Wednesday, September 19, 2012

Interplanetary flight using solar sail material as a light concentrator

Something I've been mulling over the last few days: The big problem with using light sail cloth as a reflector for solar electric propulsion is in holding the shape of the reflector when the spacecraft is accelerating. A parabolic dish reflector would be far superior than a "trough" shaped reflector, but as soon as you accelerate a flexible dish in any direction other than towards the Sun, that acceleration is going to have the effect of distorting the shape of the reflector, spilling much of the sunlight it intercepts. This to me seemed likely to severely limit the ideas practicality, but then it occurred to me that, if one wanted to travel quickly (delta v in the low hundreds of km/s) from any planet to any other planet most of the time a close pass to the Sun would be the optimum trajectory, the spacecraft would accelerate towards the Sun, make a close solar flyby choosing the track to send it to its target, turn as it passed the sun, and then again accelerate towards the Sun to slow its speed for planetary rendezvous. Material of only 0.1g/m^2 and suitable for solar sails now is being developed, so a km^2 of such material would mass only 100 kg, and could theoretically collect over a GW of power at 1 AU from the Sun, even at 20 AU (Uranus) a 400km^2 reflector of the same material could theoretically still collect a GW, and mass only 40 tonnes. Perhaps manufacturing imperfections would still be a problem in a large reflector, even though it's shape, only subjected to the constant forces of insolation, and the spacecrafts acceleration, should be stable in flight, in that case perhaps a system similar to adaptive optics, changing the temperature of parts of the reflector by reflecting light onto the back of it to slightly warm and expand over-tight sections of the reflector could be used. A GW of thermal power for 100kg or 40 tonnes compares rather well with near future nuclear systems, if an electric generation system to turn that thermal energy into electricity to power VASIMR (Ve ~200km/s) or similar electric propulsion could be built to produce 10 or even 20 KW/kg you'd have quite a ship, and one that could be built privately without the complications involved with nuclear energy systems, and because plenty of propellant is still aboard until deceleration starts there's still shielding available during the close solar flyby. Without using aerobraking a large fragile deep space reflector could be furled loosely behind a smaller more robust near Sun reflector, with the deep space reflector still useful as a true solar sail (not needing to hold near perfect shape) in the inner solar system - especially to accelerate the ship at an angle to the Sun to reach or decelerate from the various planets solar orbital velocities. Such a ship would be the ultimate in using ISRU, both the energy and propellant required is available throughout the Solar System, and extended missions without resupply and involving the rendezvous with several planets and smaller bodies would be possible, something even a nuclear system couldn't achieve, because even fission materials would mass too much to supply the TJ's of energy that would be required.

Wednesday, August 15, 2012

How large payload orbiters could fly from commercial airports

This is more like a 3rd generation passenger carrying system, one that's built on an established market (a 747, not a DC3).

Previously I've advocated Aerial Propellant Transfer as a form of Assisted Single Stage To Orbit, now, if commerce and travel between Earth and space colonies were to grow to a similar scale to that which we see between cities today, the volume of traffic would require orbital craft as large as todays largest passenger jets.

It would be convenient if the arrival and departure facilities for such spacecraft were integrated into the existing passenger system.

Because of the cost of increasing runway length and width, the growth in aircraft size is now being restricted, if that situation continues, even many decades from now commercial aircraft could be no larger than todays aircraft.

Using APT, an orbiter the physical size of a modern jumbo would require thousands of tonnes of fuel, so while the orbiter could fly from todays airports, the tanker aircraft could not.

Rather than building super runways to accommodate such giants, perhaps flying them from convenient lakes, far from populated areas, will be the solution.

The orbiter and tanker get airborne at about the same time, the former from an airport, the latter from a body of water perhaps hundreds of km distant, they rendezvous at altitude, thousands of tonnes of propellant are transfered, the orbiter carries on to orbit, the tanker returns to its base to be refueled for the next orbiter.

The tanker:
(a) Not being a passenger carrying aircraft,
(b) operating away from populated centers
(c) flying from bodies of water tens of km long
(d) only required to fly for an hour or so to altitude and then return,

could be a relatively simple wing design, not especially refined for fuel economy etc, mounted over a pair of pontoons.

Tuesday, August 9, 2011

Philosophizing on Morality.

Morality is a product of evolution, it’s a characteristic that all social animals have, and that solitary animals do not have.

The foundations of morality are based in our instincts, social moral codes are built on top of these more basic, instinct based, codes.

People who look at current liberal moral codes as being superior to past, typically stricter, moral codes; who think that morality has, in some objective sense, improved or gotten better, don’t get that these changes in moral codes are simply a product of changing technology and greater wealth, that with the loss of that wealth and technology, stricter moral codes would reappear.

In wealthy societies whose inhabitants perceive little threat to themselves as a whole or as individuals, those strict practices that worked to unite a society through common beliefs and practices (beliefs and practices that in themselves created a clear differentiation between each society and other, outsider and potentially hostile, societies), weaken, as the survival pressure to maintain that ridged unity are not so great.

Greater wealth also means that the cost of expensive and destructive practices by those within the group are more affordable to the group as a whole, for this reason divisive and destructive acts, and those who commit such acts, are more tolerable to the group as a whole.

A useful illustration of how strict adherence to accepted codes can serve to strengthen a social group can be seen in military discipline, where the groups survival is enhanced through sacrificing individualism. Military units that lack strictly enforced internal moral codes don’t survive unless they’ve significant advantages in other respects that offset this lack of enforced cohesion.

Within a society there are two groups of individuals serving different roles that in combination enhance the survivability of the group. The first group is the high status establishment leaders, those representing the status quo of the group, are typically more wary of the threat posed by both insider and outsider groups to that status quo, the second group are those wishing to move up in the pecking order, looking for opportunities to advance their own status, this group is less focused on potential external threats to the group, in fact because the former group uses the external threat to justify the status quo, the second group rates that threat as less significant.

It can be seen that religion is a useful tool in providing a group with many characteristics advantageous to survival, it provides defined common moral codes, codes that differentiate it from outsider groups, and moral codes that, because of the continuity of leadership policies provided by a deity, provide stability. It’s also advantageous if the religions moral codes are appropriate for the environment the followers live in.

All morality that runs deeper than social customs, morality that is always the same throughout human history no matter what cultures might exist, is based in human instincts. Other sentient creatures, for example, those in which, for biological reasons, the female parent eats the biological father as part of the reproductive process, would have their own moral codes that are independent of what humans might think is “right”.

In times of bounty a society as a whole may be better off accepting greater diversity amongst it’s members,taking multicultural societies as an example, I think in times of prosperity they can be stable, but in times of hardship it’s very easy for rabble-rousers to create division. In times of extreme hardship societies fragment and collapse, and cultural differences are one of the first fault-lines to split open.

Monday, July 4, 2011

The hanging gardens of space

I've been thinking about how things would be arranged to make the best use of the room inside a Stanford Torus. The depictions that we get in books promoting space colonies usually show something similar to suburbia, but without the cars, but when it comes down to it, those designing the internal layout are going to do it based around the requirements imposed by the loads and properties inherent in the physical structure of the colony. On Earth, everything we do, we do from the ground up, whereas in a Stanford Torus everything, everything will hang from the axis of rotation. It will make far more sense to support structures and surfaces as directly from that axis as is possible, as that'll reduce the amount of weight required, both because it's a more direct way of transferring loads, and because structures built under tension are far lighter than structures built under compression.

So rather than the boring flat geography we usually see depicted, perhaps the inside of space colonies will be steep, with lots of nooks and crannies, with winding paths through a landscape covered with wild vegetation to hide the engineers suspension cables.

Tuesday, June 21, 2011

Interplanetary Flight Times.

Too often when we talk about future interplanetary flight we assume the trajectories used will be Hohman transfer orbits, elliptical orbits that intersect of both the orbit of departure and the destination tangentially, in the case of Mars this involves flight times of around 250 days. But even with todays chemical rockets the mass ratios for flight times of 150 days may be optimal for cost, and with thermal systems using hydrogen as the propellant flight times of just 70 days to Mars may not only be reasonable, but sensible, with hydrogen propellant and exhaust velocities of 10km/s a delta v of 6.5km/s is achievable with a mass ratio of only 2. All it requires is a thermal energy source to heat the propellant, aerobraking at Mars, and a propellant source on Mars for the return.

Sunday, June 19, 2011

Space Mirrors

I think mirrors are incredibly underrated by people theorizing about future space technology.

On Earth a mirror has to be heavily constructed to holds its form, in space with little or no gravity and no wind to contend with they can be as lightweight as soap bubbles.
The lightest mirrors that are now being studied are intended as solar sails, 3 grams a square meter has been achieved, if we round that down to 1 gram a square meter, how much light can be collected by 10 tonnes of mirrored material at various distances from the sun?
Well, 10 tonnes, at 1 gram a square meter, would give you an area of 10 square kilometers:

from sun___intensity___10km^2

1__________1.3_________13,000.0 (Earth)
2__________0.325________3,250.0 (Mars)
5__________0.052__________520.0 (Jupiter)
10_________0.013__________130.0 (Saturn approx)
20_________0.00325_________32.5 (Uranus)
30_________0.00144_________14.4 (Neptune)
40_________0.0008125________8.1 (Pluto)

So even at the distance of Pluto, just 10 tonnes of mirrored material gives us the ability to concentrate 8 megawatts of sunlight, if solar cells will give you 1KW/KG at Earths distance from the sun, using light-weight mirrors to concentrate the sunlight, they'll still give you about 0.44KW/KG even at Pluto's distance from the Sun.

Saturday, June 18, 2011

Blowing up asteroids

When we've gotten ourselves established in space throughout the Earth-Moon system the next place I'd like to see us target for exploitation are the Near Earth Objects (NEO's), asteroids that orbit the sun at about the same distance that the Earth does.

Because they're close, and because of their small size and low gravity, these objects hold the promise of resources within relatively easy reach, resources that even if not richer than those of the Moon, will in many respects be different to those offered by the Moon, and different and richer can mean the same thing.

Lets imaging our prospectors arriving at what they hope will be a gold mine of rich rare earth elements, water and other volatiles, heavy metals and whatever else that has a market value. Question: how to use the asteroid to shelter yourselves from the radiation of space, How to find out what's inside, and how to get out any resources that are there?

Here's an idea that I haven't come across than might solve all three problems at once - blow it up! No, not with explosives.

Small asteroids are not held together too well gravitationally, if they're loose rubble piles even kilometers inside them, even at their core, the pressure from the weight of material will be very small, so in principle it would be possible to drill a single hole, shove a balloon down it, blow air in, and spread the entire mass of the asteroid out over the balloons inflated surface. Then you move inside, hell, move the whole family inside!

OK, you still want solar reflectors to warm the interior, and you'll probably want more than just the balloon used to inflate the asteroid for accommodation.

If we take an asteroid that's a Kilometer in diameter, and inflate a balloon that's a kilometer in diameter inside it, if the material of the asteroid were spread out evenly, it would form a layer about 125 meters thick.

Friday, June 17, 2011

Rotovator assisted Aerial Propellant Transfer

In my post Aerial Propellant Transfer I concluded that with APT each flight could costs as little as "$1.5 million per flight plus the operators profit. Which comes to $75,000/tonne or $75/kg payload".

But by combining APT with hypersonic rotovators we can substantially improve even on that $75/tonne. Using a 600km long tether we can get the maximum velocity required from the orbiter spaceplane from orbital velocity of 7900m/s down to 4030m/s, a reduction in the required delta V of 3870m/s. We still have the ability to transfer 340 tonnes of propellant to the spaceplane from a tanker at 12,000meters, and because we've gone with APT rather than using the A380 (or similar aircraft) as a booster for air launch, we won't have the compatibility problems that would certainly have occurred in trying to mate a different spaceplane to the booster.

In my previous example post refueling the spaceplane needed a delta V of 8400m/s, by knocking 3870m/s off that requirement delta V comes down to 4530m/s, now we still separate from refueling with 340 tonnes of propellant, and our rocket engines are still H2/O2 with an exhaust velocity of 4500m/s so using the rocket equation a mass ratio of 2.75 gets us a delta V of 1.011 times exhaust velocity, ie. 4500 x 1.011 = 4550m/s. total mass to orbit is 1/2.75 of total separation mass which is 340/1.75 = 194.2tonnes, because total mass at separation has increased (from 400 tonnes to 534.2tonnes (33.6%) the unladen mass of the spaceplane will also have increased, working on a 33.6% increase, unladen mass goes from 40 to 53.4 tonnes, and payload increases from 20 to 140 tonnes. While we will have to increase the engine power of the spaceplane by 33%, the time those engines fire will also be reduced, the tanker requirements are unchanged ($500,000) allowing an increase of 33.6% in orbiter costs (servicing and capital) takes that to 1,336,000 total 1,836,000 divided by 140 tonnes brings cost/kg down to about $13.10, with rotovator energy costs of around $2.50/kg (the rotovator uses electric propulsion to maintain momentum, accelerating 1kg of payload by 4km/s 0.1kg of propellant is expelled at 40km/s, using E=1/2MV^2 that's 80 million joules or 22.2 KWh, at $0.1/KWh thats $2.22/kg of payload plus energy loses) totaling $15.60 plus the servicing and capital cost of the rotovator(?)

Thursday, June 16, 2011

The Stanford torus

Back in the '70's when dreamers were dreaming big dreams about Colonizing Space, one of the most popular colony designs was the Stanford Torus, named for it's torus design and its designers place of work.

The Stanford Torus is a tidy wheel design that spins at 1RPM, it's the shape of a bicycle inner tube 2km in diameter, and provides an artificial gravity as strong as that of Earth to its residents, who live in the inner tube with their heads pointed towards the axial, their feet planted on the inside of the outer circumference of the inner tube wall. The original design had a tube with a width of 130 meters which, with the tube circumference of 6.2 kilometers gave an area for the inhabitants of 832,000 m^2 or 83 hectares of living space.

The population of this colony was proposed to be 10,000 people, plus their pets and livestock, it was originally suggested that it be at L4 or L5, though it was later suggested that the ideal location would be at an orbit about Earth in 2:1 resonance with the moon so as to be positioned between the Moon - the source of raw material - and geosynchronous Earth orbit - the destination of its manufactured product - solar power satellites (SPS).
Being so deep in space the colony would be without the protection from the Earth's magnetic field that protects people and facilities in Low Earth Orbit, so the colony was to be protected by a several meters thick layer of slag or waste from the lunar regolith processing facility, this shield would weigh about ten million tonnes.

I like the Stanford Torus, it is easily the lightest of the various designs that the Stanford team studied, and unlike a cylinder design, it didn't have a pointless kilometers deep atmosphere, and the small diameter "inner tube" meant far lighter construction than a big diameter cylinder. it's an elegant design that's an efficient use of mass, and it caters for a population that I think is realistic, a smaller population would struggle to support a feeling of being a civic community, a much larger population would take time to build, and such a large colony would require a far bigger investment than would be necessary for a first step to establishing a community that was truly living in space.

I like the Stanford Torus, but I don't think colonizing space will be done on the back of a huge investment in mining the Moon, launching the mined regolith into space, catching and processing that regolith into solar panels, microwave emitters and girders, and then constructing SPS's tens of kilometers across which are then moved to GEO. It's all too big a hurdle, too much risk, too much investment all at once, every step of which has got to work.

The Stanford Torus I remember was a place I'd want to live, even if a small section cost ten million dollars, I imagined that there would be no shortage of super rich people, bored with the sterility of Beverly Hills, who'd jump at the opportunity to spend far more than ten million to live there, and I remember thinking that even if I couldn't live there, I'd sure want to visit it some day.

Wednesday, June 15, 2011

Aerial Propellant Transfer.

There are I guess four basic types of schemes for getting into orbit (putting aside the nuclear option which for political reasons and genuine safety reasons isn't worth wasting time discussing), there's the disintegrating totem poles we've always used (eg. Falcon, Shuttle), there's the really big capital investments schemes with banks of lasers, or banks of microwaves emitters to get the effective Isp of some tiny launch vehicle up enough to get it to orbit, thers's space elevators, and then there's some sort of fully reusable self-contained launch system (eg. Skylon).

If you want to see airline type operations into orbit forget Falcon, expendable or nominally re-usable - more like refurbishable - launch systems will never get the cost down low enough, they're like throwing 747's away after a single use.

Maybe one day we'll see the laser launcher or a space elevator, but if we do it'll only be after there's already lots of stuff going up via other means, people need to be realistic that big mind numbingly huge capital investments aren't going to get the ball rolling.

So we're left with the Single Stage To Orbit (SSTO) and Two Stage to orbit (TSTO)fully reusable options that have been proposed again and again over the last 40+ years.

Why hasn't one of the many variations on these methods ever been seen through?

SSTO has the problem with mass ratios, even with the most energy dense rocket propellant in use, 90% of your launch mass has to be propellant and the other 10% is structure, leaving ~0% as payload. You can look to improving that mass ratio with air breathing engines, and if jet engines could be built with the same thrust to weight ratios as rocket engines have, SSTO would be a done deal, but the reality is that the best jet engines only have T/W ratios of about 13:1 (verses about 50:1 for rocket engines), and you end up taking all the mass of those engines to orbit, and one tonne more of engine means one tonne less of payload.

TSTO has the problem of shear size, building a winged orbiter to carry 20 tonnes to orbit would in itself be a manageable financial expense, but to build the ~800 tonne winged booster for a twenty tonne payload isn't quite so manageable. When they were looking for a reusable system to replace expendable launch vehicles in the 70's the designs were often of two stage fully reusable vehicles, and Congress always said "no - too expensive".

If you've seen many of my comments on various blogs recently you probably know that I've become a bit obsessed with Aerial Propellant Transfer for spaceplanes. The reason is that I see it as a route to achieve today what could not be done 40 years ago. APT means you can dispense with that 20 or 30 billion dollar specialized booster and replace it with a $350 million dollar (plus modification) commercial aircraft.

Probably one of the first reactions to the suggestion of a "booster" that only goes to Mach 0.85 and 12,000 meters is that it's not enough to make a difference. But it actually counts for more than commonsense would suggest; when the orbiter begins its climb to orbit its engines are working with far less atmospheric back pressure and so are more efficient, the orbiter can take off from the ground with only enough propellant on board to reach the tanker, and so requires lighter landing gear and possibly smaller wings, and the atmospheric drag from 12,000 meters is far less than that from the ground half the atmosphere is below you.

That start from altitude is worth about 1km/s, it reduces the propellant mass ratio from about 90% to about 85%, and if 11% of your light-off mass is orbiter your payload goes from less than 0% to 4% of the orbiters total mass, even better, aircraft landing gear typically makes up about 2% of lift-off mass, if you're refueling in the air the weight of the landing gear can be halved - so you've increased payload by another 1% of the orbiters total mass to 5%.

If want to get a 20 tonne payload to orbit, how big does the orbiter need to be? 20 tonnes X 100/5 = 400 tonnes, of which 85%, or 340 tonnes is propellant and 40 tonnes orbiter.

So we need a tanker that can transfer up to 340 tonnes (minus any excess that the orbiter has carried up) to the orbiter at an altitude of 12,000 meters.

That's a lot of propellant, it's more than can be carried by a 747-8, and even the A380 freighter only has a take-off weight of 590 tonnes total, of which 250 tonnes is plane, which would leave only 340 tonnes for both rocket propellant and the A380's own fuel.

I've tried sounding aircraft engineers out on this, the A380 as a tanker has far more space in the fuselage than would be required, my thoughts are that perhaps a fuselage based on that of an A340 - some models of which are actually longer than the A380 - could be attached to the flight surfaces of the A380, that would shave about 80 tonnes off the total mass, but even doing this would still require that some of the rocket propellant (LOX) be carried in the tanker aircrafts modified inboard wing tanks as the structural payload (payload that can be carried in the fuselage, even with the lighter fuselage, would still only be 230 tonnes.

So while heavy lift to orbit using APT might be possible now, the aircraft to do it didn't exist just a few years ago, and even the A380 would need big modifications for a heavy lift orbiter.

Another point I want to emphasis is that using even the A380 as a booster for air launch wouldn't put anything like a 20 tonne payload into orbit, even with sticking on an A340 fuselage to reduce weight, the maximum weight of the fueled orbiter would be 230 tonnes and the increased drag and high center of gravity would bring in more complications, maybe 9 tonnes of payload could be carried to orbit.

ATP has an advantage over a system which uses a direct ascent from a runway, there are far more launch windows because the ascent from 12,000 meters can take place anywhere within perhaps a thousand kilometers of the runway.

So how much would each launch cost?
The cost of flying a jumbo on a long haul flight is around $500,000, a third of which is fuel, that's also about the cost of a USAF KC135 flight to refuel other aircraft a thousand km from base. So excluding the cost of the propellant transfered, that's probably a reasonable figure for the tanker.

The cost of the propellant LH2 and LOX will be around $250,000

The cost of the orbiter excluding propellant will be servicing and payback on capital invested, with the servicing aerospace engineers tell me that with the experience gained from the SSME's it's possible to build H2/O2 that can be turned around in with almost no servicing between flights if that applies to all of the orbiters systems, servicing of $450,000/flight seems reasonable (that's about 20 times the servicing cost/tonne of airliners).

With the payback on capital invested, the development of aerospace vehicles that don't require radically new technology is around a hundred million dollars a ton dry mass, the numbers quoted for Skylon are much higher than this, but everything about Skylon is new technology, from the engines, to the active thermal control system, to the truss frame construction. The ATP would weigh about 45 tonnes dry, so I'm putting development cost at $4.5 billion, if 10 are initially built and per unit manufacturing cost is $100 million that's $550 million each. If they're good for one thousand flights that's $550,000/flight.

Adding those all together works out at $1.5 million per flight plus the operators profit. Which comes to $75,000/tonne or $75/kg payload.

That seems ridiculously cheap compared to the SpaceX Falcon, but then they're still throwing hardware away with every flight.

It also seems ridiculously low compared to what REL are talking about for Skylon, by then Skylon is really pushing the envelope with new technology, and it's the last 10% that takes 90% of the effort, an ATP using the Skylon structural system would in theory weigh half as much dry and carry twice the payload.