You're forgetting acceleration time. Ultimately, there is no 'maximum speed', because it's based all on fuel. You accelerate until half of your fuel is gone, coast, then turn around and decelerate before you reach your target. That also means that over short distances, a ship with a lower "top speed" but better acceleration will apparently go faster than a slow-acceleration, high "top speed" ship (because the former will get up to speed before the latter).

Jsnead, the dude who designed EP's ships, (IIRC), posted specifications with D-V#'s [url=http://www.eclipsephase.com/space-naval-combat-segway-antimatter-thread?... this post.[/url] (for which I am still, very thankful.) If you don't want to account for situations like aerobraking, oberth maneuvers, initial velocity, capture velocity, propellant (NOT "fuel") reserve, etc. you can assume that 1/2 Delta V is the maximum velocity of the ship. Go to [url=http://www.projectrho.com/public_html/rocket/torchships.php]this page on Atomic Rockets[/url] for cool stuff like explanations and equations of brachistochrone flights between planets and deriving the D-V needed for those trajectories, As well as fictional accounts of their use by Heinlein and Niven :)

You're welcome, give some luv to mr. snead. that is one of the EP forum threads I keep archived in several places. It should be stickied IMO

However, continuous thrust is unlikely - you run out of reaction mass very quickly. I did some calculations in one of the earlier spacecraft threads and got the estimate that most ships accelerate for a few hours at departure and arrival. You want to have as much velocity as possible during the cruise phase and not waste reaction mass by lugging it with you. You can get very reaction-mass cheap flights by exploiting gravity to travel on the "interplanetary highway" - chaotic orbits through Lagrange points linking different bodies together. But these are quite slow: they move with velocities like planets (a few km/s) but take multiple orbits around them to get anywhere.

nezumi.hebereke wrote:

I imagine there's a nice business in tugs. After all, tugs stay close to home, so they don't have that fuel penalty for hauling the fuel necessary to accelerate their fuel (to such a degree). They carry enough to accelerate for a burst and return home.

But returning home requires you to match velocities. You are not home if you are at your doorstep moving ten times faster than a bullet. I think tugs are important, but using them is complicated (and most players don't want to deal with the niceties of celestial mechanics). One role is to catch incoming ships that for one reason or another have too little reaction mass to get the right delta-v. The tug matches velocity, and then turns the orbit so they will return (possibly slowly) to the destination.

Arenamontanus wrote:

nezumi.hebereke wrote:

I imagine there's a nice business in tugs. After all, tugs stay close to home, so they don't have that fuel penalty for hauling the fuel necessary to accelerate their fuel (to such a degree). They carry enough to accelerate for a burst and return home.

But returning home requires you to match velocities. You are not home if you are at your doorstep moving ten times faster than a bullet. I think tugs are important, but using them is complicated (and most players don't want to deal with the niceties of celestial mechanics). One role is to catch incoming ships that for one reason or another have too little reaction mass to get the right delta-v. The tug matches velocity, and then turns the orbit so they will return (possibly slowly) to the destination.

When does tugs make sense? The fuel needed to decelerate the incoming ship is the same. The ship could carry that fuel from home (which takes fuel). Or the tug could take the deceleration fuel and move it out from the destination, then match velocity and position with the incoming ship (which takes fuel, potentially a lot since it seems you'd need either a very long path or you basically have to fly away from the destination, then turn around and move up to high speed towards the destination to match up with incoming ship). It seems the tug option is more fuel expensive, but with factors such as differences in escape velocity and the relative motion of the bodies involved I don't trust my intuition much.

That's an interesting thought, Holy. What you're saying in essence is that the propellant--in EP that would be Hydrogen--is less expensive in Mars orbit due to Mars' lower gravity. To make it more EP specific you could factor in Mar's space elevator. However; in that calculation you have to include the cost of boosting the mass of the tug out to a mars capture orbit with the extra payload of fuel for the incoming ship. You also have to factor in the risk of mid flight refueling. I havn't done the math either, (cause I don't know what a tug masses), but I suspect you don't really break even, let alone make a profit.

NewtonPulsifer wrote:

I'd build a space elevator that was tethered to a couple of stations/weights on each end - 100,000km long tether. Spin it a 33km/sec (1.1 Gees). It would then "grab" and "release" ships/cargo. That way you could conserve your delta-v.

That would work as a solution... for certain definitions of "solution". This would qualify as one hell of a feat of engineering (we're talking a structure that is, if I'm understanding your proposal, either one third or two thirds of a light second across), between, well, off of the top of my head: the tensile strength required for holding it together the sheer mass of material required managing tidal stresses on such a large structure from any nearby gravity wells (i.e. where people live and where the shipping would be) the highly intricate engineering needed to keep the arms from getting tangled and managing the vibrations of the structure resulting from repeated catches and launches the compromise between flexibility and rigidity in the arms (too rigid, you risk a fracture from any bad landing, too loose and you have a buildup of inertia in one arm that sends it out of synch with the others) and so forth. You get the idea. In other problems, it would be a massive no-fly zone within the spinning area, not just because of the chance of collision, but because of the chance of sabotage as well; this thing would be one giant target for anyone with a grudge against the creators, and it would be so fragile as compared to the destructive means available in EP (and if you reinforce it, that's more cost of materials and mass that has to be supported in tensile strength). It would certainly work, but as a solution to the problem, it's very similar to proposed megascale engineering here in the present, such as the Transatlantic Tunnel, in that we possess the technical capability to build it, but the construction, operation and maintenance of it would be so impractical as compared to existing solutions that there would be no net benefit from constructing it. That being said, as a giant flywheel concept to conserve delta-v as inertia, it would work... but as a "simple" solution, in terms of where it stands on the Kardashev Scale, it's in the neighborhood of suggestions that I've seen for spinning up Venus: by building space elevators on its surface with solar sails at their peaks, constructing a giant space shade at the Venus/Sun L1 point that lets through enough light to hit the solar sails on the western limb of the planet and spinning the planet up to speed, much like one of those glass bulbs with the black and white vanes. It could work, but as a matter of practicality and efficiency, not so much. (Keep in mind that transhumanity is not yet a Kardashev I civilization; they're almost there, but not quite).

I believe Newton is talking about Hans Moravec's "Skyhook" concept. It's a design that doesn't have the cargo capacity of the space elevator but it solves some problems involving coriolis forces on a tether type elevator and at smaller scales is technically easier to achieve. Considering EP's "Lunar Skyhook" went in an entirely different and much more complex direction, I think the Moravec orbital skyhook is fully doable by EP's standards Here's the wiki http://en.wikipedia.org/wiki/Skyhook_%28structure%29 Here are some better links http://www.colonyworlds.com/2009/04/are-traditional-space-elevators-the-... http://www.nss.org/settlement/L5news/1983-skyhook.htm

NewtonPulsifer wrote:

bibliophile20 wrote:

....[i] lots of valid engineering concerns [/i]

All of these concerns are valid, but they also apply to regular old space elevators. Even more so. Since there's space elevators in EP....this thing should be very do-able. EDIT: Also, there's no reason you can't have 100 separate (or meshed) tethers between each of the endweights. That's harder to engineer in a space elevator (dealing with a mesh). We don't have to deal with gravity - it dramatically simplifies things.

Heh. Alright. I'll grant you that, but I'll add/amend/extend my former concerns: First off, existing space elevators in EP are significantly shorter than your bolastation (nice name, very evocative, I like it); the Earth elevators are/were some of the longest, at approximately 40k kilometers in length; Mars' and Luna's elevators are shorter. You're talking about making a structure that's between 100k and 200k in length and spinning it at 33 km/sec, for 1.1 G at the terminal ends. This is a structure that is at least two and half times the length of the Earth elevators, under an even greater tensile load at each end. Doing some rough back-of-the-paper math, your entire structure, but especially the center of mass, would be under at least three to five times as much tensile load as the Earth elevators would be, depending on exactly the mass loads of the structure are distributed. As a solution to relative arm drift/stationkeeping, having a web of interconnected fibers between the arms to keep them at static distances apart would work nicely, but figuring out how they relate to the tensile problems is a hell of a wild card. Also note that their mass will be significantly more than the arms, which will cause additional strain and require additional engineering reinforcement on the center of rotation to avoid catastrophic failure. There's also the issue of having to deal with tidal forces of any nearby gravity wells; since the bolastation is designed as a transshipment point, it, by definition, will have to be located near a population center to be effective in its purpose. And all of the major population centers that would be worth such an investment are located at the bottom of gravity wells called planets and moons. A structure of this size would have tremendous issues with tidal forces as it spun through the gravity wells of any nearby objects. For consideration, the Earth elevators have to deal with tides caused by the Sun and Luna, but Earth tides are at least negligible, as they are relatively at rest to their host planet. The bolastation, however, is not at rest to its host object(s); it is moving dynamically, which will cause significant strains on the object (for examples of what tidal forces can do to in terms of energy input and strains on a large structure, I present Io as Exhibit A). If you put it in orbit at a Lagrange point, not only have you essentially declared that Lagrange off limits to any further development, due to collision risks, but due to its sheer size, the bolastation will almost certainly extend beyond the bounds of the Lagrange Point and be subject to tidal forces from both parent bodies! However, those are just engineering issues, and ones that can be solved with sufficiently strong materials and clever engineering, no question there. What really kills this project as a practical consideration, and why I brought up the Kardeshev Scale in my first post regarding the idea, is the energy budget required. The bolastation is essentially a flywheel for the purposes of conserving momentum, and for that purpose, it is elegant. However, there is one serious issue: Getting it up to speed in the first place. It will need to have a certain amount of mass to be able to absorb the inertia of fast-moving spaceships without tearing itself apart, and that mass must be accelerated first. And due to the bolastation's dynamic nature, the builders cannot do this by "borrowing" rotational inertia from the host body, which is the case for building a simple space elevator (did I just type "simple space elevator"? The mind boggles...). In the case of the bolastation, every bit of rotational inertia that it possesses will require external input from its builders, which will require such an input of energy using EP's propulsion technology as to be an appreciable fraction of the energy budget of the entirety of transhumanity to accomplish in a reasonable period of time--and would cost so much energy that any benefit from using it as a flywheel to conserve momentum, thrust and fuel would be overshadowed by the immense cost of getting it up to speed to begin with. Ironically, the best option for getting it up to speed is to reverse-engineer the Factors' reactionless drive and use that to get the station spun up to speed... but once you have a reactionless drive, using a flywheel for conserving momentum seems... rather silly. :) It's a cool idea, and I'm tempted to borrow the idea for a dead exoplanetary civilization that gatecrashers might come across, but as a practical solution to the inefficiencies of space travel, it quite simply isn't one. :( Yet another engineering solution killed by economics.

This bolastation idea got me looking into capture orbits. It looks like capture orbits can dramatically reduce the delta-v required to go from one planet to another. More than I realized: http://hopsblog-hop.blogspot.com/2012/06/inflated-delta-vs.html#!/2012/0...

Holy wrote:

Is the point from the Hop's Blog valid? I would think if you are in a highliy eliptical orbit around a planet you still need to decelerate alot at periapsis to rendezvous with any space station, ship or elevator in orbit.

The idea is to decelerate using aerocapture or aerobraking. Your altitude of an Earth braking/capture maneuver is going to be limited by your proximity to interdiction defenses around Earth, and aerocapture isn't going to work on say Luna.

Most rough estimates I'm seeing are 15% of entry mass for the heat shield. If 66% of your mass was propellant, that's like 5% of the total launch mass. And EP has *way* better materials science/superconductors/supermagnets - so EP engineers can likely get that 15% amount reduced. And then there's tricks like taking empty fuel tanks and reconfiguring them as a disposable heatshield while on your trip.

Circularizing a capture orbit with multiple aerobrakings is definitely doable, especially with EP style material technology. Aerobraking can shed a km/s or two per pass, but of course you need a decently thick atmosphere - Luna and Mercury are out. Earth is inaccessible (darn defenses!), and Jupiter is a bit risky (lots of radiation and small margins of error - whoops, wrong weather report, too much density, now you're doomed!) Note that a capture orbit is slow - an Earth capture orbit is roughly Lunar orbit sized and takes about a month to traverse. So doing a few aerobraking manoeuvres may take a few weeks. Just like the interplanetary transit system cheap orbits are slow ones.

Hmm. I question the value of single pass aerobraking at either end of the trip. I'm thinking that building a ship to handle 22G stress and vibration then pushing that mass up to speed is less economical than waiting an extra month or two or just burning propellant in several Oberth maneuvers of a closer orbit.

OneTrikPony wrote:

I believe the basic problem is; how many tons of propellant mass would aerobraking save? Is that mass more than the mass of shielding? Is the cost of that saved mass enough to cover the increased cost of building a ship that can take the stress of aerobraking?

Start mass / final mass = e^(delta V/Vexhaust) If you're using hydrogen and oxygen for propellant, each 3 km/s added to the delta V budget will about double the start mass. An extreme example of aerobraking was the space shuttle's re-entry. It would shed 8 km/sec within an hour. Circularizing Mars capture orbit takes about 1.4 km/s. Since energy scales with square of velocity, this takes about 1/32 of the joules per kilogram that the shuttle must shed. Moreover, shedding those joules are done over more than an hour. After each periapsis pass, the ship has more time to radiate off the heat gained. The Mars Global Surveyor as well as the Mars Reconnaissance Orbiter successfully used periapsis drag passes to lower apoapsis and both did this with with relatively tiny shielding. Time is an issue, though. The period for a Mars capture orbit is about 54 days. An aggressive drag pass shedding .2 km/s would substantially lower the apoapsis of a capture orbit to an orbit taking a little more than a day.

NewtonPulsifer wrote:

Keep in mind Mars in EP has an atmosphere 33 times denser than our mars due to terraforming.

Presently our use of aerobraking is timid and tentative. This is because we have incomplete knowledge of the state of our target planet's atmosphere. Atmosphere density at a specific altitude is variable. And a slight aerobraking mistake could result in lithobraking. If Mars is settled and terraformed, it'd likely have weather and surveillance satellites. The state of Mars atmosphere at a given time would be well known. If this is the case, more aggressive drag passes are doable. Given a well characterized, denser Mars atmosphere, I believe aerobraking could be used to circularize orbits over a week or two.