Monday, September 19, 2016

EM Drive Blues

We have thgis information on the radio frequency drive or EM drive or whatever you want to call it.

300 micro newtons are generated per kW of power.

-or-

3.5 kW generates 1 newton of thrust

A Scout ship has a thrust of 2000 tons or 2,000,000 kgs or 20,000,000 newtons

This would require 70 million kilowatts or 70 gigawatts. If the EM drive worked (I'm not saying it DOES in the real world, Rocket Cat call off the hit.)

Fusion reactions generate

3.5 x 10^14j from one kilogram H2 and H3

One kilo of fuel allows the Scout to thrust for (3.5 x 10^14)/(7 x 10^10) seconds

One kilogram of fuel provides thrust for 5000 seconds or five turns. The two tons in the power plant will thus provide 2000 * 5000 = 10 million seconds of thrust or 10,000 turns or 11.57 days of thrust. There's stuff I didn't get into of course. Like fusion isn't 100% efficient. Let's say the EM drive has increased in efficiency to offset the losses due to the fusion plant's inefficiencies. Even Steven.

This gives the ship a delta v of 10 million * 20m/s^2 = 200 million  meters or 20,000 kps. Figure you use half to speed up and half to slow down 10,000 kps cruising speed. That's 1,000,000 km in a little over a minute and a half. That's an A.U. in 4.1 hours.

You don't want to use all your fuel to speed up because aerobraking just isn't an option at 10,000 kps. Not that it doesn't work, it works too well. Boom. Say you accelerate to 5,000 kps. That will take about 42 hours. This might cause problems for a freighter on a schedule.

But it's worse than that. The Scout has to accelerate at 20 meters per second. That mean no you will not get to your top velocity on this flight. Instead you must use the time honored tradition, accelerate halfway, flip and decelerate. At a constant 2 gees acceleration this will take about 4.6 days. That can out a crimp in a ship's schedule. It's even worse for freighters. They can manage 1 gee and can make the trip in 6.5 days. That's most of your downtime between jumps. This explains why merchants do not often use the gas giant option unless there is one fairly close by and the ship has a fuel refinery. Oh yes, refining fuel takes time also. So trucking in fuel for the mainworld or providing it for ungodly markup becomes understandable. A trader (35 Mcr. ) mortgage runs 5000 cr. a day or so. A ship with one refinery onboard can process 20 tons of fuel a day. The difference between refined and raw fuel is 400 credits per ton so you're saving 8000 credits but the day of downtime 'costs' you 5000 credits. Our savings are only 3000 cr. That's before figuring life support, salaries and complaining passengers who have business elsewhere. If you are using a gas giant reaching it's jump limit can take some time as well 3-4 times as much as an Earth-sized planet.

I'm saying most inner system and nearer gas giant runs can be made no problems with the acceleration. It's economic considerations that keep us based at those terrestrial main worlds. This is why they attract settlers as much as their often breathable atmospheres.

Also pirates hang around gas giants sometimes.

No reserve the gas giant jaunts for quick refueling stops on longer journeys when you don't need to land.You're harder to predict then.

Of course like any reactionless drive it becomes a weapons of mass destruction. You have a chance to stop a Scout ship say at 10,000 kps. But that was just using 2 tons of fuel. If it goes into jump fuel you could easily make a 1 parsec jump and wind up accelerating to 120,000 kps and that my friends is starting to crowd photons. You might want to limit your 'cruising mode' to one gee of acceleration unless you're in a real hurry. Higher accelerations are less effective for the fuel cost. Double the acceleration (and delta fee and fuel only gets you there in 70% of the time. You need quadruple the acceleration to halve the trip time.

Maybe there are other factors limiting the EM drive's use. Maybe you need to pass through an atmosphere to discharge your boson capacitors or the doubletalk generator needs to be taken out and reground. Or the drive works by pushing against massive bodies like planets and stars. In that case your top speed might be limited by the size of either the body you depart from or the body that is your destination. For example going from Earth to Mars you'd figure your top speed based on Mars. Yes if you left Earth you could reach a greater speed using it as 'propellant' but you still would only have Mars' mass to slow down at your destination. You do want to slow down, right? We had this talk before.

A good approximation might be 1000 kilometers per second per UWP size code. A ship boosting from Earth to Mars would use Mars' size code of 4 and have a top speed of 4000 Kps. A ship moving from two size 8 Earth like worlds would have a top delta-vee. A size A world will let our Scout reach its maximum velocity. An asteroid could limit a ship to 1d6x100 kps. 

NavigatIon becomes something more of a craft with using planets and moons to push off. Ships might use a number of flybys to build acceleration if they launch from a small body and want to make good time in an anti-grav version of the Oberth maneuver.

It's all fun and games until someone builds a flying fuel tank farm and uses a neutron star to boost. I wouldn't but there are nutcases out there and some of them can plot courses.