Mass Drivers on Mars - "Space gun" to shoot stuff to Earth???

Byron · 2002-10-23 14:32:59

After reading RobS post, I think rail guns on the Tharsis volcanos would indeed work, especially if you only need 5.5 km/s to reach escape velocity...that's a pretty low number. Sure, drag in the first few seconds of flight would be a bit of a problem, but nothing that would be insurmountable by any means...after all, the Shuttle regularly drops though Earth's much thicker atmosphere at nearly those kinds of speeds, and it seems to do just fine with the protective heat-resistant tiles, etc.

The question I have, if you're attempting to reach a velocity of 5.5 km/s at 150 km of altitude, how much of a "premium" velocity would you need to build in to account for the slowing effects of drag as the object soars through the atmosphere (what little there is)? My guess it wouldn't be much..perhaps a total of 7 km/s of delta-v would be needed, if even that much...

I've also thought about the idea of excavating the rail gun tube to cut down on drag...good idea, but how do you do it? If you leave it open at the top end, the air will still "fall in" at the top and create the same pressure as outside at the base of the rail, which presumably would be at a much lower altitude. Maybe there would be a trap door at the top of the tube that would pop open a millisecond or so before the capsule reaches the end? Yet another challenge for the engineering types..after all, this is what we pay 'em for...

B

Palomar · 2002-10-23 15:40:33

*What is a mass driver, anyway? Can someone refer me to a web link with a photo or artist's conception of it, or whatever? It'd be appreciated.

--Cindy

Number04 · 2002-10-23 16:26:26

try these
1
2

It's an object that accelerates others. The barn-yard logic is this:
(I could be wrong)

There are two rails and the object is placed in between. A dead short circuit is created across the rails and object. The resulting magnetic field launches the object.

turbo · 2002-10-24 08:44:14

The barnyard logic is close!

The object provides the electrical connection between the rails. A high current input generates two powerful magnetic fields of the same polarity in the rails. Newton's equal and opposite reaction gets twisted into the "Lorenz Effect", where the two opposing fields force the object to move perpendicular to the direction of the fields. The stronger the fields, the faster the acceleration. The railgun used by the University of Texas has its own power station to provide 6 MEGA-Amperes (yep, six million) of current, so the 2 Kg ball achieves hypersonic speed.

The biggest problem with a railgin is the projectile must be moving when it gets on the rails, a standing start normally results in the projectile being quickly welded to the rails. Coil guns can do standing starts, but are more complex because of the timing circuits needed.

turbo, who has a diassembled railgun in his van until lab partner has storage room in his garage.

RobS · 2002-10-24 13:14:59

One way to get a maximum figure for the amount of atmospheric drag is to calculate the energy it takes to accelerate the 0.5 tonnes (or whatever it is) of air to the terminal velocity desired (5.5 km/sec to escape, 6.4 km/sec to Earth). If you are launching a 1 tonne object to those speeds, 0.5 tonnes of air requires an additional 50% more energy. Of course, the amount would really be less because you are shoving the air out of the way instead of accelerating it to the same velocity. But this assumption at least allows one to calculate a maximum figure. If you assume the air is accelerated to 10% of the final velocity of the payload--which may be more correct--that reduces the energy you are wasting quite a bit.

If the payload is more like 10 tonnes, rather than 1 tonne, the amount of energy wasted on air friction would be pretty small.

-- RobS

Shaun Barrett · 2002-10-24 17:40:24

Sounds about right, Rob!

Are we agreed we'd need to evacuate the tube? If so, there'd have to be a seal at the top end, resistant to air pressure outside but presenting no barrier to the emerging projectile. As Byron suggests, this should be the easy bit!

There could be a flaw in the logic being used here, though. It's one thing to talk about shuttles entering Earth's upper atmosphere, or accelerating half a tonne of Martian air to the terminal velocity required. But both of these are relatively gradual events. What we're talking about here is a projectile, travelling in a vacuum, suddenly hitting the air at maybe 7 kms/sec.
Even though the air is very thin, are we still going to experience an explosive shock at the instant of egress from the tube? And, if we do, how destructive is it likely to be and can we build the projectile to lessen the effect or withstand the shock?

Number04 · 2002-10-24 18:49:32

I Is there any way to gradually add air? Instead of an object hitting a wall, could you gradually increase the air density in the tube so by the time it gets out it's at atmospheric pressure?

You will lose velocity this way, but it's allot safer. But I could be wrong.

Tom Jolly · 2002-10-24 19:56:27

I don't think you would necessarily WANT to plunge from a vacuum into normal air pressure. Accelerating in the air would build up the surface-effect buffer that you'd want to protect yourself from the static air, and establish aerodynamic stability as you accelerated, rather than having to deal with a discontinuity in air pressures.

From a purely kinetic viewpoint, of KE = 1/2 mv^2, moving through air at 1mB versus Earth's 1Barr, 5000m/s would give you an equivalent of 500m/s at sea level on Earth, that is, about 1100 mph. Of course, you'd be outside the atmosphere in a few seconds, anyway. So, I think the Martian mass driver would be do-able.

Someone mentioned that "you'd be outside the Mars atmosphere in 150 miles". I think that's very high. Earth, for example, drops down to less than 1 mb at 50 miles. I'm sure there's a pressure gradient for Mars posted on the net somewhere.

The drag problem is quite complex for a number of reasons, one of them having to deal with supersonic shockwave calcs. I'm sure someone has already researched supersonic drag characteristics for spheres, it's just a matter of digging it up, but I truly don't expect this to be a problem.

On another note; 300,000 G's in a rail gun? Are you serious? The best I'd heard to date was SSI getting 100G's out of a mag rail. I'd think just about ANYTHING would disintegrate at 300,000 G's. Are you (Turbo) sure about those numbers?

Tom Jolly · 2002-10-24 20:20:45

I was just reading bits of a site on rail guns. Very interesting research. It's at;
http://www.powerlabs.org/railgun.htm

There was actually a gun that launched a 0.1 gram mass at 16 kilometers per second. Wow. And another that did 1.6kg at 3.3kps. Lots of pictures, and very informative. University of Texas has a site, too, that talks about their research.

Preston · 2002-10-26 10:43:32

Who says the railgun has to be a tube? Maybe it can be open air, and this might lessen the shock, if that turns out to be a real problem.

A couple simple calculations show that a 15 km rail gun could acclerate to 9 km/s in 3.33 seconds at 276 g's, assuming constant acceleration.

x=0.5*a*t^2
v=a*t

turbo · 2002-10-26 13:00:20

The beast I'm still hammering away on is an open-air type of railgun, just like MIT's version. So far, the simplest one I've seen is two copper strips fastened to a board and fed through a capacitor bank. The little one launches pennies and sections of copper wire.

The best railguns so far throw titanium "darts" encased in an aluminum sabot. The sabot provides the contact between the rails so the magnetic fields can build up. Once clear of the rails, air resistance opens the sabot and the dart continues with little loss of energy, just like some artillery shells.

The powerlabs.org site is one of the first we used when researching the feasibility of building a rail gun, and one of the big reasons we started almost a year prior to the senior project due date. Bear in mind this is at a rate of one day available per week to work on the thing for about 95% of the project duration.

Tom, I didn't do the calcs for the G forces, I only reported so far the fastest I know of is a 2 Kg sphere fired at 6 Km/second on what I recall as being the University of Texas' 18 foot railgun. Check their site as the original data gathered for my team's railgun is with my partner at the moment.

There were other sites that showed the Lorenz Effect, my favorite was the tube that launched a large metal ring placed around it. Lately alot of "fringe technology" has disappeared from the Web (no surprise) so I'm not sure what is still available.

turbo, who loves fringe technology

Preston · 2002-10-26 13:41:06

6 km/s over 18 ft. implies 340,000 g's. (!) I've heard that when you drive a really high current through whatever is pushing your payload, it can turn into a plasma. This isn't a problem for condunctance, since plasma conducts well, but I don't know how well the plasma can push the payload...?

turbo · 2002-10-26 14:46:02

During our initial research, there was one statement that the entire team kept in mind. I'm not sure exactly where it came from but : "A railgun is good for about one hundred shots, plus or minus one hundred."

If the projectile is not moving fast enough, it can easily weld right to the rails. The idea is to run the current through the whole system fast enough to induce the fields and shoot before the system slags. My team is still out to try a slightly different approach, but we still run the risk of our rectifier releasing what we called in school the "magic smoke".

We also found that graphite is frequently used on the rails as a lubricant. I think a railgun on Mars would be prepared by running a quick blast of low current on the rails to attract the fines in place of graphite.

I still never expect a railgun to place any living creature in space. But one quirk of railgun technology may help: Under a high current, water itself becomes explosive, as some budding high energy tinkerers have found when touching huge capacitors and having most of the water in their bodies explode. Maybe Mars ice could become fuel, I'd just hope the huge capacitors could be made from local materials.

turbo

RobS · 2002-10-27 00:22:09

Here's a bit more data. The book *On to Mars* (available through Amazon, contains highlights of papers from the 1998-2001 Mars Society annual meetings) has a CD rom in its back with additional papers, and one is about mass drivers. It says the top of Olympus Mons has only 0.67 millibars of air, and that air has a mass of 18 kg per square meter of surface area. My calculation, assuming 1 millibar, was 500 kg per 20 square meters or 25 kg per square meter, so I was about right. This means there's even less air to deal with.

The mass driver that was proposed was 400 kilometers long, so that it could accelerate passengers as well (which require less than 3 gees of acceleration). They were figuring on losing about 100 meters per second of velocity to the air. I suppose this was not a serious proposal; such a device would be massively expensive.

More practical for transportation to and from low Mars orbit, according to another article on the same CD, is a rotating tether 300 km long. It would rotate counterclockwise as it went around Mars so that its tip, when it "touched down" on the surface, was moving westward at the same speed as the counterweight in orbit goes eastward, so that it would be stationary relative to the touchdown point. This also greatly reduces atmospheric drag. While the tip is stationary one can drop off one payload and attach another. Tethers less than 300 km long generate over 3 gees of acceleration as they lift things to Martian orbit (not at take off, obviously, but later in the tether's rotation). Longer tethers generate less gee force (a 700 km tether generates 1 gee of maximum acceleration). The 300 km tether would touch down something like 22 times along the orbit, about every 900 km along the Martian equator. Thus settlements spaced that far apart would all have access to each other and to orbit. Longer tethers could be built that would touch down as far north or south as 55 degrees latitude, though those tethers have to be 10,000 kilometers long.

Tethers work best if the total mass in them stays constant. Thus if you have a "car" of 10 tonnes, plus passengers, to take up to orbit, you have to drop off a car of the same mass on the way down. So you have to plan your transfer of masses carefully.

-- RobS

Number04 · 2002-10-27 15:44:30

What would the center of the tether be attached to?

Shaun Barrett · 2002-10-27 19:08:23

With regard to rotating tethers, Rob S writes:-

While the tip is stationary one can drop off one payload and attach another.

It seems like you'd have to be pretty sure of where the tip was going to 'touch down'. Would the system be too vulnerable to slight drifting of the tether's orbit?
Also, how much time do you get to detach and attach the payloads, and does it matter which operation is performed first?

I've probably got it all wrong but it sounds like it could be a frantic exercise for the ground crew!
Please forgive my irreverence but I could almost imagine it as the basis for a Laurel and Hardy comedy sketch! No offence intended.

Number04 · 2002-10-28 16:13:26

Just an idea here, what about mounting some kind of Scram jet on the object we are trying to accelerate with the mass driver? Could that offset the shock of hitting air pressure with a boost from the jet? Of course we are missing O2 in a Martian atmosphere, but hey, I sure there is someone smarter then me out there that could find away around that :>

RobS · 2002-10-29 11:28:16

Regarding tethers, I think they need a counterweight that stays in space, and presumably has engines to stabilize the orbit. I have no idea how one avoids the Laurel and Hardy scenario; it's a good image of what could happen! The tether enthusiasts probably have an answer, but then they haven't built one of these things yet, so one can wonder whether the answer will work.

Regarding Martian scram jets on the mass driver: of course, that's a great idea! Silane burns in carbon dioxide with a specific impulse of about 280 seconds or so. The details are in *The Case for Mars.* Your mass driver could be used to accelerate the payload to about half of orbital speed--say, two kilometers per second--and accelerate at three gees or less. That'd keep the track short. Then the scram jet could do the rest, or most of it. Rather than shooting the payload vertically, you'd want to send it closer to horizontal so as to use the atmospheric CO2 for your scram jet.

-- RobS

Phobos · 2002-10-29 23:28:10

Here's a bit more data. The book *On to Mars* (available through Amazon, contains highlights of papers from the 1998-2001 Mars Society annual meetings) has a CD rom in its back with additional papers, and one is about mass drivers.

What did you think of that book? I've been thinking about buying it.

Tom Jolly · 2002-10-31 09:09:11

One major problem that I rarely see discussed is atmospheric friction with the tether, and the amount of fuel required to maintain your momentum/altitude. Even if your tether is only a centimeter wide, you still have 150 kilometers (or so) dangling off your hub as it orbits the Earth. Granted, some of the time the tether is relatively stationary with relation to the atmosphere, but the rest of the time; you'd have 1500 m^2 of surface area swinging through the atmosphere, often at extremely high speeds, and continuously. I don't have to tell anyone here that it's going to take a hell of a lot of fuel, continuously, to keep the hub in orbit when it's losing that much energy to the atmosphere, jet-airliner quantities, 24/7.

A much more frugal approach is to keep the rotating, orbiting tether on the fringe of the atmosphere so the tip never comes inside the 50 mile zone, and connect to it with ballistic launches. Of course, you still have the whole "mass exchange" problem.

RobS · 2002-10-31 13:28:47

The tether information I gave was for Mars, and the idea was to have the tether swinging in such a way that it would come straight down, at least in the lower Martian atmosphere. I suppose it ends up swinging sideways in the upper atmosphere. But Mars's atmosphere is pretty thin.

The Earth's atmosphere would generate a lot of drag and the tether would have to be much thicker, because of the greater gravity. Tether proposals usually involve a tether coming only to the upper atmosphere, where a high-speed jet or rocket aircraft would intercept the hook and transfer cargo to it, already moving at several thousand miles per hour. That sounds pretty complicated to me.

At least the Earth has a magnetic field. A tether with solar panels at the counterweight could generate a magnetic field in the tether to push against the Earth's magnetic field and boost the tether's orbit. A few years ago NASA experimented with this idea, but the short experimental tether broke. They were interested in the technology to maintain the ISS's orbit.

-- RobS

dicktice · 2002-11-20 18:54:18

If your rail is long enough, then you can keep the acceleration within tolerable limits for humans. But how long is long enough? That depends ... !
If you need to send people to Earth from the surface of Mars, you need a final velocity of 5000m/s (Martian escape velocity). Let's assume that an average human can tolerate a 5g acceleration for a limited time. At 5g acceleration, it's going to take 100 seconds to reach 5kms/s. So far, so good.
But, to accelerate at 5g for 100 seconds, your rail needs to be a whopping 250kms long!
Even if you only want to transport people from the surface to LMO (low Mars orbit), you will need a final velocity of about 4000m/s. At 5g acceleration (this time for 80 seconds), your rail still needs to be 160kms long!
And I'm not sure how many people could cope with 5g acceleration for 80 seconds. Is this tolerable for most humans?
My biggest concern is atmospheric drag and heating. Even up on the higher slopes of Olympus Mons, I believe the atmospheric pressure is still 1 or 2 millibars. This doesn't sound like much but, at a velocity of 4 or 5 kilometres per second, I think it would produce significant problems.
Atmospheric drag will probably be the show-stopper for any kind of rail-gun launching system on Mars, even for LMO-bound cargo which can be accelerated at 50g (which, incidentally, would still require a rail 16kms long! ).
The Moon is the ideal place for a rail-gun system with its low escape velocity and negligible atmosphere. But for Mars, unless the Podkletnov gravity-modifying device gives us a new way of overcoming gravity wells, I'm still a space elevator fan!!

I am strongly opposed to a space elevator for Mars, since it would necessitate the elimination of Phobos and Deimos, which are "God-given" platforms of inestimable value for future LMO operations. The much-to-be-desired alternative, of kilometres-long launch rails up the slopes of Olympus Mons, in any direction, would seem the ideal solution to reach any LMO. It should eliminate, besides, the need of boosters for inanimate payloads, such as surface manufactured space-platform modules and/or consumables. Launched Eastwards, the lesser acceleration to gain LMO might even permit boosterless astronaut shuttle operations. Single-stage boosters could be added to shuttles carrying non-astronauts, or where acceleration by rail alone might turn out be too great even for astronauts, to reach polar LMO for instance.
But for goodness sakes, don't propose anything that will do away with the natural moons of Mars!

Shaun Barrett · 2002-11-21 01:08:40

Point taken, Dicktice!

If a space elevator on Mars can only be achieved by destroying one or both Martian moons, I would be very reluctant to proceed without exceptionally good reasons.

I realise that the raw materials for fuel and air that Phobos and Deimos almost certainly carry in abundance are an absolute treasure trove!
Until the "Podkletnov Drive" is in common use, you're right that we can't afford to be too hasty about these things!!

Byron · 2002-11-21 06:41:52

I am strongly opposed to a space elevator for Mars, since it would necessitate the elimination of Phobos and Deimos, which are "God-given" platforms of inestimable value for future LMO operations. The much-to-be-desired alternative, of kilometres-long launch rails up the slopes of Olympus Mons, in any direction, would seem the ideal solution to reach any LMO. It should eliminate, besides, the need of boosters for inanimate payloads, such as surface manufactured space-platform modules and/or consumables. Launched Eastwards, the lesser acceleration to gain LMO might even permit boosterless astronaut shuttle operations. Single-stage boosters could be added to shuttles carrying non-astronauts, or where acceleration by rail alone might turn out be too great even for astronauts, to reach polar LMO for instance.
But for goodness sakes, don't propose anything that will do away with the natural moons of Mars!

I agree with you 100%...

Although some would argue that it would make sense to build a Martian space elevator before putting up one on Earth, a la KSR, as it would far smaller than its terran counterpart...I say baloney. There are far far resources available here on Earth to build a space elevator, whereas on Mars, the new settlers will focused on such things as building habs, domes, etc.

But the real show stopper is having the moons in the way, of course...and the idea of eliminating them (the sheer horror of it! ) or having the elevator perform that oscillation...playing dodge-ball with a mountian-sized chunk of rock...no thanks!

Achieving LMO will always a much easier proposition than than reaching LEO, due to the .38 gee and likely lack of political opposition to nuclear-powered propulsion systems, and of course, there's the rail-gun option, which I think would be used for cargo, at least in the early going, or perhaps a split system could be used...use the gun to achieve half the delta vee needed to achieve LMO, with a final boost of a rocket engine to go the rest of the way to orbit (this would likely be the cheapest and most practical option of getting humans to LMO, if a mass driver is ever constructed.)

B

berty · 2004-03-16 16:15:18

The use of a mass driver on Mars would be very energy expensive. It will be far easier to mine asteroids and perhaps use a mass driver to send materials to Mars, the Moon or near Earth orbit (with careful aiming!) or perhaps utilise lagrange points to store them until needed.
Of course the effect of the mass driver on the orbit, position and velocity of the asteroid would have to be calculated and allowed for.
:;):

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#26 2002-10-23 14:32:59

Re: Mass Drivers on Mars - "Space gun" to shoot stuff to Earth???

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#36 2002-10-26 13:00:20

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#37 2002-10-26 13:41:06

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#39 2002-10-27 00:22:09

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#40 2002-10-27 15:44:30

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#41 2002-10-27 19:08:23

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#42 2002-10-28 16:13:26

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#43 2002-10-29 11:28:16

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#44 2002-10-29 23:28:10

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#45 2002-10-31 09:09:11

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#46 2002-10-31 13:28:47

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#47 2002-11-20 18:54:18

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#48 2002-11-21 01:08:40

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#49 2002-11-21 06:41:52

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