Off to Alpha Centauri Bb ! ? - Page 3

Dubslow · 2012-10-22, 22:59

Quote:

Originally Posted by Uncwilly

So, you choose not to design a mission that fits within the constraints given. (note, total travel time is not one of them, deliberately so.)

Thanks for playing.

Next contestant!

Quote:

Originally Posted by Uncwilly

[LIST][*]You have $10,000,000,000 to spend on the project.
(This includes cost of monitoring the mission until it crosses the orbit of Neptune, but not the rest of the journey
[assumes that this becomes a line item in a budget]).[*]You have 15 years until it must leave earth orbit (with 2 years to extend it if need for planetary slingshot alignment).

His point is that these constraints are sufficient to place a minimum ROI period of hundreds of years, which is out of the question for almost anybody.

science_man_88 · 2012-10-22, 23:43

Quote:

Originally Posted by xilman

Indeed.

However, in 1961 we could calculate that the journey time to the moon would be of the order of a month, that almost real-time communications would be possible throughout the mission and that the energy requirements per kilogram of payload would be only a few times that which had already been demonstrated.

For your mission, the journey time will be at least a decade (itself not unreasonable, cf Voyager). Real time communication becomes infeasible long before the mission leaves the inner solar system.

If the journey time is to be less than a century, and I really doubt that anyone these days would want to embark on such a speculative project with a payback time of more than that --- especially as you posit that a mere 15 years would cause a fatal loss of interest --- the probe needs to travel at approximately 0.1c. That speed is 30,000 km/s or something over a thousand times the delta-v needed to get to the moon and back. Kinetic energy scales as the square of the velocity, so well over a million times the energy per kilogram is needed as for a lunar mission. Simple chemical thermodynamics tells you that chemical rockets won't provide that unless the fuel/payload mass ratio is prohibitively outside your $10G budget.

With technology which can be built in the next 15 years, that would appear to leave sails, nuclear thermal rockets, nuclear electric rockets, gravity assists and electromagnetic rail guns (*). The first three have very low accelerations at present technology levels; if they are to be used for long periods you either need extortionately large supplies of reaction mass for the rockets or (near) earth-based beamed power for sails to be useful much beyond a few astronomical units. There is no way that terawatt average-power lasers are going to be built and fielded in 15 years. I've already pointed out that gravity assists aren't going to get you much faster than a coasting speed of around 100km/s.

As for electromagnetic rail guns, do you really think that we can build a linear motor long enough and high power enough to accelerate even a kilogram to 0.1c in the next 15 years? Assume that the payload can withstand 10,000 g acceleration. To a reasonable approximation, the boost phase to 0.1c will take 0.1 / 10,000 years (using the handy approximation that c = 1 g-year). That comes something over five minutes, during which time the probe will travel about an astronomical unit. Again I ask, do you really think that we can build a linear motor long enough in the next 15 years?

Now examine the energy requirements for a 1kg probe (how is such a thing going to be able to send results back?). Neglecting relativistic increase of mass (down in the 1% noise at this scale of approximation), the kinetic energy is 0.5 * (3e7)^2 Joules, or around 0.5PJ. In more common units, this is close to 100 kilotons of TNT. The kinetic energy alone of a tiny 1kg probe is markedly larger than that of all the Apollo missions put together.

Using known physics, only fusion rockets show any hope of reaching the nearby stars in less than a century. Unfortunately, fusion rockets aren't ready to be flown in a 15 year time scale.

Slightly further down stream might be ramjets, but they're even higher technology than "standard" fusion rockets. First you need to get them up to speed (see earlier problems) and then you need to do something with the material collected. For a start the ISM is principally hydrogen, much harder to fuse than D/He3. Even if you use it only as reaction mass accelerated by an on-board power supply, the ISM is very dilute and there are very severe problems with inlet stagnation at the throat of the collector. Building the collector and the PSU to drive it (and accelerate anything collected if H fusion is too hard) is going to take a serious mass budget; all that mass has to be accelerated to high velocity before it starts being useful.

(*)Well, a few others. Magnetic tethers, for instance. Unfortunately the galactic magnetic field isn't strong enough to give the accelerations needed.

for one kilogram to get up to that speed I get we would need to totally use all the energy possible from 5 grams of matter. good luck getting 100% of it and using it effectively.

Uncwilly · 2012-10-23, 00:29

Quote:

Originally Posted by Dubslow

His point is that these constraints are sufficient to place a minimum ROI period of hundreds of years, which is out of the question for almost anybody.

Why not build in a TAU mission? Would that not provide some ROI earlier? There are some other side missions that could be part of it.

fivemack · 2012-10-23, 00:43

So: you're saying that we can pass the cost of monitoring the probe after it passes 30AU to an organisation with an infinite budget. Alpha Centauri is 280,000AU away; 7AU/year is just about attainable with a hot launch, Jupiter and Saturn flybys, and an only marginally unreasonable supply of ASRGs powering ion engines that we know how to build.

An RTG of the ASRG type has a half-life of 80 years, weighs 20kg and produces 140 watts; unfortunately it has moving parts. If we use something with a half-life of 8,000 years instead (americium-243 or curium-245), it would weigh two tons for the same power output; and after 40,000 years we'd still have three watts.

(of course this is insane, nothing with moving parts will last 40,000 years, a convincing explanation of why a proposed design could be expected to last 40,000 years would probably exhaust the $10^10 in experiment alone, arranging that the last three watts are still usable at 40,000 years is absurd)

Voyager 1 has a 23-watt radio and can be heard reasonably happily at 120AU with a 3.7-metre dish at its end and a 34-metre DSN dish at ours. Our radio has one ninth the power, so would be audible at 40AU without antenna gain.

Let's add some antenna gain. There's a satellite currently in orbit (TerreStar-1) known to have an 18-metre deployed antenna; there's a satellite USA-202 currently in orbit which is suspected to have a 100-metre deployed antenna. Let's get the NRO director some really good cigars and get a spare one of those; doubling the antenna diameter at either end doubles the range, so with our 100m transmitting antenna and our 3-watt radio we'd be audible with a 34-metre DSN dish at 1000AU.

Which means the dish that the infinitely wealthy organisation has to have at Earth has to be about 10km; this is at the absurd-but-not-ridiculous level.

So this is a probe consisting of two tons of ludicrously over-specced radioisotope generator and let's say two tons of huge antenna (and some kind of sensor pack, but that's a trivial weight); it's assembled in orbit, and attached to a last stage containing four tons of Pu238 ASRGs at 7W/kg, which provide 28kW of power to a cluster of twelve NSTAR ion thrusters (which weigh 50kg each and provide 92mN thrust each using 5g/hour of xenon each), and tankage for thirty tons of xenon. Two Falcon Heavy launches for the probe and the xenon; call the stack fifty tons since we do need some kind of spacecraft structure.

And then you use the remainder of the budget on launching Falcon Heavy second stages and stacking them as the departure stage - $100 million per second-stage delivered to the assembly site is almost reasonable. Deliver 40 of them; they weigh 50 tons each, 90% of which is fuel, and each provide 445kN of thrust for 345 seconds.

We're starting off in the ISS's orbit at 7.7km/s orbital velocity

Fire the first 27: 12MN of thrust applied to 750 tons of payload plus 1300 tons of engines-and-fuel, accelerate at 6m/s for about 300 seconds; off we go at 2km/s.

Fire the next nine: 4MN of thrust applied to 300 tons of payload plus 450 tons of engines-and-fuel, accelerate at 6m/s for about 300 seconds; 2km/s more, and we've reached Earth escape.

Fire the next three: 1.3MN of thrust applied to 150 tons of payload plus 150 tons of engines-and-fuel, 2km/s more, we're on the way to Jupiter.

Fire the last one when you're at perijove for the Oberth manoeuvre, then light up the ion engines; thirty tons of xenon gets half a million thrust hours on a 1-newton engine on 50 tons of vehicle and an additional 36km/s which is our 7AU/year.

The laughter as you propose this to a NASA congressional hearing can clearly be heard at the other end of the Mall.

science_man_88 · 2012-10-23, 00:54

Quote:

Originally Posted by Uncwilly

Why not build in a TAU mission? Would that not provide some ROI earlier? There are some other side missions that could be part of it.

lets say one robot is TAU to earth and one is TAU to the other star system this only cuts down on the transmission time by about 277-278 hours and that assumes the robot around earth is in control of the whole mission if earth control is needed it adds back 499 004.784 seconds of transmission time at very least which is 138-139 hours.

Uncwilly · 2012-10-23, 01:29

Quote:

Originally Posted by science_man_88

lets say one robot is TAU to earth and one is TAU....

The purpose of a TAU mission is not to be a relay, rather for scientific studies.

science_man_88 · 2012-10-23, 01:44

Quote:

Originally Posted by Uncwilly

The purpose of a TAU mission is not to be a relay, rather for scientific studies.

what's your time constraint again (a close look shows 4.37 light years/(0.1 light years/year) = 43.7 years to get there at 29979245.8 meters/second) ?

Andi47 · 2012-10-23, 05:48

Just two questions:
* What was the cost of Apollo 11 in today's money?
* What was the cost of the Curiosity mission (let's say until the end of primary mission, not taking in account any extension of the mission)?

xilman · 2012-10-23, 06:46

Quote:

Originally Posted by jyb

Once again, I know this doesn't change your conclusion so I apologize for picking nits, but the terminal velocity wouldn't be achieved until the end of that 5 minutes. I believe the actual distance would be about 4.6 million kilometers.

Y'know, ${1 \over 2} at^2$ and all that.

Fine. You're working to two significant digits, I'm working to the nearest factor of two at best and to the nearest order of magnitude much of the time. You didn't pick up, for instance, that at 0.1c it takes only 43 years travel time. The figure 10,000 g acceleration was picked almost arbitrarily as something which ruggedized hardware might be able to withstand for minutes at a time; I've no idea whether that's plausible. Even a tenth of that may be infeasible.

xilman · 2012-10-23, 06:49

Quote:

Originally Posted by Uncwilly

So, you choose not to design a mission that fits within the constraints given. (note, total travel time is not one of them, deliberately so.)

Thanks for playing.

Next contestant!

Oh, I can design one all right, well under budget and well within the time limit. The point is that the mission is then pointless. Throwing an upper stage rocket to Alpha Cen is straightforward and it will get there in a few tens of kiloyears. So?

xilman · 2012-10-23, 06:58

Quote:

Originally Posted by Uncwilly

So, you choose not to design a mission that fits within the constraints given. (note, total travel time is not one of them, deliberately so.)

Thanks for playing.

Next contestant!

Why, then, did you place a tight constraint on the time to start the journey?

2012-10-23, 00:43	#26
fivemack (loop (#_fork)) Feb 2006 Cambridge, England 14351₈ Posts	So: you're saying that we can pass the cost of monitoring the probe after it passes 30AU to an organisation with an infinite budget. Alpha Centauri is 280,000AU away; 7AU/year is just about attainable with a hot launch, Jupiter and Saturn flybys, and an only marginally unreasonable supply of ASRGs powering ion engines that we know how to build. An RTG of the ASRG type has a half-life of 80 years, weighs 20kg and produces 140 watts; unfortunately it has moving parts. If we use something with a half-life of 8,000 years instead (americium-243 or curium-245), it would weigh two tons for the same power output; and after 40,000 years we'd still have three watts. (of course this is insane, nothing with moving parts will last 40,000 years, a convincing explanation of why a proposed design could be expected to last 40,000 years would probably exhaust the $10^10 in experiment alone, arranging that the last three watts are still usable at 40,000 years is absurd) Voyager 1 has a 23-watt radio and can be heard reasonably happily at 120AU with a 3.7-metre dish at its end and a 34-metre DSN dish at ours. Our radio has one ninth the power, so would be audible at 40AU without antenna gain. Let's add some antenna gain. There's a satellite currently in orbit (TerreStar-1) known to have an 18-metre deployed antenna; there's a satellite USA-202 currently in orbit which is suspected to have a 100-metre deployed antenna. Let's get the NRO director some really good cigars and get a spare one of those; doubling the antenna diameter at either end doubles the range, so with our 100m transmitting antenna and our 3-watt radio we'd be audible with a 34-metre DSN dish at 1000AU. Which means the dish that the infinitely wealthy organisation has to have at Earth has to be about 10km; this is at the absurd-but-not-ridiculous level. So this is a probe consisting of two tons of ludicrously over-specced radioisotope generator and let's say two tons of huge antenna (and some kind of sensor pack, but that's a trivial weight); it's assembled in orbit, and attached to a last stage containing four tons of Pu238 ASRGs at 7W/kg, which provide 28kW of power to a cluster of twelve NSTAR ion thrusters (which weigh 50kg each and provide 92mN thrust each using 5g/hour of xenon each), and tankage for thirty tons of xenon. Two Falcon Heavy launches for the probe and the xenon; call the stack fifty tons since we do need some kind of spacecraft structure. And then you use the remainder of the budget on launching Falcon Heavy second stages and stacking them as the departure stage - $100 million per second-stage delivered to the assembly site is almost reasonable. Deliver 40 of them; they weigh 50 tons each, 90% of which is fuel, and each provide 445kN of thrust for 345 seconds. We're starting off in the ISS's orbit at 7.7km/s orbital velocity Fire the first 27: 12MN of thrust applied to 750 tons of payload plus 1300 tons of engines-and-fuel, accelerate at 6m/s for about 300 seconds; off we go at 2km/s. Fire the next nine: 4MN of thrust applied to 300 tons of payload plus 450 tons of engines-and-fuel, accelerate at 6m/s for about 300 seconds; 2km/s more, and we've reached Earth escape. Fire the next three: 1.3MN of thrust applied to 150 tons of payload plus 150 tons of engines-and-fuel, 2km/s more, we're on the way to Jupiter. Fire the last one when you're at perijove for the Oberth manoeuvre, then light up the ion engines; thirty tons of xenon gets half a million thrust hours on a 1-newton engine on 50 tons of vehicle and an additional 36km/s which is our 7AU/year. The laughter as you propose this to a NASA congressional hearing can clearly be heard at the other end of the Mall. Last fiddled with by fivemack on 2012-10-23 at 00:51

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2012-10-23, 05:48	#30
Andi47 Oct 2004 Austria 2·17·73 Posts	Just two questions: * What was the cost of Apollo 11 in today's money? * What was the cost of the Curiosity mission (let's say until the end of primary mission, not taking in account any extension of the mission)?