The Prophesy Chronicles - Discussion Thread

[TheBigDog]

The workhorse of the Prophesy Space Program is the Hercules Rocket. It a one stage to orbit Goliath with the ability to hurl 20 million kilos into orbit at a punch. I have two numbers as my starting point here: 20 billion kilos as the payload, and 20 billion kilos as the dry weight of the rocket system.

As a Hydrogen/Oxygen rocket, I estimated a requirement of 200 billion kilos of fuel to complete a maximum payload launch. To carry that much Oxygen and Hydrogen in a tank similar in shape to the Space Shuttle main fuel tank, the dimensions would be something like 40 meters in diameter and 180 meters high. Inside that tank would be two cylinders with rounded ends holding the two liquids; a large one for Oxygen and a smaller one for Hydrogen. Throw in another 20 meters at the bottom for the rocket motors and you have an empty rocket that stands 200 meters tall. Some of the payloads will be 100 meters tall, so at launch you will be sending essentially a 300 meter tall building into space. And repeating this process twice per week for ten years.

Sounds simple enough! The industry to keep this supply chain flowing would be in itself one of the largest efforts in human history.

Bill

[Donk]

Originally Posted by TheBigDog

I am planning on sling-shotting to help build velocity. I was also thinking along the same lines as you about resupply, etc, but the fact is that it takes as much or more energy to resupply than it does to just stack on more fuel from the start. Also, this is a one way mission, there is no crew changing. Communications will last for a long time, but what is onboard is what they have to work with.

As you say, fuel or food resupply would not be viable. I was thinking of small items - computer chips, nanotechnology items, new seed varieties, germ plasm. Plus any items that come under the generic heading of "ohmigod, we forgot to bring a can opener!"

It would take a lot of fuel to travel from Earth orbit to the gas giants, matching speeds with Prophesy at the end of the trip. Carrying enough fuel to reverse the process would increase the costs massively. An unmanned craft could take its time going back, using the same slingshot technique to work its way slowly into Earth orbit. Manned, it would have to get there much more quickly, and would need to carry all that heavy life support stuff. So I suspect the trip would be mostly robotic, and possibly one-way only.

If a special individual is needed on the expedition, no doubt it could be done, at horrendous expense - which could make an interesting plot point along the way.

The reverse could be even more interesting. The chief designer/engineer is persuaded to stay aboard to deal with the many last-minute problems. S/he is given an absolute guarantee that a way will be found to get him/her off after no more than five years. Unfortunately...

[CraigD]

Originally Posted by TheBigDog

That brings me to a question about Gravity Slingshot. If we use slingshots inside the solar system to maximize our velocity before heading into deep space, can we use another star to accelerate even more?

Yes, absolutely. Orbital mechanics is orbital mechanics, depending little for the scale and composition of the bodies, or time scales, involved.

It gets even more fun when you consider the possibility that there may be more giant planet-to-small star-mass objects between visible stars than we can currently see – brown dwarfs, or, being a bit more speculative, more exotic bodies, like quiet neutron stars with anomalous velocities, or perhaps the ultimate gravity assist bodies, fast-moving black hole with little or no accretion disks.

Detecting and plotting gravity assist maneuvers with one or more of these could yield some big delta-V – and works as well with a humongous spacecraft as with a tiny one.

We’ll have to do some real, albeit approximate, physics to see what sort of speeds can be had achieved via gravity assists from various bodies – at this point, I can’t hazard a guess to within a factor of a thousand. For me, this’ll require some serious sit-down time – and give me an excuse to dust of and resume work on my beloved simulators.

[TheBigDog]

Originally Posted by Donk

As you say, fuel or food resupply would not be viable. I was thinking of small items - computer chips, nanotechnology items, new seed varieties, germ plasm. Plus any items that come under the generic heading of "ohmigod, we forgot to bring a can opener!"

It would take a lot of fuel to travel from Earth orbit to the gas giants, matching speeds with Prophesy at the end of the trip. Carrying enough fuel to reverse the process would increase the costs massively. An unmanned craft could take its time going back, using the same slingshot technique to work its way slowly into Earth orbit. Manned, it would have to get there much more quickly, and would need to carry all that heavy life support stuff. So I suspect the trip would be mostly robotic, and possibly one-way only.

If a special individual is needed on the expedition, no doubt it could be done, at horrendous expense - which could make an interesting plot point along the way.

The reverse could be even more interesting. The chief designer/engineer is persuaded to stay aboard to deal with the many last-minute problems. S/he is given an absolute guarantee that a way will be found to get him/her off after no more than five years. Unfortunately...

I like the way you think. the thing to remember is this is a real lumbering beast. It is going to move away from earth very very slowly. Remember in the Movie "The Jerk" when Steve Martin left home and started hitch-hiking? His family is sitting around the dinner table that night and wondering how he is doing, so one of his brother's opens the window and shouts the question to him; he is still by the front gate with his thumb out. This ship is going to make that type of exit. It is possible that it takes years or more simply to exit earth's orbit, unless a temporary chemical rocket system is employed to push it away in one stupendous shove. I am not sure what the delta V is for going from LOE to escape velocity. But that is a good question...

How much energy is needed to accelerate 19.82 billion kg from orbital velocity past escape velocity? Subtlety is welcomed, but brute force is not out of the question.

Bill

[TheBigDog]

Originally Posted by CraigD

From a story plot perspective, that the ship would be a long time – likely at least 1, maybe several human generations – within reach of Earth by small spacecraft, is a major twist, going against the old space fiction cliché of short goodbyes and the ship as an isolated worlds unto itself. Such a generation ship might not be more than partially crewed for its first few generations. Its “shakedown” could be the work of several lifetimes. And – perhaps most dramatic and hard to imagine and plan for – its technology – the parts that are small and replaceable, such as its computers – will likely be generations more advanced than its originals.

Challenging stuff to write, as the key characters are likely to be a succession of great-great-etc. grandchildren – challenging to stir identification and empathy for – though immortal characters, especially artificial, computer-based ones, are reasonable.

One of the things I was considering is the ship is hundreds of years from earth when they finally arrive at the planet and find that it has already been settled by a ship that was built 200 years later and arrived 100 years earlier. But that is a long way off.

[Donk]

Originally Posted by TheBigDog

One of the things I was considering is the ship is hundreds of years from earth when they finally arrive at the planet and find that it has already been settled by a ship that was built 200 years later and arrived 100 years earlier. But that is a long way off.

You might want to check out A.E. van Vogt "Far Centaurus" (1944), in Destination Universe. Four astronauts reach Centaurus after 500 years spent mostly in suspended animation. When they get there they find a thriving human civilisation, with ships that can do the trip in a half hour

In the same book there's another possibly-relevant short story, "The Sound". A massive spacecraft which will take decades to complete. The people building it have the right technical skills, but they also have children who explore the growing ship and are allowed - encouraged - to go just about anywhere. The crew will be formed from those children. Larry Niven used a similar plot point in Oath of Fealty. The only people who don't feel lost and intimidated by the giant building they live in are the guy who designed it and the children who are growing up in it.

[CraigD]

Originally Posted by TheBigDog

How much energy is needed to accelerate 19.82 billion kg from orbital velocity past escape velocity? Subtlety is welcomed, but brute force is not out of the question.

Energy calculation for rockets can be less useful than one might expect.

This question, for example, has an easy answer, if one assumes 100%-ish efficiency in whatever does the accelerating: it’s equal to negative of the ship’s GPE,

Assuming the ship starts in a low Earth orbit of 350 km altitude (about ), this gives about – about the same as the electric energy used at present by the US in one year.

However, most near-Earth spaceflight isn’t 100% energy efficient, but closer to 50%. For example, you could get a ship from LEO to escape with the 1.2 EJs calculated above by, say, tying it to a 46,746 km (10,960 km above geostationary orbit) space elevator, then run it up to the elevator’s “Earth escape terminus” (the energy would be in the form of whatever ran the various winches, elevator motors, and elevator prevent-from-getting-pulled-out-of-whack systems needed) – but first, you have to build a giant space elevator. Using something like VASIMIR engines () and Earth escape speed (), efficiency will be around 60% (

), and will require about 0.22 billion kg of reaction mass (

) – not too daunting, expecially given the cost of lifting the 20 billion kg ship into LEO, though this is just the first leg of the voyage.

With rocket-propulsion, there’s an inverse relationship between energy efficiency, and the amount of reaction mass need to accelerate a payload, so any rocket design for a ship like the Prophesy 2 will almost certainly seek to have even higher – and thus exhaust speed – that a VASIMIR, in order to minimize reaction mass requires, at the expense of energy efficiency. In this case, in an inversion of ordinary Earth engineering, energy’s cheap, inert mass expensive.

(sources: wikipedia articles ”propulsion efficiency”, “specific impulse”)

PS: if you value an approximation of accuracy, check my calculation!

[TheBigDog]

Only 2.2 billion KG! I thought it would be a really really high number. Hell, we have that in the reserve tanks alone. Of course it will still take a long long time. We would be in an ascending orbit, increasing our altitude while we continue to spiral further from the earth until we reach a point where we exited on a long, arching tangent. Then we begin slingshot moves through the solar system until we achieve the velocity that will carry us to our first extra-solar checkpoint.

Bill

[freeztar]

Hmmm...

I was checking Craig's math and I'm getting 2.2 x 10^8 rather than 2.2 x 10^9. Can someone else verify this for the last division of M_E?
(note, I've only checked that one equation so far...)

[CraigD]

Originally Posted by freeztar

I was checking Craig's math and I'm getting 2.2 x 10^8 rather than 2.2 x 10^9.

You're right. I think I got 2.2e8, then wrote it "2.2 billion" instead of "0.22 billion".

That I'm using specific impulse calculations correctly to set up the calculation still needs checking, but the result makes intuitive sense - a gigantic VASIMIR engine / array of engines can get a ship from LEO to escape spending about 1% of its total mass as reaction mass.