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Velocity ranges

Posted by: Ekkehard Augustin - Tue May 17, 2005 12:31 pm
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Post Velocity ranges   Posted on: Tue May 17, 2005 12:31 pm
I had to think about velocities a little bit and got the result that considerations about them show the upper boundaries of what an engine technology must be able to achieve.

One special velocity often subject of posts of mine is 0,12c. Ulrich Walter quotes it as maximum velocity Daedalus can achieve.

Daedalus has been designed as interstellar vehicle -and such vehicles mostly are not of interest here and currently.

Is 0,12c of interest within the solar system? 0,12c is around 36,000 km/s. a vehicle going by that huge velocity would go 360,000,000 km within 10,000 seconds - less than three hours. The time required to accelerate up to 0,12c is 3,600,000 seconds - 1000 hours.

I didn't do any further calculations yet. 100 hours to accelerate to have 0,12c for much two hours and then decelerating 1000 hours long seems to be of no interest. - especially under the aspect of the weight of propellant required - the micro-fusion pellets of a pulsed fusion engine.

If the velocity would be 360 km/s such a vehicle would require 1,000,000 seconds for 360,000,000 km - less than 300 hours or a little bit more than 16 days. 36000 seconds or 10 hours would be required to accelerate plus the same time to decelerate again later. This seems to be quite more reasonable to me.

What about other such upper boudaries of required capabilities, capacities etc.? What ranges do you consider to be reasonable?



Dipl.-Volkswirt (bdvb) Augustin (Political Economist)

PS: Peter, as already said elsewhere I got the 0,12 c from Ulrich Walter and from the Daedalus project and quoted the number and so I am correct in this. There may be new results in between saying that 0,12 c is not achievable but I don't know them. I will look for them but you are incorrect in using your information because you don't list your source(s) which would enable to analyse it...


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Post    Posted on: Tue May 17, 2005 3:26 pm
Ah, a post to my liking, with lots of easily calculated numerical quantities!

Using the simple formula X=1/2AT^2 where X is the distance covered, A is the acceleration and T is the time, I plug in A=0.01 km/s^2 (that is 1 g), T=3,600,000 seconds and solve for X to get 6.48E10 km, which is over 400 AU, waaaaaay past Pluto. So, no, speeds of 0.12 c are not of interest in the solar system. However, as space cowboy has pointed out earlier, a space craft with such a capability would be very nice to have because you could go anywhere in the solar system in just a few days and enjoy Earth normal gravity all the way. For example you could go to Mars (at opposition) in less than 4 days, although your maximum speed on the trip is only 750 km/s (0.0025 c). With such high speed craft you don’t need to worry much about planetary orbits because compared to those speeds the planets are practically standing still.

However, I think the dream of such a capability is very far removed from reality in my lifetime. Certainly far beyond the X-prize and orbital tourism.


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Post    Posted on: Wed May 18, 2005 8:43 am
It is very interesting to be able to be at distant locations of the solar system within days - but they higher the desired velocity the higher the propellant requirements.

It seems that in sapce propellant is of use for two purposes only:

1. acceleration/deceleration and
2. manoevers like changing course/path.

The higher the desired velocity the higher the propellant consumption and required volume of propellant storage - which wouldn't be a real tank in the case of the pellets for a pulsed fusion engine.

This means that the focus of drive/engine development should be on

1. least propellant consumption for a given rate of acceleration and deceleration,
2. least propellant consumption for manoevers,
3. tendency of specialisation for distinct classes of distances.

What velocities are reasonable for which distances?



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Post    Posted on: Wed May 18, 2005 2:12 pm
First off, all maneuvers are accelerations. It's just that we tend to think of them in aviation terms: the direction in which you're heading is "forward", and so we say that an acceleration in that direction will cause you to "speed up". An acceleration in the direction opposite that of motion will cause you to "slow down". An acceleration perpendicular to the direction of motion will cause you to change the direction of motion. That's just some physics so you get what Peter and I say when we talk about accelerations and space maneuvering.

In essence, what a high-velocity, constant-burn capacity drive can do is keep you from having to change direction: you figure out where your target is going to be when you get there, point in that direction, and go. You suck fuel worse than a 426 Hemi 'Cuda with a bad carburetor -- but hydrogen is cheap, so you don't really care. Because it's a constant, mid-thrust burn, the gravitational effects of the sun and planets are basically negligible, so you can safely approximate your trajectory as a straight line. This is a far cry from concepts such as the Interplanetary Superhighway, which require practically no fuel, but take forever and a freakin' day to get to the destination.

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Post    Posted on: Wed May 18, 2005 2:40 pm
The reason for which I made the distinction between manoevers and acceleration/deceleration is the theoretical chance that after departure for a certain destination it might be decided to go for another destination. The reason may be an unexpected event at the new destination, aboard the vehicle or at the departure location.

Clearly this is interesting mainly in a future more than 100 years away when there is significant traffic between several locations in the solar system. But it may be of meaning for the Mars mission too that is part of the Bush plan - but that mission won't have a vehicle that can achieve 360 km/s.

If the first manned Mars mission is followed by a permanent Mars station, by tourism to Mars or/and mining and industrilalization of Mars this may generate regular traffic between Earth, Moon and Mars. This situation may be sufficient to generate alterings of destinations.

I was aware of the different image from the physical and engineering view but cannot neglect the image of traffic and its logical consequence that in traffic manoevers are different from speeding up and slowing down. This makes it complex...



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Post    Posted on: Wed May 18, 2005 3:46 pm
I was going to say how acceleration, deceleration and maneuvers like changing course/path are all exactly the same thing, but spacecowboy beat me to it. You can keep the image of traffic or changing destinations in mind, but don’t neglect the difference between what is possible and what is impossible to do to avoid traffic or change destinations.

This can be demonstrated by a simple game I learned from some other physics students. On a piece of graph paper draw a curving road that wanders all over the page and put two dots on the starting line. Each player starts by drawing a line from his dot to any adjacent line intersection and makes a new dot. The line represents the velocity of his “car” and the new dot represents the position of his “car” after 1 second. On the next turn each player finds the intersection where another line from the second dot in the same direction and with the same length would be, but he has the option of drawing the line instead to any other adjacent intersection. This represents acceleration. So the second line could be 2 units long, or it could change direction. The race continues until one dot ends up off the course or crosses the finish line. No fair counting the squares more than one move ahead. Here is what it looks like.
Image
The green player got greedy and tried to go too fast. He didn’t have enough deceleration (down direction acceleration) to stop before running off the road. This is really more like a space race since tire friction on corners is not exactly like straight ahead acceleration or breaking, but it is an exact representation of space, similar to the rocket’s movement in the old Asteroids video game.


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Post    Posted on: Wed May 18, 2005 6:53 pm
Regarding such problems there will be solutions provided which are topics of the Regulations section:

1. Safety of the passengers of the others vehicles,
2. saftey of the passengers of "this" vehicle and
3. reduction of ergency rescue requirements.

The first two are repeated only - the last one is a new topic for the Regulations section but in this case I will say something about it here because it has a technological aspect too perhaps.

In Germany car traffic is regulated not only to keep all participants of the traffic safe but also to keep a special kind of "emergency" as low as possible: Each car driver is required to provide as much gasoline as is needed to reach the next gasoline station. If he doesn't the police can punish him. Each driver is required to have at least one canister in the car full of gasoline. And the canisters are regulated too.

In short the regulations say that redundant amounts of gasoline have to be provided.

I am a rower and often participated in boat travels - and a few of them went along the danish cost. In danmark each boat travel has to accom,panied by at least one rower who has got aspecial eduction by the danish rowers' organization and I have that eductaion. From this I know that each boat has to keep a distamce from the shore of no more than 300 meters. Only exception is crosiing a bay - in that case a distance of no more than 2,5 km is allowed. If the boats don't keep these distances and end up in emergency and the rowers must be rescued by a helicopter - then the rowers have to pay the costs of that rescvue completely: 250,000 Euro. This would be huge amount of money for them and they would loose their fortunes. The reasons are danish experiences with too much rowers who didn't be cautious.

Such regulations are to be expected for interplanetary traffic too.

The technological aspect of all this is that by computer and sensors measuring the amount of propellant each departure of a vehicle providing insufficient redundancy of propellant may be prevented - and that the regulation agency may check each vehicle if it has such a prevention euipment and if it is working. If a vehicle doesn't have it the regulation agency may remove it from the traffic.

But please let's keep of these questions from this thread - we shouldn't discuss it here regardless of the image of traffic - I seem to remember that it is topic of at least one other thread.

What's really interesting here are reasonable velocities. The term reasonable also includes the consumption of propellant. It should be made sure that there is sufficient propellant left for manoevers and/or to reach the next station where new propellant can be got.

That all the locations are moving has been topic of another thread already.(Maaping Courses for example...)

The faster the vehicle the more propellant required to change the direction by 1° it seems. And propellant means weight. If I again take the pulsed fusion engine as example then the more pellets have to be provided the longer the required acceleration nad decelration is. Which in turn menas that the possibel payload-weight is reduced because of volume restrictions - an additional criterion for reasonability of velocities.



Dipl.-Volkswirt (bdvb) Augustin (Political Economist)


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Post    Posted on: Thu May 19, 2005 12:51 pm
I like that game, Peter. I'm going to have to remember that for GT. 8)

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Post    Posted on: Thu May 19, 2005 2:57 pm
Ekkehard Augustin wrote:
In Germany .... Each driver is required to have at least one canister in the car full of gasoline. And the canisters are regulated too.
:o I didn't know that! About 10 years ago some friends and I drove around Germany for about 2 weeks and we never carried a can of extra gassoline. Good thing we didn't run out!


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