luck with that (IMO there's no way around NASA when it comes to Mars, not even for Musk).

[Robotbeat]

MCT will take to get to Mars (~100 days).

6 months (180 days) is sensible for a all-chemical mission (assuming MCT will use methane/LOX). 100 days would require some radical new propulsion system...

I've done the math. 100-120 day trajectories are doable with chemical propulsion if you refuel in Earth orbit (which SpaceX has said they'll do), perhaps a high orbit. No radical propulsion system is required (you're thinking of 40 day trajectories). SpaceX has mentioned /possibly/ using solar-electric propulsion (a technology which they will need to perfect for their huge satellite constellation) especially for hauling propellant from LEO to a higher orbit (where you can refuel the MCT), but given the ubiquity of solar-electric propulsion these days, calling it radical and new doesn't make any sense. Remember how cheap propellant could be if launched on a high-launch-frequency reusable super-HLV like the BFR.

Or maybe it's only for the passengers and 100 days of supplies, and cargo will take a longer path?...

Yes, that is logical. The trajectory for cargo only needs to be fast enough to get the MCT back before the next launch window so it can be reused.

But if the cargo doesn't travel inside the MCT, you could go much longer than even 180 day transfers:
Some cargo could be sent on a ballistic trajectory toward Mars--where it would be captured by a lander--with the MCT turning around immediately back to Earth after Trans-Mars-Insertion so you can use just a couple MCTs (refueled many times) to launch dozens of loads of cargo every synod. (The cargo would proceed to Mars without anything more than perhaps some cheap cold gas thrusters to assist in capture by the lander/MCT at Mars.)

[Robotbeat]

Problem of Mars Direct is that you have to launch a 20t+ habitat plus an Earth reentry vehicle (Orion is 21t incl. fuel, you may get away with less) from Mars to Earth, that is roughly 7km/s. Meaning you have to land a huge rocket on Mars.

Of course you can design some spartan mission with astronauts living like rats in a cage, but you have to convince NASA first, good luck with that (IMO there's no way around NASA when it comes to Mars, not even for Musk).

Orion is way overweight for that role. Think something more the size of a Soyuz (just slightly larger to squeeze in another astronaut). Soyuz reentry module is ~2.8 tons, you could maybe increase it to 3.5 tons for 4 astronauts or use modern tech to reduce the weight back to 2.8 tons.

Also, Earth to Mars can be less than 7km/s (6-6.5km/s, remember you have rotation helping you). Also, that rocket doesn't need nearly as much thrust as an Earth rocket needs and has the benefit of near-vacuum at take-off for further thrust and Isp advantage. With methane/LOx, the rocket wouldn't actually have to be that big, and with near-zero aero loads on the way up, you can make it squatter instead of tall.

Also, NASA once sent 2 astronauts in an absolutely tiny Gemini capsule for 2 weeks when they couldn't even get out of their suits.

[Pipcard]

Problem of Mars Direct is that you have to launch a 20t+ habitat plus an Earth reentry vehicle (Orion is 21t incl. fuel, you may get away with less) from Mars to Earth, that is roughly 7km/s. Meaning you have to land a huge rocket on Mars.

Of course you can design some spartan mission with astronauts living like rats in a cage, but you have to convince NASA first, good luck with that (IMO there's no way around NASA when it comes to Mars, not even for Musk).

The MCT system is speculated to land an even larger vehicle on Mars, and some even think it's going to be one vehicle that the people ride in from Earth-Mars surface and back (surface habitation would probably be separate and pre-landed), which is why I'm asking about something less, a four-person round-trip (whether it involves the ERV going to the surface or remaining in Mars orbit).

Because of the 7 km/s Mars-Earth Δv, the Mars Direct ERV has two stages.

Also, you said "MCT can be anything, really" (that post is what inspired me to start this thread, by the way)

[Robotbeat]

It doesn't have to be quite 7km/s. 6.5km/s is enough. Besides, you can easily do 7km/s in a single stage, especially if you can get by with less thrust and you get vacuum Isp boost from the start and use good mass fraction. TSTO launch vehicles do it all the time for the upper stage. A challenge to do reusably, but not in the least bit impossible.

And if it's really too hard to do it all in a single stage (it isn't), you can always refuel in Mars orbit on the way back.

[Robotbeat]

...
The MCT system is speculated to land an even larger vehicle on Mars, and some even think it's going to be one vehicle that the people ride in from Earth-Mars surface and back (surface habitation would probably be separate and pre-landed),...

Musk said that the idea of MCT is to "land the whole thing." That pretty much only makes sense one way.

And Musk explicitly said that for the early trips, MCT itself will function as a surface hab.

[MATTBLAK]

On Gemini 7, Borman and Lovell had special lightweight suits that they did in fact remove for their 'long duration' mission. Such suits were an explicit recommendation of Cooper and Conrad who had flown nearly 8 days on Gemini 5 with the standard suits.

[Oli]

which is why I'm asking about something less, a four-person round-trip (whether it involves the ERV going to the surface or remaining in Mars orbit).

How big it would have to be is highly dependent on structural mass fractions. If I look at deep space methalox stages, I have a hard time finding anything better than 15% (note for the Mars Direct Return Vehicle stages it's around 20%). So I don't know, you're maybe looking at a 2-stage rocket with 40t dry mass for launching a 30t payload to Earth (could go up to 80t for 20% smf). Your Mars lander might be volume limited though.

[Burninate]

MCT will take to get to Mars (~100 days).

6 months (180 days) is sensible for a all-chemical mission (assuming MCT will use methane/LOX). 100 days would require some radical new propulsion system...

I've done the math. 100-120 day trajectories are doable with chemical propulsion if you refuel in Earth orbit (which SpaceX has said they'll do), perhaps a high orbit. No radical propulsion system is required (you're thinking of 40 day trajectories). SpaceX has mentioned /possibly/ using solar-electric propulsion (a technology which they will need to perfect for their huge satellite constellation) especially for hauling propellant from LEO to a higher orbit (where you can refuel the MCT), but given the ubiquity of solar-electric propulsion these days, calling it radical and new doesn't make any sense. Remember how cheap propellant could be if launched on a high-launch-frequency reusable super-HLV like the BFR.

Or maybe it's only for the passengers and 100 days of supplies, and cargo will take a longer path?...

Yes, that is logical. The trajectory for cargo only needs to be fast enough to get the MCT back before the next launch window so it can be reused.

But if the cargo doesn't travel inside the MCT, you could go much longer than even 180 day transfers:
Some cargo could be sent on a ballistic trajectory toward Mars--where it would be captured by a lander--with the MCT turning around immediately back to Earth after Trans-Mars-Insertion so you can use just a couple MCTs (refueled many times) to launch dozens of loads of cargo every synod. (The cargo would proceed to Mars without anything more than perhaps some cheap cold gas thrusters to assist in capture by the lander/MCT at Mars.)

Could you present us your BOTE math on a 100 day transfer?

[Robotbeat]

I did it a while ago using the Java porkchop program. It's in a thread somewhere here. Some synods are much worse than others for the same transit time, but those times you just do ~120 day transfers.

[Burninate]

I did it a while ago using the Java porkchop program. It's in a thread somewhere here. Some synods are much worse than others for the same transit time, but those times you just do ~120 day transfers.

Are you talking about this?  http://forum.nasaspaceflight.com/index.php?topic=33642.msg1140519#msg1140519

What were the burns involved?  What mass ratio or dV does "Doable with chemical propulsion" mean?

[Robotbeat]

"Doable" is fairly flexible, but a c3 of 50 km2/s2 shouldn't be too bad with chemical (actually, should be much better than that due to Oberth effect). Less than that is ideal.

[Burninate]

"Doable" is fairly flexible, but a c3 of 50 km2/s2 shouldn't be too bad with chemical (actually, should be much better than that due to Oberth effect). Less than that is ideal.

So that implies an LEO chemical burn of 3.2 + (50^0.5~=7.1) = 10.3km/s to set aphelion?  And then later whatever is required for Mars capture and Mars EDL?   If you can get from that insanely fast transfer to the surface for 2:1 (doubtful), the overall wet to dry mass ratio ends up being ~32:1.  I don't know about the diminished limits of microgravity construction, but existing Earth LV tankage AFAICT doesn't do those ratios, dry mass grows to such dimensions that potential payload shrinks below zero...

unless you do, in fact, stage.  Which Mars Direct does, so sure, but MCT isn't supposed to.

Let's assume you get that ~3200m/s escape velocity for 'free' using regular deliveries to EML2 at 5000s Isp SEP, and a multiyear spiral.  Then you can drop chem dV down to something manageable.  My inclination after that is to pack a giant ship on a 6-9 month Hohman transfer (less than 1km/s dV) , and capture using SEP, and EDL with a MAC and heavy propulsive involvement (less than 2km/s dV).  Your inclination, as I understand it, is to pack a tiny ship on a 100-day transfer using 7.1km/s dV, then... what... direct aerocapture and EDL?  I guess after that you need a ground propellant depot to fuel your tiny MAV for a nearly immediate ~14km/s hyperbolic chemical return, in order to establish 1 synod per mission per vehicle?

[Burninate]

I would split the mission up wherever possible in such a way that each mission can be a new Mars base location, at least at first.  No leg greater than maybe ~5km/s, propellant resupplied using Mars ISRU (self contained), a tug to HEO, LEO deliveries, and a tug to LMO.  An expectation of 2 synods per mission per vehicle, or possibly 3.  It's trying to square Musk's plan on reusability and passenger count with Raftery's plan on capabilities and vehicles.