Author Topic: Space cannons & mass drivers, launching into specific orbital planes?  (Read 5235 times)

Offline Nilof

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Mass drivers are possibly the most practical option for non-rocket space launch. The Startram generation 1 design promises launch costs on the order of 40-50$ per kg ammortizing the construction cost over ten years, assuming ten 35 ton launches per day (which is admittedly very high compared to present launch rates).

The upfront construction cost is listed as ~20 billion in 2010 dollars, which is very high but less than the inflation-adjusted development budget of the space shuttle, and within a factor of two of Skylon's proposed budget. Max g-load would be 30 gee, which isn't usable for humans, but is quite flexible for cargo when compared to space cannons.

One thing that got me wondering however, is what inclinations you can launch into. I'm assuming the mass driver is at ~35 degrees latitude, directed straight eastward, and can launch into any altitude between 200 and 800 km. Clearly, the orbital plane you launch into depends on the time of the day, and launching ten times a day would mean you launch into ten different orbital planes. This is a significant downside if you want to launch a lot of payloads to a fixed given orbit.

You can use nodal precession to match orbital planes with the same inclination. However, I'm getting a ballpark figure of ~1 year for the inclination of a body in a 200 km orbit to "catch up" to a body in an 800 km orbit if they were launched 12 hours apart (please correct me if I am wrong). This is acceptable for cargo, but it's still a significant limitation and requires some delta-v in for a 200 km to 800 km altitude Hohmann transfer.

What options exist for changing the inclinations of a large number of payloads from one fixed orbit to another fixed orbit? Could momentum exchange tethers do this?

Another option would be to focus on an equatorial mass driver and use equatorial orbits for everything. I recall that equatorial LEO was a particularly benign environment for a space station, in terms of radiation?
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Hanelyp

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Presuming you don't use delta-V for plane changes, you could potentially launch into any inclination passing over the launch site by orienting the mass driver for that orbit.  But most mass driver concepts would need to be built in a specific orientation, and would not be easily moved.

Offline DarkenedOne

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Space guns are a great technology, but the problem is the market.  Space guns would not be able to launch sensitive systems.   They would only be able to launch things that can withstand the G forces.  Only things like fuel, water, oxygen, some foods, radiation electronics, and etc could be launched this way. 

The only things that a space gun would be able to service are space stations and fuel depots. 

Offline Hanelyp

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A "space gun" could launch with less acceleration, but it would need to be a lot longer.

Offline Rei

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Space guns are a great technology, but the problem is the market.  Space guns would not be able to launch sensitive systems.   They would only be able to launch things that can withstand the G forces.  Only things like fuel, water, oxygen, some foods, radiation electronics, and etc could be launched this way. 

The only things that a space gun would be able to service are space stations and fuel depots.

HARP did a pretty good job demonstrating that when properly encapsulated, sensitive payloads can survive far higher G-loads than 30g.

I do think there's more interesting use for mass drivers on asteroids and moons that lack atmospheres (made with telescoping, collapsibe, or rollable trusses, of course; there's no realistic way you'd make such long structures out of components that can't pack down).  I particularly like the concept for asteroid mining. As with one of the NIAC Phase 2 concepts, if you sinter regolith, you can end up with an optimal-shaped aeroshell for aerobraking - the material to be recovered arriving as its own aerobraking and entry vehicle. But beyond that, it has elemental iron, meaning it can be accelerated by a coilgun or quenchgun (the latter being generally superior for the role) - a small propulsive stage would only be needed for guidance.  dV requirements can still be significant, however, which means a very long quenchgun. I'd imagine that gravitational assists (Mars, Jupiter, etc, depending on the asteroid's orbit) might help get lower dV returns to Earth, and I'd think that trojans would offer some especially cheap return trajectories (gravitational assist with the parent and/or its moons, aka, you only need to escape L4/L5).

At least in microgravity, aiming should be much more achievable than on Earth.  Very slow, but you don't need much of a slew rate.

On bodies where you have a significant atmosphere, however, you either have to deal with huge atmospheric resistance, or build an a very massive vacuum tube - not so much of a problem for its own mass, but of the mass of the megastructure that has to support such a huge thing.   You can hold up a vacuum tube without preposterous support mass and/or materials requirements if you go with kinetic suspension, like a Lofstrom loop, but that comes with its own technological challenges.  You could float it with balloons, but the needed size would get huge, fast with increasing altitude.  I guess if something like that "Martian evacuated balloon" NIAC concept works out, a tube might be able to float itself, and only need stabilization cables. But that's really pushing the bounds of materials engineering, and the tube would have to constantly change in design, or exponentially expand, with increasing altitude.
« Last Edit: 04/12/2017 08:40 AM by Rei »

Offline as58

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To be honest, even the quoted $20 billion cost estimate sounds optimistic.

Offline Robotbeat

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.
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Offline CameronD

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Space guns are a great technology, but the problem is the market.  Space guns would not be able to launch sensitive systems.   They would only be able to launch things that can withstand the G forces.  Only things like fuel, water, oxygen, some foods, radiation electronics, and etc could be launched this way. 

The only things that a space gun would be able to service are space stations and fuel depots.

If you think that's all they're good for... then if you do happen to build one, (a) please don't tell your military about it and (b) please don't aim it in the direction of North Korea! (..or the Middle East, Russia or anywhere else on Earth, pretty much) :o


« Last Edit: 04/13/2017 01:25 AM by CameronD »
With sufficient thrust, pigs fly just fine - however, this is not necessarily a good idea. It is hard to be sure where they are
going to land, and it could be dangerous sitting under them as they fly overhead.

Offline Rei

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.

Very well might. Although it's hard to say, as we don't yet have examples of either  ;)  (although we're certainly closer to the former).

IMHO my favorite non-rocket launch system is the launch loop, which I don't feel gets enough attention versus older, more showy things like space elevators (which require unobtanium).

Offline Rei

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Space guns are a great technology, but the problem is the market.  Space guns would not be able to launch sensitive systems.   They would only be able to launch things that can withstand the G forces.  Only things like fuel, water, oxygen, some foods, radiation electronics, and etc could be launched this way. 

The only things that a space gun would be able to service are space stations and fuel depots.

If you think that's all they're good for... then if you do happen to build one, (a) please don't tell your military about it and (b) please don't aim it in the direction of North Korea! (..or the Middle East, Russia or anywhere else on Earth, pretty much) :o

Not exactly. Countries have built giant guns over the years, but as a general rule they've proven impractical as weapons - slow firing rates, highly vulnerable to attack, etc.  Gerald Bull, for example, wasn't assassinated over the Babylon supergun, which the Mossad didn't really care about, but for his work on improving the accuracy of Iraq's scuds (a project he didn't have any particular interest in, but was part of the agreement for getting his gun project built)

Offline Robotbeat

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.

Very well might. Although it's hard to say, as we don't yet have examples of either  ;)  (although we're certainly closer to the former).

IMHO my favorite non-rocket launch system is the launch loop, which I don't feel gets enough attention versus older, more showy things like space elevators (which require unobtanium).
Space elevator doesn't require unobtainium, it's just that with current materials (which require an AREA taper ratio of on the order of a million, so a diameter ratio of 1000), the space elevator would be a meter in diameter in the middle but for only a small payload (100kg), so would never pay for its own mass because a space elevator is really slow.

And actually the taper ratio improves a lot as you increase specific strength, so it's possible we'll reduce the taper ratio to 10,000, but even then, the really long travel time kills you.

Rockets FTW. People underestimate how efficient rockets are. ITS, for example, would be about 10% efficient at converting chemical energy of methane into potential and kinetic energy of reusable payload in orbit (15% if we're talking expendable). Considering it runs on methane, which is the cheapest source of energy on Earth right now at 1/kWh, you could beat the energy cost of a laser-powered space elevator or any sort of electrically powered launch method that has to pay 10/kWh for electricity. (Of course there are costs to properly condition that chemical energy so it is cryogenic, etc, but that's true for converting electricity into laser/rail gun power, too.)

ITS is supposed to get the cost of propellant in LEO down to just $9/kg. Nine. Dollars.

And ITS isn't the end-all, be-all, either. It's possible to improve efficiency by increasing chamber pressure, modifying mixture ratio so you run either oxygen-rich or stoichiometric, improving mass fraction, using hydrogen on the upper stage to better match the Isp to the flight speed, perhaps using a lower speed launch rail to accelerate the first little bit where it's not possible to lower the Isp far enough to match the flight speed, launch from high altitude, etc.

But for a first shot at a truly large, full, rapidly reusable RLV, ITS (and possibly whatever Bezos has planned for New Armstrong) would already demolish the $50/kg benchmark that these alt-launch concepts are struggling to meet.
« Last Edit: 04/13/2017 04:04 PM by Robotbeat »
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Offline DarkenedOne

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A "space gun" could launch with less acceleration, but it would need to be a lot longer.

Yes that is always true, but you have to remain within a reason length.  In order to accelerate a mass up to 5 km per second with  max G force of 50g you would need a barrel 25 km long. 

Offline Rei

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.

Very well might. Although it's hard to say, as we don't yet have examples of either  ;)  (although we're certainly closer to the former).

IMHO my favorite non-rocket launch system is the launch loop, which I don't feel gets enough attention versus older, more showy things like space elevators (which require unobtanium).
Space elevator doesn't require unobtainium, it's just that with current materials (which require an AREA taper ratio of on the order of a million, so a diameter ratio of 1000), the space elevator would be a meter in diameter in the middle but for only a small payload (100kg), so would never pay for its own mass because a space elevator is really slow.

And actually the taper ratio improves a lot as you increase specific strength, so it's possible we'll reduce the taper ratio to 10,000, but even then, the really long travel time kills you.

Most of the proposals I've seen for space elevators that try to come up with plausible cost figures have a density around that of graphite and a tensile strength of 100-120 GPa.  But individual carbon nanotubes (let alone CNT bundles, let alone bulk fibers) usually measure in at 60 GPa or less.  It's not entirely clear that it's even physically possible to make a material with that sort of strength to weight ratio, let alone achievable.  And space elevators are awash in problems beyond that (not just travel time). Oscillations (they're undamped), micrometeoroids, atomic oxygen, lightning (both tropospheric, and sprites, and the potential to build up a charge on its own), etc, etc.  The worst is power transmission - space elevators are horribly inefficient because of the combination of how distant the receiver is with how small it is and how light it must be.  And the throughput is horrible too, because of the combination of travel time with how few climbers the elevator can support at once.

A launch loop avoids all of the problems of space elevators. I see no reason at all favour the latter over the former. But as you say...

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Rockets FTW.

At least in the foreseeable future, that's the bet that the good money is on  ;)  Technically, a launch loop should beat rockets, with over 50% efficiency, efficiency not dropping off with increased dV beyond that needed for LEO, virtually no wear-and-tear (versus the great stresses of rocket launch, particularly on the engines), and crazy-high throughput. But right now it's nothing but a concept, while rockets are pushing hard in the direction of increasingly affordable reusability.  :)

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ITS is supposed to get the cost of propellant in LEO down to just $9/kg. Nine. Dollars.

I will say that while I *very* much hope they can, I'm not holding my breath  ;)

Quote
And ITS isn't the end-all, be-all, either. It's possible to improve efficiency by increasing chamber pressure, modifying mixture ratio so you run either oxygen-rich or stoichiometric, improving mass fraction, using hydrogen on the upper stage to better match the Isp to the flight speed, perhaps using a lower speed launch rail to accelerate the first little bit where it's not possible to lower the Isp far enough to match the flight speed, launch from high altitude, etc.

And propellants aren't done advancing, either.  Let's not forget that ITS is intended to be the first large scale methalox rocket in history - I mean, they're looking to advance propellants even in that case.

The big technology jump will be if ITS can succeed with composites. We've been stuck with aluminum because it works with LOX and, historically, composites don't. But there's been a lot of work trying to change the latter. The strength to weight ratios on composites are far higher than aluminum, and there's significant potential for further advancement; aluminum's strength to weight ratio is not going to advance much with time.

Offline Robotbeat

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I like composites, but aluminum lithium alloy is pretty great. Aluminum 737 has lighter dry weight per passenger than composite 787, for instance. There are actually some manufacturing advantages to composites, which is why you have fiberglass boats and wind turbines instead of just aluminum. Composites are easier to make compound curves, for instance. That may actually be as much of a reason as strength to weight.

Anyway, I don't think that's the big enabling technology. They could build an aluminum ITS if they wanted to. Raptor is more important. EDIT: trying to keep on topic: New Armstrong could be nearly as cheap even if it's just aluminum and the lower performing BE-4. The REAL trick why rockets can beat alt-launch concepts is because of rapid reuse. Reusing the booster 1000 times and the upper stage hundreds of times with zero refurb on the booster and virtually none on the upper stage. Rapid reuse and the ability to use LOx and LNG, both ridiculously cheap, is the biggest threat to alt-launch.
« Last Edit: 04/13/2017 06:22 PM by Robotbeat »
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Offline DarkenedOne

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I have now come to believe that high flightrate RLVs would beat both gun launch and even a space elevator in cost to orbit, at least for Earth.

Very well might. Although it's hard to say, as we don't yet have examples of either  ;)  (although we're certainly closer to the former).

IMHO my favorite non-rocket launch system is the launch loop, which I don't feel gets enough attention versus older, more showy things like space elevators (which require unobtanium).
Space elevator doesn't require unobtainium, it's just that with current materials (which require an AREA taper ratio of on the order of a million, so a diameter ratio of 1000), the space elevator would be a meter in diameter in the middle but for only a small payload (100kg), so would never pay for its own mass because a space elevator is really slow.

And actually the taper ratio improves a lot as you increase specific strength, so it's possible we'll reduce the taper ratio to 10,000, but even then, the really long travel time kills you.

Rockets FTW. People underestimate how efficient rockets are. ITS, for example, would be about 10% efficient at converting chemical energy of methane into potential and kinetic energy of reusable payload in orbit (15% if we're talking expendable). Considering it runs on methane, which is the cheapest source of energy on Earth right now at 1/kWh, you could beat the energy cost of a laser-powered space elevator or any sort of electrically powered launch method that has to pay 10/kWh for electricity. (Of course there are costs to properly condition that chemical energy so it is cryogenic, etc, but that's true for converting electricity into laser/rail gun power, too.)

ITS is supposed to get the cost of propellant in LEO down to just $9/kg. Nine. Dollars.

And ITS isn't the end-all, be-all, either. It's possible to improve efficiency by increasing chamber pressure, modifying mixture ratio so you run either oxygen-rich or stoichiometric, improving mass fraction, using hydrogen on the upper stage to better match the Isp to the flight speed, perhaps using a lower speed launch rail to accelerate the first little bit where it's not possible to lower the Isp far enough to match the flight speed, launch from high altitude, etc.

But for a first shot at a truly large, full, rapidly reusable RLV, ITS (and possibly whatever Bezos has planned for New Armstrong) would already demolish the $50/kg benchmark that these alt-launch concepts are struggling to meet.

You can deduce that space guns and space elevators will likely beat RLV if you reason by first principles.  Suppose you had the perfect RLV.  For the sake of the scenario lets define the perfect RLV as one that is infinitely reusable, requires no maintenance, and is completely automated so there are no labor costs.  The cost of using such a RLV would be in fuel alone.  Elon Musk said that the fuel costs for the Falcon 9 are about $200,000 per launch.  Sure you can use a cheaper fuel like methane, but you still have to deal with the rocket equation.  At the end of the day you will still consume fuel at a 10:1 fuel to payload ratio if you are really lucky. 

The perfect space elevator would only require an amount of energy roughly less than the orbital energy of GSO.  The perfect space gun would only require the orbital energy minus the drag the projectile initially experiences.  In order to put it in perspective the orbital energy for LEO is about 33 MJ/kg, which is less than what is in a kg of gasoline.

Fact of the matter is that if it comes down to energy space elevators and space guns will always win over RLV.
« Last Edit: 04/13/2017 08:40 PM by DarkenedOne »

Offline Robotbeat

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That is due to a misunderstanding of the significance of the rocket equation. If you adjust the Isp of the rocket to match your flight velocity, you can get PERFECT efficiency. Now, as chemical rockets cannot get more than 5km/s (vs 7.5km/s orbital speed) there's a limit to the efficiency, but you're still able to get maybe 75% of the way to perfect efficiency just with chemical rockets.

Guns have to deal with going at full speed in the dense atmosphere and still needing circularization. Additionally, since it isn't expanding into vacuum, there is inherent loss there as well.

With space elevator, you have loss due to conversion of electrical energy to laser energy then conversion back to electrical energy. That alone gives you maybe 25% efficiency.

And a big problem with gun and elevator cost is the dry mass that the payload carrier brings with. That is pure loss. Suppose your elevator climber is equal in mass to the payload it carries. That's ANOTHER 50% loss, so you're ALREADY back to the same efficiency of ITS, PLUS your source of energy (electricity) costs an order of magnitude more than ITS's methane. And actually dry mass in orbit, this is one thing that rockets (especially multistage rockets) can excel at. The F9 upper stage has a dry mass of about 4.5 tons, and supposedly can put 22 tons payload in orbit. That's an incredibly good ratio. ITS is supposed to do similar while also being fully reusable.

Again, we get hung up on the exponential part of the rocket equation and tend to ignore everything else. Rockets do dang good. Gun launch cannot compete even with finely divisible & g-tolerant payloads.

(This may be different for the Moon, by the way.)
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Offline Robotbeat

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Let me repeat: the rocket equation does not preclude perfect efficiency.
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Offline Robotbeat

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A gun is also not perfectly efficient. Maybe 30% efficiency is typical. Good rocket engines are better, with some oxygen-rich Soviet engines approaching 50% in vacuum (and you can get 80% or even 99% efficient with high enough pressure and big enough area ratio). And that's ON TOP of the inefficiency induced by the dry mass of your sabot, payload carrier (with heatshield), and crazy friction.
« Last Edit: 04/13/2017 09:10 PM by Robotbeat »
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Offline KelvinZero

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A "space gun" could launch with less acceleration, but it would need to be a lot longer.
Yes that is always true, but you have to remain within a reason length.  In order to accelerate a mass up to 5 km per second with  max G force of 50g you would need a barrel 25 km long.
There is an possible workaround to this that can be exploited in some cases. It might be hard to apply to direct launch from earth but could be used in other cases.

Rather than a huge rail gun used only momentarily, you could have a much smaller rail gun that fires a stream of momentum carrying elements. Then you would need a similar rail on your vehicle to catch this stream and send it back to the first rail gun in a loop. It would sort of be equivalent to having a temporary magnetic rail that stretches over say a million kilometers.

If you do not attempt to recover these elements then the catcher on the vehicle can be lighter than the launcher. In the extreme case with smaller elements this becomes a variation on beamed powered propulsion.
https://en.wikipedia.org/wiki/Beam-powered_propulsion#Direct_impulse

Offline ChrisWilson68

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A "space gun" could launch with less acceleration, but it would need to be a lot longer.
Yes that is always true, but you have to remain within a reason length.  In order to accelerate a mass up to 5 km per second with  max G force of 50g you would need a barrel 25 km long.
There is an possible workaround to this that can be exploited in some cases. It might be hard to apply to direct launch from earth but could be used in other cases.

Rather than a huge rail gun used only momentarily, you could have a much smaller rail gun that fires a stream of momentum carrying elements. Then you would need a similar rail on your vehicle to catch this stream and send it back to the first rail gun in a loop. It would sort of be equivalent to having a temporary magnetic rail that stretches over say a million kilometers.

If you do not attempt to recover these elements then the catcher on the vehicle can be lighter than the launcher. In the extreme case with smaller elements this becomes a variation on beamed powered propulsion.
https://en.wikipedia.org/wiki/Beam-powered_propulsion#Direct_impulse

Good luck aiming the slugs to hit the rail on a vehicle a million kilometers away.

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