BBC Focus Magazine

[M Paul Lloyd]

Whilst we dream of flying to distant worlds in ‘warp’ powered starships protected by exotic deflectors, force fields and all kept in check with artificial gravity and inertial damping systems we must not lose sight of the reality that in the not too distant future we will hopefully be making our first tentative steps on a journey of manned exploration and colonization of our local solar system a journey which, save for some miraculous discovery or invention, will be achieved in rather more conventional, slow and vulnerable vessels.
To this end we need to think very carefully about how this might be achieved within the limitations of current or near viable technology and consider, very carefully, the risks imposed by solar radiation, possible meteorite collision and the vast distances and journey times involved. It is also going to challenge some of our attitudes towards the sort of lifestyle we have at present against what we might expect from the restrictions and moral dilemmas that circumstances will impose upon our space borne descendents.

Construction.
The total mass of any given space craft is important as it governs the power needed to accelerate and decelerate said mass and thus the energy needed to propel a given volume and mass of propellant required to achieve that end. Basically the more fuel you carry the more fuel you need to propel that additional fuel and so on escalating to quite staggering proportions. Structurally this poses quite a problem in that our space ship will have to have the ability to carry such huge amounts of fuel, but low structural mass and high structural integrity. We are, in effect, looking for the best compromise between the low mass of an airship and the structural strength of a submarine, which it has to be said, is not going to be an easy task.
This vessel also has to be designed in such a way that in can be built/assembled fitted out, supplied and ultimately navigated around the Solar System as efficiently as possible. It is presumed that such construction would take place in earth orbit with major components being lifted into position by conventional rocket systems although in time it would be hoped that such work might be possible in purpose built space borne facilities in Solar orbit where resources from asteroids and such, might be acquired with relative ease?
So let us have a look at the major points on an individual basis.

Accommodation.
If this vessel is intended to transport human explorers, prospectors and colonists around the solar system we are going to have to consider where these people are going to be housed. According to research conducted some years ago it is considered important, for an individual’s mental and physical well being in such an isolated and enclosed environment, that the minimum habitation volume per individual be no less than 10 cubic metres which is 2x2x2.5 metres (not much more than a large cupboard really) set aside as private space.
This research also suggests that a communal area relative to at least one ‘private’ space per occupant is important although this could be incorporated into a multifunctional work area/canteen/conference room and exercise area. This means that for a crew of five we will have to allow 100 cubic metres of habitable space even before we address such environmental necessities such as plumbing, recycling, air supply, food production and/or storage.

Supplies.
Storage will be limited as it adds to the total mass of the ship pushing up propellant requirements and ultimately the need for more propellant and structural containment for same. This makes the idea of a fully recyclable system, which can be incorporated into some form of crop production, a highly desirable proposition.
Although it has to be appreciated that such a system would still require the same basic mass of elements within the system as large capacity storage might in any case and it is worth considering the pro’s and con’s of both systems before committing entirely to one source of consumables.
Back in the 1970’s someone at NASA (sadly I cannot recall who exactly) calculated that the average human requires a bare minimum of 24 square metres of life supporting agriculture. This would supposedly provide the 2,000-2,500 calories and 2 litres of fresh water every 24 hrs required for a basic subsistence diet as well as forming a significant part of an environmental management system. However I have done some research of my own and found that a small holding farmer in the Northern Hemisphere will come up with a figure of at least 450 square metres per person. Now this does make allowance for a sustainable and seasonal rotation of crops with storage to see over the colder seasons along with the inevitable day/night solar power fluctuations. So I think it is reasonable to assume, given that we can intensively grow and harvest our ‘hydroponic’ garden 24 hours a day 365 days of the year, we could get away with considerably less, but, I suggest, still somewhat more than the NASA figure? After all even the best planned and intensive hydroponics farming system could be prone to possible crop failures and the need for a small surplus against times of possible shortage makes the 24 square metre figure seem far too optimistic. Looking at it from a purely practical viewpoint 10x10 square metres does not seem unreasonable but even so this is quite a lot to accommodate on our ever growing spaceship!! The most economical way in which this might be installed would probably be a system of racks to save on space growing high yield crops that could be processed into a variety of reasonably palatable foodstuffs with 100 square metres represents a set-up of ten shelves (with adequate access for husbandry an harvesting) one metre deep by ten metres long and for a crew of five could be accommodated in a space 5x10x20 metres. This would also have be supplemented by a recycling system as the crews waste products cannot be used as a growing nutrient source without some prior treatment. Recycling of human waste will be the main source of nutrients for on board food production and indeed food production could serve as an important part of the waste recycling system and environmental control mechanism.
Basically we need a suitable sewage treatment plant which will virtually double the size of our agricultural component along with its total mass.
Now consider how much actual storage that additional mass/volume of food production and recycling would provide for any given voyage duration?

Operating such a closed loop system on a minimal production basis is risky at best and it would prove more beneficial if food production exceeded the basic needs by a small margin as a stop gap against any unforeseen set backs. Experiments with this type of system on Earth have so far proved worryingly negative and much research still needs to done in this area. The main problem with on board food production is that it would require as much, if not more, space than would a storage facility of prepared food stuffs that would last many months, but this still presents us with a suitable method of waste disposal/storage. The current solution is simply to eject said waste overboard but in our future space borne environment such ‘waste’ is actually a valuable commodity and storage of same for eventual retrieval might be a serious consideration.

An alternative possibility is that in the years to come genetically modified plant strains may prove the best option despite their current unpopularity with the wider population. It is all very well for us to sneer at such use of technology from the relative comfort of the home world but faced with the harsh realities of life in deep space GM might just prove an essential necessity.
It is also possible that animal protein could be grown as a genetically modified cell culture providing a high protein yield food source, which would require very little space to produce. This would be in effect meat in a test tube and probably not something that will appeal to everyone, but faced with trying to produce the required daily calorie count per crewmember based entirely on what would essentially be a vegan diet it does offer a possible alternative that should not be overlooked. http://www.telegraph.co.uk/science/scie ... first.html
However it will be up to those who choose to travel around the solar system as to what types of food source they find the most acceptable and we cannot prejudge the moral issues involved in such decisions.

Propulsion systems.
Propulsion is a product of Newton’s third law in that ‘for every action there is an equal and opposite reaction’ thus an expelled exhaust will result in a proportional thrust in the opposite direction. Given that the energy involved is a result of the expelled mass times its actual velocity we can see that the faster the exhaust is expelled the less mass is required for a given amount of propulsive/kinetic energy.
The benefit of ion or plasma type propulsion over more conventional liquid fuel thrust chamber rockets is one of duration, whilst a rocket can impart great thrust and acceleration with low velocity/high mass exhaust it can only do so for a very limited period of time as its fuel supply will be expended very rapidly. An Ion motor on the other hand, despite using only a fraction of the thrust mass of a rocket, can operate at much high exhaust velocities for many months at a time slowly building up to far more practical velocities without the detrimental side effects of the rapid acceleration that characterizes the thrust chamber rocket motor.
Ion drive motors of this type have been employed successfully on a small scale and although they do not lend themselves to being scaled up on an individual basis I see no reason why they could not be used in multiple installations for a larger space vehicles. Alternatively we could use something very much simpler such as water in a steam jet system, although using such a valuable resource as a propulsive medium does seem insanely wasteful in my opinion but it does present a possibility that cannot be easily overlooked.
As to a propellant we could consider a rather experimental plasma drive whereby an inert substance say Lunar soil could be heated using microwaves or an electric arc (plasmatron) until it becomes a high energy gas or plasma and then ejected out of a magnetic tunnel (to avoid nozzle erosion) that would by virtue of the mass and velocity of the discharged plasma accelerate the vessel in the opposite direction. This type of propulsion has actually flown in the form of an ‘Arc-Electric’ or Electric Propulsion system, the advantages of which are simple construction, ease of control, simple power requirements, ability to use inert propellants, low cost, relatively high thrust and overall efficiency.


Power Source.
Mention this at any meeting of the Interplanetary Society and it will not be long before someone mentions fusion power, and yes, ideally such a power source would be most useful but it overlooks a couple of key issues. First of all no-one has yet managed to get fusion power up and running for more than a few nano seconds and even when they do the installations are going to be simply too massive for our short term needs. The other thing to bear in mind is that power, although essential to the smooth running of our vessel and its environmental systems, is only of any use as part of a propulsion system if we have a suitable propellant in abundant supply.
To this end I still believe that an ‘ordinary’ nuclear fission reactor, along the lines of those employed in nuclear powered submarines. Stripped of its non essential shielding such a device could conceivably be raised to Earth orbit using a conventional heavy lift rocket. This would make a fine source of energy for our purposes, radiation is a serious pollutant here on Earth but space is awash with such stuff and our meager contribution will have no detrimental impact to anything but ourselves. We have many years of practical experience with these types of nuclear generation systems, which on the whole have proved to be (generally) safe and reliable for heat/electrical power generation.
Space is generally pretty cold but their will be occasions when waste heat could become a problem although this could be disposed of via large radiators that, if properly designed, could also provide a small amount of additional thrust as any expended energy will do this if its direction can be controlled.

Velocity.
The operational velocity of our space ship is an important factor when considering a crewed mission over many millions of kilometers as total flight time places a severe strain on stored resources and recycling systems not to mention the crew’s health and general wellbeing. Average velocity is dependant on acceleration which in turn is reliant on the ratio of thrust to total mass and with the systems envisaged this is likely to be somewhat less than the 1g (one Earth gravity at 9.8 metres/sec/sec) that might ideally be hoped for which would in turn create a comfortable internal gravity equal to that of Earth. However even an acceleration of just 0.1g would produce adequate speeds for a trip to Mars within a matter of weeks especially if given an initial boost with more conventional liquid fueled rockets. At a rate of just 1 metre/sec/sec acceleration sustained over a period of six weeks would result in a velocity of around 350 kilometres per second, it may be only a fraction over one thousandth the speed of light but it is still 1,260,000 kilometres per hour, that’s 30 million kilometres every 24 hours!! Also acceleration would increase as propellant is expended reducing the total mass of the ship resulting in potential journey times measured in weeks rather than years.

Shielding.
As already mentioned space is awash with radiation which is sadly quite detrimental to organic life. We have two possible solutions to this problem, first of all we could make the hull of our vessel with very dense radiation proof layers to physically shield us from the radiation, but although this works well enough in theory the actual mass of such a shield would prove quite prohibitive. The other possibility is an electromagnetic field not unlike the one that protects the Earth from such detrimental radiation, only on a smaller scale. Much work has already been done in this area with successful test models constructed so the practical application of such a system is no longer entirely the realm of science fiction.
This also brings us to the vexed subject of collisions.
A drawback of travelling at high velocity is that a collision with even a very small object can cause tremendous damage to a space vessel. This has been witnessed on many occasions during orbital Space Shuttle missions with something as small as a flake of paint cracking a window in one incident alone. Encounters with anything larger than fine dust seems likely to be rare in the local solar system and it would be hoped that anything large could be avoided by making minor course adjustments at an early opportunity. However being able to locate and track these efficiently will require some sophisticated radar systems which will only be practical at speeds below 10% of light speed. Anything faster than this and by the time the projected wave has been reflected back the resultant manoeuvre would be have to be so violent as to destroy the ship are surely as any collision. Even at the seemingly sedate speed of 1% light speed we are going to have to provide the ship with some sort of armoured shield complete with shock absorbers to reduce the inevitable vibration the energy of even the tiniest impact. Consider that a tiny grain of dust with a mass no greater than 0.1 gram travelling at just 100 kilometres per second will easily pass through 10 centimetres of solid aluminium alloy and the resultant plasma plume would still posses a fair degree of damaging kinetic energy.

Conclusions.
So, as you can see, quite a lot to think about, but putting aside some of wider the political, financial and possibly even moral concerns I think it is actually possible for us to do this in the foreseeable future, if we have the will to do so.

M Paul Lloyd. April 4th 2009