# How far can we go with space drives and the third law?

1. Aug 31, 2016

### Lothlorien

Hi all.

I'm new here and I appreciate any help on this question:

Under the premise of E=mc^2, m=E/c^2 and the more energy you gift your propulsion device and the faster the exit speed I'm trying to figure out exactly why making some sort of high speed mass driver system as a propulsion drive is not feasible.

Granted, this question is for making a few decisions on a SCI fi script and I'm just trying to work out how far to stretch the windows of possibility, say, in 50 years, could we have drives that still eject mass but with sufficient efficiency and impulse (with whatever methods) so as to allow adequate interplanetary travel, at least for the inner solar system.

Cheers.

2. Sep 1, 2016

3. Sep 1, 2016

### Lothlorien

Adequate being that it is within a craft's abilities to perform multiple course corrections, not require gravity-assists and not have to rely on meticulously precalculated trajectories to allow the easiest possible route. In other words, the craft would have enough propulsion and enough endurance to go from, say, Earth to Mars regardless of position / aphelion of the two planets - and I guess one could throw in the ability to turn back at any point for whatever reason. This, to me, is what constitutes a space-faring societies minimum requirements adequately.

4. Sep 1, 2016

### phinds

So if it took 10 years to get from the Earth to the moon, that would be OK?

5. Sep 1, 2016

### Lothlorien

Well, if a craft had the capabilities I described earlier, for example not requiring carefully precalculated trajectories, then this means it has to be able to move at a decent pace for extended periods of time. Otherwise the thrust would not be adequate to move the craft against the natural inclination of space time. It would mean it would have to be able to resist the gravitational pull of incidental nearby planets if they would counteract the intended progress of the craft. This implies a fairly strong thrust (compared to, say, an ion drive) which is at least capable of moving the craft in this manner. With this level of thrust I would think it would cover the Earth Moon trip in hours or days. Not years.

6. Sep 1, 2016

### phinds

Yes, but "I think" is not a specification. What I am trying to get you to realize is that you have not really laid out enough specifics for your question to have any kind of realistic answer. What EXACTLY do you want it to be able to do? How much payload should it carry? How long to the target? What IS the target. How many crew? Duration of stay? You have not even begun to answer the kinds of questions that are needed in order for this to be a anything more than a fairly meaningless discussion.

7. Sep 1, 2016

### Lothlorien

I had a feeling you were doing this for such a reason. That is why I precluded my post with an explanation on why I am enquiring. It is for a SCI fi script and thus I'm more after a general opinion rather than a specific answer.

8. Sep 1, 2016

### phinds

Then just make stuff up and gloss over it. It's the story that matters, not the technology.

9. Sep 1, 2016

### Lothlorien

I was hoping for something a little better than that which is why I'm trying to go the extra mile. As long as the underlying general concepts are sound then yes I am more than happy to gloss over the specifics. However in trying to describe the general principles behind a futuristic propulsion system and assuming that no further breakthroughs into more advanced methods were discovered I was just curious as to how far we could push the current method of "shooting stuff out the back to go in the other direction". but thank you for your time.

10. Sep 1, 2016

### phinds

You seem to think that technology is the issue. It is not. COST is the issue, and you have to answer all those other pesky questions before you can answer that question. And underlying the question of cost is the question of motivation. With the very small widespread motivation we have now, improvements will be very slow. If we had unquestionable data showing that the Earth was going to be obliterated in say 15 years or so if we did not develop a massive space capability beyond what we already have, that would change the motivation.

11. Sep 1, 2016

### Lothlorien

Ok I understand where you are coming from. I was surely under the impression that I didn't need to answer these things in that order, especially if it is not for a real world proposal, engineering design or project? If you'd like to fill in those blanks yourself and qualify your answer that way, that would be fine by me. All I'm interested in are speculated examples of what people think is feasible and realistic.

PS: I also now realise I have posted this in the wrong forum. This belongs in the Writing Forum. My apologies.

12. Sep 1, 2016

### Staff: Mentor

E = mc2 isn't what's important here. First let's look at kinetic energy and force:

Ke = 1/2mv2
F = ma

Take the example of a 1,000kg probe. If it expels 1kg of propellant at 100m/s then the force = 1kg * 100m/s = 100 newtons. Divide that by the mass of the probe and you get the final velocity = 100 / 1,000 = 0.1m/s. If that's all the fuel it has then 0.1m/s is the Delta-V

How much energy did it take to do that? The kinetic energy of the fuel = 1/2 * 1kg * 100m/s2 = 5,000 joules. That's not much, but it also doesn't get us much. At 0.1m per second our probe isn't going anywhere fast, it's not even getting out of Earth orbit. There are two ways to solve this: expel the propellant at a higher speed (i.e. increase the exhaust velocity) or expel more propellant at once (i.e. increase propellant flow).

Lets go with the first scenario and this time have the exhaust velocity be 1000m/s instead. Running through the same calculations:

F = 1 * 1000 = 1000n
Dv = 1000 / 1000 = 1m/s
Ke = 1/2 * 1 * 10002 = 500,000J

Notice that we've increased the exhaust velocity by 10x but the energy required has gone up 100x. This is because kinetic energy increases with the square of the velocity. It doesn't take long for this to become extremely unmanageable: requiring very advanced power systems on the ship, engines that can use them effectively and a host of support systems like heat radiators. All of that adds complexity and mass. And don't forget this is the theoretical minimum if the rocket is perfectly energy efficient, which it wont be!

What about the other scenario of keeping exhaust velocity the same but increasing propellant flow? Energy increases linearly with that, throw out ten times as much propellant at the same speed and it will cost you ten times as much energy. On the face of it that seems more sensible except you now have to carry a lot more mass. We've been ignoring the mass of the fuel so far but obviously it has mass. If you add 10kg of fuel then the probe now weighs 1010kg and so we need to bring a little bit extra for that extra. To find out how much we can use the Tsiolkovsky rocket equation. That's a bit more complex to work through so I won't do it here (I advice looking through it and putting it into a spread sheet to play around with) but a 1,000kg probe with an exhaust velocity of 100m/s aiming to reach 1m/s needs to carry 10.05kg of fuel. If you want to go 10m/s and keep exhaust velocity the same the probe needs 105.7kg of fuel. So you've increased the speed by 10x but increased the fuel requirement by 10.6x. This problem only gets worse the faster you go.

This is the fundamental problem with space travel. It requires ludicrous amounts of mass and energy to go anywhere "fast". As you want to have technology not be much greater than what it is now you're going to be stuck with this problem. The very best we could probably do with current technology would be to use a nuclear thermal rocket which were investigated decades ago but never developed. If you have a little wiggle room you could use a plasma rocket but even plugging the estimated capabilities of those into the rocket equation won't end up with people jetting round the solar system in a matter of days. Years for the outer-system, maybe a few months for the inner.

Last edited: Sep 1, 2016
13. Sep 1, 2016

### Lothlorien

Thank you for this. I will have to investigate nuclear thermal. If this is the orion project then that has huge design issues all on its own iirc Finally, there seem to be a few advancements on the horizon for ion drives at the moment in terms of allowing higher thrust yet maintaining the efficiency. I guess that may, depending on how far they can take that tech (perhaps even an array of them) be another possibility even though it's a stretch.

Cheers.

14. Sep 1, 2016

### Staff: Mentor

No nuclear thermal isn't project orion. The idea behind nuclear thermal is that you heat up propellant by passing it over a nuclear reactor. The heat causes it to massively expand and shoot out of the nozzle at high speed.

We could probably help you more if you answered some of the details that Phind's was getting at. What kind of world are you trying to write about? Is it one in which small teams of astronauts are sent on months-long missions to other planets or something much more advanced like the general public taking short holidays on Mars?

Knowing the size of the craft and the intended duration of an average journey will help in answering what technology is needed.

15. Sep 1, 2016

### Lothlorien

Ok sure. I'll give the quick few-paragraph rundown: It is set in the near future. Earth and Mars are colonized with various trade routes and consumers have space corridors (series of mass accelerators / decelerators) that allow mass transit during certain earth/mars orbital positions. So there is trade, commerce, travel and piracy in these stories. Due to pirates, all craft alter their course multiple times every hour to a random vector along their actual trajectory with reasoning being a ship on a constant course could easily be destroyed or intercepted with ballstic matter set up hours or days in advance. As for the propulsion: Currently we are depicting ion drives with fission reactors for the larger ships. I have the larger ships run solely on ion drives, smaller scout ships run on ion drives with a chemical rocket booster (as they are used to spy on enemy ships in the story) to get out of areas really quick.

Military craft main weaponry are mass drivers. Area of engagement is in the less-than-300,000km range for kinetic bombardment. Military craft also use spinning gyroscopes to alter where they are pointing in space instead of expelling precious propellant. Sudden evasive maneuvers are performed with what could best be described as a space anchor that shoots out of the craft in various directions and the craft can quickly alter its location by several hundred meters very quickly, in an attempt to evade the enemy bombardment. Basically its 1800s battleships and cannons again, just in space.

Edit: Summarised and took the useless bits out of my original post.

Last edited: Sep 1, 2016
16. Sep 1, 2016

### Lothlorien

Currently they are way too big to be feasible. Aircraft carrier sized. So this is where we are digging in and ripping the script to shreds and seeing what we can do to improve its realism, so I'm guessing if we can work out how far in the realm of possibility we can achieve some of these storyline battles in space, I'd adjust the ship size downward as much as possible. I hope the realistic number isn't too, erm, small :O

17. Sep 7, 2016

### Staff: Mentor

How do your space corridors work? Are they some sort of megascale magnetic catapult? Have you looked into the idea of a Mars cycler, they might fit what you need. The idea is to have a large space craft that orbits the sun in such a way that it regularly intercepts Earth and Mars. Smaller craft/cargo would jettison off of it to be caught by some sort of breaking system and outbound craft/cargo are accelerated up to dock with it.

Pirates...are a tricky one to justify in space. There is no stealth in space[1] (at least without really contrived circumstances). Pirates could be easily seen and tracked back to wherever they come from. Even if there was a space station that would allow them to dock and hide them it would be trivial for a belligerent faction to attack or blockade it. Perhaps instead the political situation could be such that you have customs officials acting like pirates. Various Mars factions may sponsor these groups to carry out technically legal "random inspections for contraband and tariff control" when really what they're doing is identifying potentially rich targets, corruptly auditing them and finding a reason to "confiscate" the cargo.

Aircraft carrier mass is indeed huge. Do you really need that from a story perspective? How many actors do you expect to play ship roles? A large fast[2] military ship could be >90% fuel by mass and be crewed by a dozen people and a few robots.

[1] This also makes scout craft almost completely obsolete. With a large enough telescope array (which would be well within the means of an interplanetary economy to make, it would likely be necessary for traffic control) could could track every craft in the solar system. Space is cold, real cold, and spacecraft have to be close to 300 kelvin in heat to keep their human occupants alive. Far more if they've got nuclear reactors because they have to dump a lot of waste heat.

[2] "Fast" as in able to maintain centigee acceleration for sustained periods allowing for Earth/Mars transfer in 6-7 weeks, at the cost of an eye-wateringly huge bill.

18. Sep 7, 2016

### Lothlorien

I guess megascale catapult is a good way of putting it. I imagine many of these in series, giving a rather brisk push to the craft at each stage. I thought a nice trick would be that you need to get a "partner" coming back the other way (Earth -> Mars, Mars -> Earth) of either another craft or you pay additional tax for them to send ballast / raw materials back the other way. That way the mass drivers, after all the catapulting has been done, waste as little precious fuel as possible station-keeping themselves. Supposedly the series of relays at the other end then slows them down. Perhaps they also are used, while they are in transit, as a drop-off vehicle to certain space-corridor-units. Perhaps each vehicle drops off 2 or 3 canisters of fuel and preprogrammed relay points (Sorry, getting rather deep in the whole fantasy technobabble. The point is it's just a way of leveraging movement in both directions and offsetting a large part of the propulsion to some sort of space infrastructure, leaving the craft propulsion system to course correct I guess).

EDIT: Oh, and of course, such a corridor would only be "open for business" at certain times of the earth/mars orbit.

Indeed. I did have such contrived circumstances to allow them to exist. I initially included this in my previous response but I thought it was a whole lot of fluff and I didn't want to waste anyone's time reading about these useless details so I took it out. But now that you mention it, yes I did create such a contrived circumstance and it is basically this:

- We're going to have to assume that there are many many craft from many nations and they all have callsigns (similar to ocean vessels on earth at present) and such pirates have counterfeit callsigns so they are in fact hiding in plain site.
- Also, perhaps a lot of space junk. Man-made. From mining and other such activity. So it may be a difficult task information-wise by authorities to identify pirates.
- In other words, I tried moving the battle of the space pirate into the information space rather than actual space. Thus the tracking of space pirates becomes a problem similar to tracking terrorists in a country in today's world. The problem is not getting SWAT to their door (the easy part), it's working out who is the real pirate. Easier said than done.

I was thinking of a few ways a pirate ship could mask itself is perhaps with some carefully constructed stealth shield/dome and all waste heat of the craft is removed by a very small single opening that is pointed towards a blank part of space but from what you said I'll have to rethink this whole area. Cheers.

This is a great idea. I'm going to have to put this one aside :)

I can adjust the craft to whatever sizes are feasible. I was thinking with a few advanced nuclear reactors they'd get into the megawatt-ish range. Still a drop in the ocean in terms of the power needed to push these things with, say, ion propulsion, so I'm open to drastically reducing the craft size and/or their abilities. The thermal signature for the scouts may be occluded with some sort of advanced stealth exterior that concentrates waste heat into a single ray of particles that point to a blank region of space. I'm guessing that would make it pretty hard to track.

Plus, we have natural objects of occlusion that make it very hard to track everyone everywhere. The sun, for instance. Part of the storyline I'm working on is some-earth-faction fiddles with an asteroid on the other side of the sun, so by the time United Space whatever picks it up, it's on a collision course with country X and nobody knows who nudged it sort of scenario.

I'm not sure if I brought it up in my previous post before I edited it but, as with the point you raised, battles in open space don't happen in this fictional universe. They happen close to what I like to call "objects of occlusion". Planets, moons, asteroids, or man-made temporary clouds that occlude what can be tracked. This allows for some reasonably exciting battles. Just some military thing that deploys some gasseous substance that interferes with light. Perhaps infrared is the only Achilles heel in that system but I'm just going to have to concede that as one of those glossed over points. I'm sure the audience won't mind too much *eek*.

[2] "Fast" as in able to maintain centigee acceleration for sustained periods allowing for Earth/Mars transfer in 6-7 weeks, at the cost of an eye-wateringly huge bill.[/QUOTE]

I'm happy that a large part of the existing rocket bill (per pound of launch material in space, etc) is because the rocket programs by, say, NASA with the space shuttle had a lot of non-reusable stages. SpaceX's Falcon Heavy are literally an order of magnitude more dollar efficient. I think the total outlay of costings for fuel and other additonals comes to something very small, like only 2 million, compared with 20 million in running costs for a shuttle launch.

I will indeed rethink the space pirates role as this is looking not as rosy as before. I'm only with taking it out because I would have loved to put in a character talking about the irony of space travel being so simple back in the old days. Nowadays you gotta waste twice as much fuel twisting and turning for nothing. Or some sort of throwaway line :D

Cheers.

19. Sep 7, 2016

### Staff: Mentor

Posting from my phone so please forgive the format and brevity:

RE space catapult. In theory a long tube that fires a space craft could work, in practice it would be difficult to use on large, human crewed ships. The shorter the tube is the higher acceleration would be needed. To calculate this we can use a = v2/2L. A is the acceleration in m/s2, V is the final velocity in m/s and L is the length of the tube in metres. If we posit a 100km catapult (about 1000x the size of the international space station) and a velocity of 15km/s (IIRC the delta-V for an Earth/Mars transfer) that gives an acceleration of 1,125m/s. Divide that by 9.82 (the downward acceleration that objects on earth experience) and you get a crazy 114.5gs!

That's not survivable by humans, it would likely wreck many machines too. If we up the size of the accelerator to 1,000km (a third of the diameter of the moon) the acceleration drops to an easier 11.45gs, but that's a huge object to manage. You also need to then work out the energy required. We know the absolute minimum from the kinetic energy, if we work out how long it takes to exit the tube we know how many seconds that can be spread over. To work out the time to exit the accelerator we use t = √2L/a. For the 1,000km version that's 133.333 seconds.

To plug that all together with the energy let's assume a 100 tonne space craft. At 15km/s it's kinetic energy is 1.125e13j, or to put it another way 11.25 terajoules (enough to power the whole world for two-thirds of a second). Spread out over 133.333 seconds that leads to an energy requirement of 84GW, that's around 40 nuclear power stations today.

It's easy to see with all this why these things would cost so much money. Even if the cost to launch to orbit was lowered and space manufacturing progressed were talking about systems that need to produce as much energy as a European nation just to operate. How much do you think the bill is for that?

RE stealth and debris. Space is really, really empty. Nothing to hide behind or occlude oneself. Even around a moon or planet a very simple spread of satellites will ensure sensors covering all angles. Attempting to pump heat and radiate out in one direction is dooable in theory but it would require one incredibly hot radiator and projecting heat in a small cone is quite hard. I'm skeptical it could be done for long periods of time without serious damage to the ship and the radiator. That's not to mention the fact the exhaust is like to be visible. In terms of traffic control and call signs space is too vast for that to matter. Even in orbit space craft are going to be pretty far apart, stay on highly predicable paths or be easily trackable once they move. Hiding in a crowd or in debris is simply not possible outside of soft-SF.

A couple of quick proposals:

1) Just ignore much of this. If you're not trying for hard-SF then it's not as important as a good story.

2) Look into beam propulsion, particularly M2P2 sails. You could potentially cut a lot of your problems RE travel if you assume the difficulties in beam propulsion have been fixed.

3) Perhaps step back a bit and post (or just think about) the story you want to tell. It might be that this is all putting the cart before the horse and there's a much simpler way to tell the story without diving into the complexities of how large scale interplanetary transport (and the societal consequences) would work.

20. Sep 8, 2016

### Lothlorien

Ok thank you for taking the time to write this up. It may well be very impractical. I agree a space catapult as you have described it would be extremely costly, expensive and would have to achieve ridiculous acceleration that would not be very good for the cargo. What did you think of the space corridor idea? I'm not sure if I was clear in describing my imagining of the whole space corridor concept but the main two distinctions from the space catapult you described are:

1. These accelerator stations are in space already, so the craft they intend to push have already exited the earth's gravity well. I was thinking of something mild like 2G or 4G acceleration. With some very simple rail system (as in, literally, a track with a driver) perhaps they could extend a few km either side of the main mass of the relay. As the craft gains speed, the next accelerator station would add to the previous station's push in a series of diminishing returns.

2. There is not just one accelerator station. They are in series. Say tens of them near earth for acceleration and a similar quantity on mars. This is to give some sort of simple quick explanation of how they could potentially be managed by modularising the system and thus stations can be taken offline and upgraded / replaced. New stations could be added to the line-up etc. As opposed to one huge megastructure.

As far as the cost of operating in space, yes it is incredibly expensive. 2 things I consider when glossing over such events in my script is commercial applications that require space operation and the actual technology for space operations mature.

If we use the most optimistic estimate by SpaceX, their Shotwell initiative could bring the launch costs down to $5-7million USD. That may only be to LEO but the Space Shuttle was also LEO and its average cost was$450 million per launch. SpaceX is shooting for 2 orders of magnitude by 2019.

I'm going to reply to the others in another post as I'm just on my break at work at the moment:

Cheers!