SWM wrote:Jonathan_S wrote:But if you're a pre-impeller ship and need to build up speed before jumping into the Alpha bands you don't have much choice, with reaction rockets you can't practically bring enough fuel to reach relativistic speeds so you're stuck with a very long n-space acceleration phase.
Those old fusion-drive ships did have some problems to deal with.
First of all, they didn't have inertial compensators. That means they were limited to around 1 gee acceleration. They could probably go higher, but they have to run continuously for weeks or months at a time. So I doubt they went above 1.5 under normal circumstances.
Let's suppose they normally accelerated at 1 gee. They start out fully fueled from sources inside the home system. They push all the way up to 0.3 c. While accelerating in normal space, they want to collect as much hydrogen as they can, to top off the tanks before they transit. When they reach 0.3 c and full tanks, they transit to the alpha band.
Once in hyperspace, they can't collect any more hydrogen. If they have enough hydrogen, they might accelerate to a velocity just enough to stabilize them for the transit to the beta band. It is unlikely anyone would repeat that to get into the gamma band. Whenever they decide they can't go any higher, they accelerate to as high a velocity as they can and still leave a little bit of fuel.
When they reach their destination, they transit back to normal space, bleeding their velocity nearly down to zero. So they have to accelerate again with whatever fuel they have left. Once they start moving, they can collect hydrogen from space, again, but it won't be very much until they start moving faster. They will basically be sputtering on fumes. At their destination, they will need to refuel, either from a fueling station in the system or by carving up some ice or by flying around the system with the gas collector. Since most of these ships are exploration ships, there usually isn't a refueling station at the other end.
So we said the ship uses 1 gee acceleration to reach 0.3 c. Running some quick calculations, this ship (using hydrogen fusion drive at 100% efficiency) would require fuel massing 12 times the mass of the empty ship just to get up to that speed with no fuel left over. It's no wonder they need to replenish fuel in mid-flight. It requires a huge amount of hydrogen if it wants to bring enough into hyperspace to be useful.
The ship would take 106 days to reach 0.3 c from zero velocity, and it would travel 16 light-days distance before transiting. If we assume the average density of gas is about the same as at Earth orbit (5 particles per cubic centimeter), a square gas catcher 1000 km on a side would collect 3000 tons of hydrogen. The interstellar medium varies between 10% of that and 10 times that, with very rare regions of thousands of times that.
Another point about the older, non-impeller drive ships, or impeller drive ships without inertial compensators*, is that the hydrogen you are catching is essentially stationary, while you are moving (potentially very quickly, as high as .23c or .3c--respectively, the normal maximum safe speed and absolute maximum speed to transit from normal space to the alpha band of hyper-space).
All that hydrogen has inertia (remember, no inertial compensator) and energy will have to be expended to accelerate the hydrogen in the direction the ship is traveling. The energy loss can be thought of as drag and as the ship speed increases, so does the drag.
I remember reading an article somewhere a few decades ago about the Bussard ramjet and how the energy equations worked out. The point was made that a Roman galley, with slaves working the oars had better energy propulsion numbers. And even when the slaves were asleep, you could still get more energy out of the slaves body heat than you could collecting and fusing interstellar hydrogen.
On a different point (economic/financial), I suspect that just as today, time is money in the Honorverse. For merchant ships, they would probably fill up on hydrogen when in orbit around their destinations at the same time they are loading and unloading cargo. In support of that idea, Captain Malachai (the
captain-owner with a 50% share of the ship) of the Candida Line's RMMS
Voortrekker states to Commander Wu (captain of the RMN CL
Cometary) when Captain Malachai is notified of the imposition of the Wartime Commerce Security Act:
A Rising Thunder, Chapter 2 wrote:“Even if I don’t get hit with the nondelivery penalty, I’m going to come up short on the note, especially if I have to sit in a parking orbit somewhere in the home system between now and then,” she pointed out. “A ship that’s not moving is only a hole in space that people pour money into. It’s sure as hell not a hole money comes out of!”
Well, that’s true enough, Wu reflected. And what are you going to do if she refuses?
Italics are the author's, boldface is my emphasis.
It just makes more sense financially if the merchantman makes more money for the time spent moving cargo (after deducting the costs for hydrogen) than it does making no money but saving the cost of hydrogen for the time spent using hydrogen catcher fields to refill the fuel tanks.
Another data point that indicates merchant ships value time spent moving cargo than time spent (and fuel costs reduced) using hydrogen catcher fields to refuel is that merchant ships arriving in a star system translate from hyper, accelerate in-system, then make turnover and decelerate to achieve orbit at the destination.
If they valued the time spent and money saved on fuel costs more than the time spent making money moving cargo, I would expect them not to accelerate in-system, but to deploy their hydrogen catcher fields to gather the fuel and use the drag of the catcher fields to further reduce the velocity the ship carried across the alpha wall, as well as the inward acceleration the ship will experience inside the hyper limit that the gravity source--usually a star, but can be a gas giant as well**--that produced the hyper limit in the first place will exert.
* Hyper drive invented 725 PD, impeller drive invented 1246 PD, Warshawski sails invented 1273 PD, inertial compensator invented 1384 PD. All dates are Post Diaspora (PD), taken from More Than Honor, The Universe of Honor Harrington, (1) Background (General).
** For an example of a gas giant with a hyper limit (lying outside the star's hyper limit), see Uriel in the Yeltsin's Star system (with the moon Blackbird, and later the Grayson's Blackbird shipyard(s) inside Uriel's hyper limit.)
The Honor of the Queen, Chapter 23 wrote:His eyes switched to a direct vision display filled with Uriel’s bloated sphere. The planet was so enormous it created a hyper limit of almost five light-minutes—half as deep as an M9’s. That meant Principality would have to accelerate at max for ninety-seven minutes before she could translate the hell out of here, and Harrington might have her cruisers smoking in on a ballistic course to pick off anyone who tried to run. With her drives down, he’d never see her coming till she hit radar range, but she’d see him the instant he lit off his impellers. That would give her time to adjust her own vector. Probably not by enough for a classic broadside duel, but certainly by enough for two cruisers to reduce a destroyer to glowing gas.
Italics are the author's, boldface is my emphasis.
