February 12, 2004
The Mars Direct Plan of Dr. Robert Zubrin’s Mars Society advocates a trip with two launches. The first unmanned launch lands a return ship, which then makes 96,000 lbs of rocket fuel for the return journey. The second launch carries three astronauts to Mars, where they re-enter and land near the return ship. They then drive all over the place for 300 to 500 days, before launching directly for Earth in the fully fueled return ship.
There are several things I don’t like about this plan, chiefly that when you’re done you’ve still got no ship left up there. Just like Apollo, you can do it once or twice and still not have a functioning space program at the end of it. So in response I’ve written a rough plan that I call “Mars Indiscrete”. It’s more than direct. It’s downright rude. And at the end you still have a space faring spaceship in space that has lots of useable space onboard and is still cable of spaceflight to other places in space.
First you start with a nice, comfy, reasonably shielded ship, such as I’ve talked about here and here, that’s capable of a variety of missions, or can even serve as a space habitat. I would design it for a crew of 12 to 20 astronauts, which leaves you able to support multiple simultaneous landings, with astronauts in reserve, while keeping the ship’s systems and crops fully staffed. This actually would save money over the three man approach, because you have 9 to 17 additional astronauts who won’t realize that their paychecks aren’t actually getting deposited in the bank.
Now, once you’re safely in Mars orbit, the landings can begin. Here’s where the program gets rude. First we aero brake, re-enter, and bounce-down an ascent stage, just like with the Mars rovers. If all the system checks are A-OK, we’re ready to send down the astronaut. Yes, just one. They’re not that cheap, you know, and if you send two they might get to talking about their paychecks and figure out that they’re getting screwed.
Our 180 lb astronaut wears a 50 lb Mars excursion suit tastefully done up in red splotchy camouflage from the David Clark Corporation, to keep our hero hidden from the Martian Defense Forces. He has on a 175 lb backpack and weighs exactly same as the Mars rovers, 185 kg. So we’ll just re-use the NASA Mars Rover descent system, which includes a 348 kg lander, 209 kg back shell and parachute, and a 78 kg heat shield. The drop weight of the package is 820 kg, or 1800 lbs.
You may ask if an astronaut can reasonably survive the airbag cushioned bounce-down without injury, which is a very good question. This is why we’ll thoroughly screen all astronaut candidates on Earth, testing them much like cranberries. Only the bounciest astronaut candidates will be selected for the mission. The only drawback to the scheme is that mankind’s first words on the surface of Mars will be “Ughhh!!!” followed by “fuck”, “crap”, “damn”, “unhhhh”, and something about vomit. Unlike Neil Armstrong, who only landed on the moon once, our new explorers will later be able to brag how they made over 200 landings, the first dozen being the worst.
Now after the astronaut has taken a few small rock samples, craned his head all over the place like a chicken, planted the flag and generally ran around for several hours, he’ll be ready to return. He could stay longer, but then he’d get pretty hungry, and as there are no known restaurants on Mars and men can’t cook, he’ll need to get back to the ship fairly shortly. So he wanders over the ascent rocket and climbs on top, into a sophisticated carbon fiber chair with a 5-point harness that NASA got at a street racing store.
His consumables are mostly consumed and his tools are left on the surface because they’re Sears Craftsman and we can replace them free of charge with a good enough story. Anyway, at this point we have a 180 lb astronaut in a 50 lb suit with a 100 lb backpack sitting in a 50 lb seat. We need to give him a delta-V of about 3,500 meters per second, and we’re doing it on the cheap. So we’ll just re-use the same basic technology as the Lunar Module ascent stage, burning N204 and UDMH for a specific impulse of 311, exactly the same as the LEM ascent engine.
So let’s talk some numbers. We want to circularize an orbit around 50 miles up, so we’ll try the following, just because the spreadsheet I just scratched out in Excel tells me it will work. After all, why go to Mars using rocketry software when you can get there with accounting software? Plus, given the expense of this trip, accounting software will be crunching all the serious numbers anyway. Now I should warn everyone that there’s a bit of math ahead.
1) The ascent stage without the cargo has a mass ratio of 4.7 to 1. This is the fuel weight to dry rocket weight. If you can’t achieve such a mass ratio with a hammer and anvil then you need help. This is seriously overbuilt. This means the mass ratio of the actual ascent stage can be tough enough to survive a landing that would bruise a cranberry, if not our cranAstronaut.
2) The total mass ratio, which includes the payload, is 3.57 to 1. In re-usable Single Stage to Orbit (SSTO) concepts they have to play with mass ratios of around 30:1, so again, take a fragile experimental ship, beef it up by a factor of ten, and you’ll have an idea of the safety factors we can toss into this design. It’s this ease of launching from the Martian surface that makes Zubrin’s Mars Direct plan possible, because with even poor performance it’s possible not only to get into Mars orbit, but come all the way home, too. However, our CranAstronaut merely needs to rendezvous with the long duration and oh so spacious ship that’s still up there in Mars orbit. So we don’t have to go to the extremes of actually sweating over this design, and can instead worry about details like whether it really needs a DVD player for the in-flight movie. As rocket science goes, we’ve can fall back to a Studebaker and still have plenty of performance and reliability.
3) These numbers tell us we’ll be putting about 561 kg into orbit, which is 388 kg from the ascent stage and 173 kg from our intrepid if somewhat hungry Mars adventurer. To get to orbit we’ll burn 1442 kg of fuel, which thankfully won’t be contributing to global warming on earth because it’s being burned on a planet that needs all the greenhouse gases it can get.
4) This means the ascent stage, which doesn’t have the astronaut and all his fancy gear, weighed in at 1852 kg. Taking our Mars rovers as a guideline, we can calculate likely descent module weight. If we wrap our ascent module in a big airbag equipped landing pad, like we did with the infinitely more fragile Mars rover, the lander would weigh 3484 kg, 1.9 times as heavy as the ascent stage itself. This is undoubtedly serious overkill, so maybe it can contain a self-righting mechanism, a fake little launching pad, an onboard supply of fried bologna sandwiches, and a moped.
5) The ascent stage and lander are almost exactly ten times heavier than for the Mars rovers, so the back shell and chute come to 2093 kg and the heat shield masses 781 kg, for a drop weight of 4,726 kg. Adding in the astronaut that we drop somewhat later, and hopefully within a good treck to the ascent stage to keep our guy fit, this comes to 5,550 kg per bouncing explorer, or just 12,200 lbs. This is a less per explorer than the weight of a Saturn V by several hundred times, and we’re doing this backwards, starting from up in orbit.
6) So we’ll use a scaled down LEM engine that has 3,181 lbsf of thrust burning 10.23 lbsm of fuel per second, which means it’s no bigger than the motors used by a serious model rocket hobbyist, but which might make a fine ejection seat for a Cessna. The fuel consumption gives us a 5 minute and 8 second burn time, gulping it down at about the same rate as a real SUV driven by a man with a giant pair of balls, similar in size to those dangling from the bounce tested rocket jockey who’s just strapped himself to our absolutely insane open air ascent stage.
7) 3…2…1 blastoff! And our astronaut is hit with an eye-peeling 0.722 G acceleration, meaning he’s under less stress than when floats on his inner tube wondering if his wife is having an affair with the pool boy. At ignition the ascent stage starts pitching at 0.5 degrees per second till he’s 3:05 minutes into the flight and finally aimed horizontally, which occurs at 163,500 feet and a horizontal velocity of 3,200 mph. During this 3:05 rotation phase our explorer can listen to a Billy Joel song on the in-flight entertainment system, which is made possible because of the extremely easy design parameters involved in getting off of what to a rocket scientist is just one really honkingly huge asteroid.
8) 93 seconds after liftoff, at 52,178 feet up and traveling at 982 miles per hour, our intrepid astronaut passes through the dreaded max-Q, the point of maximum atmospheric dynamic pressure during ascent. Our astronaut is completely unprotected from the bare Martian atmosphere ripping past him at supersonic speeds, and his camouflage space suit flaps in the breeze with a force equivalent to ripping through the wind at 75 mph on Earth. In short, a guy on a Harley rides around under about the same aerodynamic loads as our worst case max-Q point, and without even risking a speeding ticket.
9) Exactly 5 minutes after launch later our intrepid hero is pulling 2.37 G’s as he reaches engine cut-off, 236,200 feet up and ascending at 17,700 feet per minute, while traveling at 7500 mph horizontally. Then 3 minutes and 45 seconds after engine cutoff, 6 minutes and 50 seconds into the flight, we burn the engine for 8 seconds more, still horizontally, to circularize the orbit at an altitude of 50.7 miles and an orbital velocity of 7988 miles per hour. Once in orbit he’d better hope that somebody comes to pick him up, because being stood up for his dinner date would just suck.
10) If we cut the incredible amount of fat we’ve built into every stage of the calculation and go with more reasonable numbers, we can easily build the ascent stage with a 6.142 to 1 mass ratio, coming in with a dry weight of 172.7 kg, exactly the same as our payload. This drops the ascent stage weight to 1,083 kg, the same as a Mercury capsule. If our landing pad weighs the same as the ascent stage, instead of the 1.8 times heavier necessary to protect the extreme delicacy of the Mars rovers, the weight of the whole ascent stage re-entry module is now only 3,334 kg, and the whole mission comes to just 4,154 kg per astronaut dropped, or merely 9,139 lbs. That’s 110,000 lbs expended to drop 12 astronauts to the surface and return them to orbit. Mars Direct expends this kind of mass for 3 astronauts, and leaves them on the surface for a year, even though both they and we will be bored out of our minds after the first couple days of 24/7 coverage.
This is where I’d like to bring up the point of diminishing returns. The first Mars rock is precious. The thousandth is just a rock. By the time humans set foot on the red planet we’ll already know the chemical composition of the most of the rocks from all our prior unmanned probes of the planet. The areas we’re not likely to have good data on are the areas where the chances of a landing mishap are high, such as the deep valleys and high mountains. These are also problematic places to get to on the ground, since you can’t drive further than the maximum walking distance from a lander without a broken axle or burnt out motor becoming a fatal situation. No matter how long you leave your astronauts on the surface, until the infrastructure is large enough to support rescue missions that have their own redundancy, nobody is going to be venturing much further than 10 or so miles from the landing site. Spending 300 to 500 days stuck in the middle of a desert while leashed to a tiny room is not something that’s going to be very interesting or very scientifically beneficial. Most of what you’ll learn will occur in the first week on site.
There’s also little benefit in building a Mars colony on the first mission. If the goal is exploration then sending subsequent missions to the same spot is pretty pointless. All they’ll see is a lot of footprints and a years worth of garbage. If you’re talking about building a colony you’ll need a whole lot more than just three people with a fixed departure date. Also, about two weeks after landing half of Congress will be debating whether a return mission is justified, and after the astronauts return from their multi-year jaunt we’ll still have zero long distance mission capable manned vehicles in space.
So just what can we accomplish with the Mars Indecent mission profile? Well, we can drop 12 astronauts on Mars while expending less than the orbit mass of a single Apollo mission. We can visit 12 sites instead of six, since after all, if the astronauts wanted some company they wouldn’t have gone all the way to Mars, now would they? With our modern communications technology they can still keep up a running dialog of clever quips, and the crew remaining in Mars orbit can stay in constant video contact.
We could drop pairs of astronauts and scale up the designs accordingly. Given the extreme ease of achieving Mars orbit, we could even design the ascent stage to carry 172 kg of astronaut and 172 kg of Mars rocks, while dropping them in pairs. This broadens the options in case one ascent stage develops a problem after the astronauts are on the surface, as they can abandon the rocks and ride up together.
Our landers are cheap and light, so we can first drop some probes and rovers to make sure they’re successful and re-entry and landing. The rover can be designed to collect rocks and fill up an ascent stage, so that can also get an all-up systems test prior to manned use. If problems appear and no work-arounds are found, the mission becomes one of dropping a large number of unmanned rovers and probes, to gather more data prior to the next mission. Since we haven’t actually trashed our expensive, heavy, long-duration spacecraft, the existence of a next mission is guaranteed.
The ascent stage functions equally well for missions to Mars, the moon, and Mercury, a planet long over looked but the next easiest target to get too, after the moon. The stages can work with manned and unmanned missions, and are small enough that they will affordable, exchangeable, and won’t take up much mass or volume in storage. They are also reusable, since once an ascent stage returns to orbit it could be refueled, stuck in a new descent stage cocoon, and dropped again. The only thing we’re expending is the descent stages, essentially heat shields, parachutes, and retro-rockets. For airless bodies like the Moon and Mercury, the ascent stage rides on a powered descent stage, which can also be designed to land things like small habitats or rovers, fulfilling the same functions as the parachutes and airbags on Mars missions.
By seperating the ascent stage from the rest of the equipment, we no longer have our engines tied to the size of any habitat we want to occupy. Those can be dropped seperately and need not contain a bunch of toxic and highly explosive chemicals. We also don’t have to launch our bed back into orbit, which is kind of a stupid design if you think about it. We also don’t have our astronauts spending half his time trying to get in and out of a tiny airlock on short visits, and we can make visits because we’re not spending a fortune building an orbit capable bedroom with multiple hatches and an airlock.
We need not have an astronaut land without knowing his ascent stage is A-OK. Given our modern electronics, computers, and navigation technology, we can pretty much guarantee he’ll land almost exactly where we want. We’ll also have a good view of the landing zone, and navigation beacon, and wind forecasts for a manned landing. We also no longer have an astronaut plunging through an atmosphere in a big fireball while riding with explosive rocket fuel. We lose the descent abort capability we had on lunar missions, but then we’ve never had that capability on Earth re-entry either. It’s lack doesn’t seem to be much cause for concern.
Our planetary and lunar landers would be completely seperated from the functionality of our long-duration spacecraft. That means our large expensive craft isn’t being expended on just a couple of missions, much less one. It can grow incrementally as we add shielding, change engines, and just keep enhancing this and that. It can serve as an Earth orbital space station, a lunar ferry, or a deep space exploratory craft, visiting asteroids, Mercury, and Mars. In between exploration missions it can also keep up continual food production to support other space activities, and generally serve as a long term component of our space infrastructure.
Since the heavy and expensive long duration ship isn’t solely dedicated to the Mars program, but to the overall space program, the incremental cost of the actual Mars mission boils down to the costs of any short term habitats we drop for the early missions, the descent stages (parachutes, shells, and retro-rockets), and the fuel consumed during the flight. It also saves taxpayer dollars, because we’re not depositing the astronaut’s paychecks while they’re gone, and all the while we’re conning them into growing their own turnips and spouting off about oxygen levels instead of sending them on the trip with several tons of frozen Hooter’s hot wings.
Anyway, that’s my Mars Indecent proposal, and hopefully it will at least prove amusing to someone.
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I just don't get all this fascination with Mars. Why not expend the capital and effort on moving civilization into orbit instead?
Here's a good refutation of Zubrin's plan.
Posted by: TangoMan at Feb 13, 2004 2:16:58 AM
Good point! I was basically pointing out that since we've got no serious plans for Mars yet, why not just drop a guy on the planet, have him poke around for a couple hours, and blast off again. "Whew! We got that out of our system! Now lets go mine an asteroid!"
Frankly, I think Mercury would make a much more attractive prospect. You can get their in a couple months, and your solar cells would be humming. Build a colony their and your solar panels are tiny, or your corresponding crop acreage is huge.
Posted by: George Turner at Feb 14, 2004 1:47:53 AM
I see that the meme hasn't yet taken hold :)
Keep coming over to spacesettlers and SSI and we'll disabuse you of your planetary chauvanism :)
Posted by: TangoMan at Feb 14, 2004 2:15:28 AM
Hehehe... No, I mean move the space colonies to where the sun is. Building on Mercury would be an exercise in hot feet, radiation, punctuated by months of freezing cold.
Posted by: George Turner at Feb 14, 2004 3:07:43 AM
The plan isn't half bad. I'd make a few modifications though. The ship in orbit should park on Phobos where it has tons of radiation shielding and where equipment can be launched ahead of time or left afterwards for followup missions.
I'd also build a bit sturdier landers than you talk about and leave them parked on Phobos for the next trip. The first couple of trips can bring landers (to ensure redundancy) and after that the lander weight can be replaced by something else.
Each trip the Phobos base gets bigger and bigger.
Posted by: yank at Aug 5, 2004 6:10:49 PM
Now that I think of it, following my slightly modified plan you could send a series of much smaller Mars direct type first stage-launches to Mars. Swap the return vehicle for a small hab, you only need enough fuel to get from Mars to Phobos, so you have a lighter, simplier plan.
No reason a team should be limited to a single landing site on their mission. In fact geologists might find it more useful to have two or three bases to work out of during their year. If you make it somewhat easy to get back and forth to Phobos most of the radiation problems fade away.
And if you have a semi-permenant base on Phobos there is no reason a team member would have to go back with the ship that brought him so if you overlapped the missions you might have some folks staying two years, etc.
Oh, and Phobos has a tiny gravity well, has geological interest as well, and its possible that hydrogen fuel could be extracted from its regolith.
Just some thoughts but I think its a better plan than Mars Direct because it leaves material behind for the next missions to build on, I think it would be safer and get more and better science, and I don't think it would be significantly more expensive.
Posted by: yank at Aug 6, 2004 10:29:50 AM
Those are very good improvements to the plan. Getting a source of orbital radiation shielding and access to raw materials would make a huge difference in the expansion rate of any sustained effort.
Your enhancement also made me think that you could be dropping a plethoria of cheap rovers from Phobos, taking advantage of the short transmission delay to control them with human operators in real time, or at least be able to get real time human intervention when a navigation obstacle is at hand.
That alone might up the amount of locations explored by a significant factor, and even allow high-risk low-cost landing attempts in some of the canals, valleys, and mountains.
Posted by: George Turner at Aug 6, 2004 10:07:01 PM