Colonising Mars and thoughts on propulsion
This blog is just a space for me to think about Mars colonisation, however if anyone with better scientific knowledge happens to read it I would welcome as many outside thoughts as possible.
SpaceX Current plan
I've read as much as I can at the moment about the ITS and whilst I think it will be great for the first few trips, I don't see it as a lasting sustainable way to colonise Mars or anywhere else in the solar system.
Space and Time
If I were going to spend over 3 months (or longer) in a space ship I would want it to be both fun and comfortable. Elon has said that it will be a pleasant trip, but when I think of a very long voyage of that kind of duration, I think of something like an ocean cruise.
Therefore if the voyage will be like that it means the ITS will essentially be like a cruise liner, or like a hotel but in space. In other words, the ITS will be filled with things like beds, rooms, furniture, entertainment of all sorts, gyms, cleaning products and all kinds of other things that you probably don't want to be ferrying up through the earths thick atmosphere and deep gravity well on each trip (not to mention the same but less dense/deep atmosphere/gravity well of Mars on the way back). Also you will need a LOT of space for the passengers to move about in and not get bored. Unsurprisingly Bigelow agrees with this.
You could argue that most of that equipment will be needed on Mars (and left behind permanently) and so is actually not a waste, and this might well be true, to start with, however in the long run the trip to Mars will be more about moving passengers and (stored) cargo.
In addition whilst astronauts on the IIS might manage to live for months without gravity, it is not good for your health. Ideally for most of the journey the passengers should be in a similar gravity strength to Mars of about 3.7m/s/s (preparing them for their upcoming adventure). Therefore the ship should ideally have spinning sections taking up even more space and weight that will definitely not be needed once at Mars. Also consider that all hotels generally have staff, and for trips to Mars to be commonplace, (if they take longer than a day) you will want to have staff servicing the needs of your clients. The staff will need to be kept healthy as they will be in space for much longer periods than colonists.
All of the above means you will really want a purpose built ship for the voyage that doesn't descend to either Mars or Earth that is almost entirely made from spinning sections (with some area for weightless playtime).
Space-liner
A purpose build space liner, filled with all those things you might want to keep you entertained and safe would be a much better option for regular trips. It would be assembled in space so that it could be bigger than you could launch from Earth, and be mostly composed of spinning sections. In short its more like a space station than a space ship.
It would serve as an ideal place for the passengers to congregate whilst the ITS style 'Earth' ships ferry up all the cargo, passengers and fuel that will be needed for the journey, which could take a while.
It would serve as an ideal place for the passengers to congregate whilst the ITS style 'Earth' ships ferry up all the cargo, passengers and fuel that will be needed for the journey, which could take a while.
These periods would be just like when a cruise liner docks up and changes passengers. A time for the staff to resupply the ship, perform more thorough cleaning. Perform maintenance on the vessel etc.
Time
Most of the above problems are caused or exacerbated by the sheer amount of time that travelling to Mars takes. The current SpaceX plan is only to go and return during the windows of shortest journey time (once every 2 years) but this still leaves journeys of nearly 4 months. Even by cruise standards, that's a long time to be on a journey. For the trip to really go mainstream this time has to come down, a lot, and that means going faster, much faster. So how can that be achieved?
Using the space travel calculator it looks like it would take a DSL of length 1,850 km 10 minutes to accelerate a vehicle to 6 km/s at 1G. Although a quick search suggests that untrained healthy humans can take 5G briefly or 3G more comfortably for longer periods of time. That would reduce the length of the DSL to 'only' 375 km (5G) or 625 km (3G).
From this website it appears you would need 4410000 newtons to accelerate a dry-weight ITS at 3G. You would need to supply this constantly for the 4.8 minutes it would take to accelerate down the 625km of track meaning a supply of 4410 KW or 4.4 MW assuming 100% electrical to kinetic efficiency. That's quite a lot of power but not impossibly high, but you might want to think about manufacturing them on the moon to save on costs. 4MW is 15000 "normal" PV panels, but unless you were launching continually you could get away with less and use batteries or capacitor or flywheels to store the energy.
Numbers:
You need 555K newtons to accelerate 150 tonnes at 3.7 m/s/s which would be 555 KW of continuous power to your DSL provided 100% DSL energy to ship transfer.
The DSL itself would only be limited in length by the engineering challenges of accelerating 555KW of debris in a sufficiently accurate manner that it would hit the spaceship
NB: Consideration has to be made to the speed of the particles. If they were only travelling at speeds relatively similar to the spaceship they would be of little use. Therefore the faster the particles the more momentum they will impart to the ship and the more efficient the process. It might be necessary to start them slower initially to stay within the constraints of the ships ability to capture the particles without being damaged. Therefore the total energy requirements would be bigger than stated. (or would it?)
Alternate Propulsion
There are lots of articles on alternate propulsion everywhere including Wikipedia, so here is a brief summary of the better known ones and a bit more in depth of ideas I've had which I haven't read on Wikipedia.
Rail gun (deep space launcher DSL)
Despite articles about using rail gun (magnetic linear motors) as non-rocket launch options, I couldn't find any discussion about using them as deep space craft launchers (from orbit). My idea is simply to make the ITS a bit more like a Hyperloop capsule, with a stator on one side that is accelerated by a liner rotor from orbit up to its cruise speed. The ITS could then use its entire fuel for decelerating once at mars. With almost no changes to the ITS this could in theory, from my rubbish calculations, at least half the journey time. Its possible that with the amount of delta-v (DV) the ITS has, you could go even faster as currently I don't think they are planning on using any DV for deceleration once at Mars.
A side effect benefit of the DSL is you could use it as a ship de-orbiter. When returning to earth the DSL could accelerate the ITS in the opposite direction killing its orbital velocity allowing it to free-fall back to Earth. (LEO orbit velocity 7.8. If the DSL could accelerate a full ITS to 6km/s it would be trivial to accelerate it the other way to 7.8). This would significantly reduce the stresses on the vehicle and wear on the ablative surfaces. (Similar to how the Falcon first stage can safely re-enter and land without any ablative heat shield because its orbital velocity is so low).
Because the DSL is fixed in space (in an orbit or on the moon) it can be as big and heavy as you like, which means lots of space and allowance for huge solar arrays and cooling panels.
Considerations:
Orbital Energy
Due to Newtons third law, the DSL would be pushed backwards with the same energy as it propelled the ITS forwards. Depending on the mass ratio of the DSL to ITS this could make the DSL be propelled into deep space instead of the ITS :). Or more sensibly, the DSL would need to be significantly more massive than the ITS. (And for pretty obvious reasons it would be).
In addition it would need to manage its kinetic energy. Every launch into deep space would cause its orbit to degrade. (But would be restored some for every craft it de-orbited back to earth).
Engines on the station would have to make up for the difference in energy ratio of orbital energy lost vs amount gained.
All of the above considerations could be mitigated by placing the DSL on the surface of the moon. I'm not sure if I'm correct on this but in my head the downside of that would be that you would limit the trajectories available to ones that aligned with your rail gun (presuming it wasn't free to rotate). In addition it may be more difficult to construct a rail gun of sufficient length whilst in the gravity well of the moon, or maybe it would be easier? In any case you would lose the de-orbiting ability of the DSL which may not be a massive issue if the PICA-X is as good as SpaceX thinks it will be.
Launching energy and size
Accelerating the 2 million tonne mass of a fully loaded ITS to 6 km/s would take quite a lot of energy and a pretty long gun.
Using the space travel calculator it looks like it would take a DSL of length 1,850 km 10 minutes to accelerate a vehicle to 6 km/s at 1G. Although a quick search suggests that untrained healthy humans can take 5G briefly or 3G more comfortably for longer periods of time. That would reduce the length of the DSL to 'only' 375 km (5G) or 625 km (3G).
From this website it appears you would need 4410000 newtons to accelerate a dry-weight ITS at 3G. You would need to supply this constantly for the 4.8 minutes it would take to accelerate down the 625km of track meaning a supply of 4410 KW or 4.4 MW assuming 100% electrical to kinetic efficiency. That's quite a lot of power but not impossibly high, but you might want to think about manufacturing them on the moon to save on costs. 4MW is 15000 "normal" PV panels, but unless you were launching continually you could get away with less and use batteries or capacitor or flywheels to store the energy.
Alternatives to electrical
If the practicalities of using electrical energy to accelerate such massive objects to such speeds are beyond our engineering, we might still be able to improve on the usual rocket design. I'm imagining the ITS attached by hoses connected to long stationary fuel tanks (375km hoses FTW). This isn't a perfect scenario as the fuel will still need to be accelerated, however it would presumably be more efficient as you would only be accelerating the fuel in the hoses which means the highest acceleration would be reserved for the latter parts of the acceleration curve with the bulk of the fuel never being accelerated much.
Or perhaps you could also do something similar by firing the fuel at the ship from in front of it as the ship accelerates (with some kind of fuel scoop to capture it). The fuel itself would only ever be accelerated a small amount (enough to hit the scoop). Energy is still lost as this mass would cancel out some of the energy gained by the engines thrusting towards the fuel but in my head it seems like it would be more efficient. (The fuel delivery system would itself be accelerating in the same direction as the ITS slightly (as a side effect of its ejection) and so would need some kind of engine to return it back to its starting place). In my head this is basically how a jet engine works (which have much higher ISP than rockets) except instead of just the oxidiser entering at the front its the fuel too.
Something tells me my lack of knowledge of physics is making me miss something here.
Constant Acceleration
If you want to get somewhere fast in space, you need to be constantly accelerating; towards your destination for the first half, and away for the second. If you could accelerate at 3.7m/s/s (handy as it means you no longer need a spinning spaceship) the journey might take as little as 3 days and at most just over a week. This means you could schedule departures any time and not have to wait for one of the 2 year windows to open.
The counter-intuitive nature of constant acceleration is pretty interesting. If you could only manage a tenth the acceleration of the above, i.e. 0.37 m/s/s you could still do the short journey trip in 9 days and the longest journey trip in 24 days.
The counter-intuitive nature of constant acceleration is pretty interesting. If you could only manage a tenth the acceleration of the above, i.e. 0.37 m/s/s you could still do the short journey trip in 9 days and the longest journey trip in 24 days.
On craft electric Propulsion (e.g. Ion engines)
With current technologies completely infeasible due to incredibly low thrust to weight ratio of the propulsion system (solar panels + engines + heat dissipation panels), not to mention the incredible cost.
So I've tried to figure out the numbers and here is what I got:
Dry mass of ITS 150000kg
Acceleration of 3.7 ms2 gives required newtons of 555,000
VASMIR 200kw ion engine has 5N of thrust requiring 111,000 of them at 22,200,000 kw of power (incoming and dissipated) or if you just had one of them it would give you an acceleration of 0.0000333 m/s/s which would take 31 months minimum :O
Needless to say 111000 VASMIR engines and the solar panels and radiators to generate and dissipate the 22,200,000 kw of power would significantly add to the mass meaning the total number of engines/solar panels/radiators would be orders of magnitude higher.
So I've tried to figure out the numbers and here is what I got:
Dry mass of ITS 150000kg
Acceleration of 3.7 ms2 gives required newtons of 555,000
VASMIR 200kw ion engine has 5N of thrust requiring 111,000 of them at 22,200,000 kw of power (incoming and dissipated) or if you just had one of them it would give you an acceleration of 0.0000333 m/s/s which would take 31 months minimum :O
Needless to say 111000 VASMIR engines and the solar panels and radiators to generate and dissipate the 22,200,000 kw of power would significantly add to the mass meaning the total number of engines/solar panels/radiators would be orders of magnitude higher.
Externally supplied momentum
I'm sure there must be other ideas along this line of thought. The idea is you accelerate large amounts of tiny particles at your spaceship from a stationary device. Its a bit like the rail gun idea but instead of shooting the spaceship itself, you shoot particles at the spaceship. Or like a light sail but with much higher DV as its not using photons but actual atoms.
The spaceship would need some way to receive the momentum from the particles without damaging it, my first idea was having the spaceship continuously producing(and recycling) an aerogel (or ballistic gelatin) shield at the rear soaking up the momentum of the particles. If you restricted the particles to solely metallic you might be able to accomplish this with a magnetic containment field, for instance it could be similar in design to the proposed 'Hydrogen scoops'. This would also make it easier for the "gun" to aim it at the ship too as a magnetic containment field would be much larger.
There is no shortage of metallic materials readily available, these could be cheaply mined from the moon, or the gun could be on the moon itself.
Ideally you would have a similar gun at the destination to decelerate the craft after the midway point.
Numbers:
You need 555K newtons to accelerate 150 tonnes at 3.7 m/s/s which would be 555 KW of continuous power to your DSL provided 100% DSL energy to ship transfer.
The DSL itself would only be limited in length by the engineering challenges of accelerating 555KW of debris in a sufficiently accurate manner that it would hit the spaceship
NB: Consideration has to be made to the speed of the particles. If they were only travelling at speeds relatively similar to the spaceship they would be of little use. Therefore the faster the particles the more momentum they will impart to the ship and the more efficient the process. It might be necessary to start them slower initially to stay within the constraints of the ships ability to capture the particles without being damaged. Therefore the total energy requirements would be bigger than stated. (or would it?)
Momentum Transfer
The universe demands that momentum be conserved, but it doesn't specify the direction or which piece of mass must preserve it.
To illustrate these points, think of a billiard ball in a stable orbit between 2 fixed masses. Its direction is changing every time it passes around one of the masses, steered efficiently by the gravity. Now if you were to put another ball directly in its path, when the orbiting ball hits the stationary ball the momentum will be transferred from the moving ball to the stationary one. Apart from losses to heat you now have a perpetual motion machine.
If you place new balls at either end of the loop each individual ball is only making the journey one way.
At usual orbital speeds it would transfer in a less than ideal way of exploding both balls, however in my fictional illustration the masses and orbits are such that the balls can survive. You could also assist this (with reduced efficiency) by placing something sufficiently elastic between where the balls will collide.
Now imagine that the balls are spaceships. Essentially passengers in these balls will be getting a (nearly) free ride from the one mass to the other (just efficiency losses).
Another way to look at it is by thinking of the traditional rocket as accelerating all its fuel in the opposite direction to its travel on its outward leg(to accelerate), and doing the opposite on the inward leg (to decelerate). This is effectively sending large amounts of gas thundering around the solar system helping no-one. By recycling the momentum from mass travelling the opposite way you use much less energy.
The problems I see are:
* Earth and Mars are constantly moving in relation to each other. In my head this shouldn't be a deal breaker but would make it more difficult that the simplistic example above.
* I cant think of a good way to change the direction of the energy without exploding the spaceships.
This sounds like a big deal but I'm pretty stupid and I'm still confused about how a bouncing ball can return to almost its starting height after hitting the earth and presumably acting on it with the same force as it (the earth) had on it, shouldn't it at most return halfway? (with the earth moving very slightly the other way?) I therefore feel that people less stupid than me could think of a good way that didn't involve artificial gravity wells or other new science.
To illustrate these points, think of a billiard ball in a stable orbit between 2 fixed masses. Its direction is changing every time it passes around one of the masses, steered efficiently by the gravity. Now if you were to put another ball directly in its path, when the orbiting ball hits the stationary ball the momentum will be transferred from the moving ball to the stationary one. Apart from losses to heat you now have a perpetual motion machine.
If you place new balls at either end of the loop each individual ball is only making the journey one way.
At usual orbital speeds it would transfer in a less than ideal way of exploding both balls, however in my fictional illustration the masses and orbits are such that the balls can survive. You could also assist this (with reduced efficiency) by placing something sufficiently elastic between where the balls will collide.
Now imagine that the balls are spaceships. Essentially passengers in these balls will be getting a (nearly) free ride from the one mass to the other (just efficiency losses).
Another way to look at it is by thinking of the traditional rocket as accelerating all its fuel in the opposite direction to its travel on its outward leg(to accelerate), and doing the opposite on the inward leg (to decelerate). This is effectively sending large amounts of gas thundering around the solar system helping no-one. By recycling the momentum from mass travelling the opposite way you use much less energy.
The problems I see are:
* Earth and Mars are constantly moving in relation to each other. In my head this shouldn't be a deal breaker but would make it more difficult that the simplistic example above.
* I cant think of a good way to change the direction of the energy without exploding the spaceships.
This sounds like a big deal but I'm pretty stupid and I'm still confused about how a bouncing ball can return to almost its starting height after hitting the earth and presumably acting on it with the same force as it (the earth) had on it, shouldn't it at most return halfway? (with the earth moving very slightly the other way?) I therefore feel that people less stupid than me could think of a good way that didn't involve artificial gravity wells or other new science.
Please help (rubber balls have been driving me mad).
So if a moving rubber ball, hits another rubber ball (in space), presumably what happens depends on the masses of the balls (provided they don't both explode). If they are both the same mass, does the originally moving one end up stationary? My intuition says it does, but maybe they both move apart at the same speed. If the originally stationary one is more massive than the other the more massive one will definitely be affected (by speed) less than the less massive one, but my knowledge of physics says this is a product of their masses and that both are affected equally in terms of energy. This means the larger one will still be pushed backwards some amount.
So in my head I'm trying to imagine a magnetic energy conversion device. An incoming ship passes into the field and its kinetic energy is converted into electrical energy which is then routed into a rail gun to accelerate an outward ship in the opposite direction. However my brain is saying that as the incoming ship is decelerated the station/device itself will be being affected by this and propelled backwards. The same effect then happens from the 'recoil' of the rail gun firing the other ship outwards, resulting in the station being pushed back even more. Then I start thinking how this is similar to how the earth must feel when its being hit by a rubber ball and I start wondering how rubber balls are even possible.
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