I know I have been absent for a couple of weeks. Partially work-related but partially because I have been playing around with a variant of Aurora, which I will probably call Aurora FTL. This isn't a change to the main game, but an optional alternative that should be available in a few months, spare time permitting. The two major differences will be a Newtonian flight model and FTL instead of jump points. The latter is partially forced by the former because speeding up and slowing down at each jump point on long journeys would use so much fuel it would render it unplayable. I am working on the Newtonian element at the moment, which is a huge change to many parts of Aurora, and then I will move on to the new universe model. This is really an experiment because I am still not convinced that a newtonian game is playable but it's something I thought about many times and it has been suggested on here a lot. I decided that I may as well give it a go because if I don't I will always wonder if it was possible

. As a taster of the final game, here is a summary of the current changes to engine design and ship design in Aurora FTL.

**Engines**The concept of Military Engine, Commercial Engine, Fighter Engine, etc has been removed and Hyper Drives have been removed. The five elements of engine design are now:

**Engine Technology: **As before, except the base values are different and those values are expressed in Meganewtons of thrust per HS of engine. One Meganewton (MN) is equal to the amount of net force required to accelerate a mass of one ton at a rate of one kilometre per second squared. For example, the Internal Confinement Fusion Drive has a rating of 2 MN per HS, so an unmodified size 5 engine would produce 10 MN of thrust.

**Base Fuel Efficiency: **This is similar to the old Fuel Efficiency, although it is now modified by other factors in engine design. The Base Fuel Efficiency is critical though and will be more important than in the past. It starts at 20 and additional technology levels will lower that figure. An Engine is rated in the number of litres of fuel per hour it consumes. This amount is derived from Engine Power x Fuel Efficiency. So an Engine with 10 power and a fuel efficiency of 12 would consume 120 litres of fuel per hour at full burn.

**Engine Size:** You can now select the size of engine from 1 HS to 50 HS. Larger engines are more fuel efficient so Fuel Efficiency is modified by 1 - (EngineHS / 100). In simpler terms, each HS of engine reduces fuel efficiency by 1%, so a size 10 engine reduces Base Fuel Efficiency by 10% and a size 25 engine reduces it by 25%.

**Thermal Reduction:** As before, this reduces the thermal signature of engines, which is equivalent to 10x thrust in MN.

**Engine Power / Fuel Efficiency Modifiers:** There are two new tech lines to research, called Max Engine Power Modifier and Min Engine Power Modifier. These establish the range within which you can change engine thrust from that provided by the base engine technology. Increasing thrust has a significant effect on fuel efficiency and decreasing thrust can provide huge savings in fuel efficiency. Power can be increased by up to 300% of normal and decreased to 10% of normal if you have the prerequsite techs. The dropdown on the design window will have options from the minimum possible to the maximum possible in 5% increments. So 40%, 45%, 50%, 55% ...... 180%, 185%, etc. Each engine power modifier percentage is accompanied by a fuel efficiency modifier, based on the formula Fuel Efficiency Modifier = (10 ^ Engine Power Modifier) / 10.

For example if you choose to increase Engine Power to 125% of normal. The Fuel Modifier would be (10 ^ 1.25)/10 = 1.7783, so for a power increase of 25% fuel use would increase by 78%. This is shown on the dropdown as "Engine Power Modifier 1.25. Fuel Modifier 1.78". For an engine with 200% of normal thrust, the fuel modifier is 10x. This is the equivalent of a FAC engine in Aurora, except now you can have different size engines and you can have more than 1 per ship.

Here is the design summary for an engine of 5HS, using Internal Confinement Fusion technology, with a 25% increase in thrust, base fuel efficiency of 8 and no thermal reduction.

**Internal Confinement Fusion Drive**Power Output: 12.5 MN Exp Chance: 12 Fuel Efficiency: 13.5151 Thermal Signature: 125

Base Acceleration: 50 mp/s (5.1G)

Fuel Use at Full Burn: 168.9388 litres per hour

Engine Size: 5 HS Engine HTK: 2

Cost: 62 Crew: 8

Materials Required: 15.5x Duranium 46.5x Gallicite

Development Cost for Project: 620RP

The Fuel Efficiency is calculated as base 8, x0.95 for engine size, x1.7783 for engine power modifier, which equals 13.5151. Fuel use in litres per hour is therefore 12.5 MN x 13.5151 = 168.9388. As that single engine alone would use up a 1 HS fuel tank in a little over 12 days, you can already see that fuel tanks are going to be a lot bigger in Aurora FTL. Based on testing so far, fuel is going to be 10-20% of hull mass for warships. A lot less for freighters as you can build some very fuel efficient engines as well.

The base acceleration is for the engine accelerating itself with no accounting for where the fuel is coming from. While this is obviously never achievable in practice, it provides a way to rate engines against each other. 50 mp/s is an acceleration of fifty meters per second squared. The 5.1G is the force a passenger on the engine would feel. This subject is covered more realistically in the ship design below. Exp Chance is based on 10% of the engine power modifier, rounded down.

Now lets look at an engine designed around fuel efficiency rather than thrust. This is an engine of 25HS, using Internal Confinement Fusion technology, with a 70% decrease in thrust, base fuel efficiency of 8 and no thermal reduction.

**Commercial Internal Confinement Fusion Drive**Power Output: 15 MN Exp Chance: 3 Fuel Efficiency: 1.197 Thermal Signature: 150

Base Acceleration: 12 mp/s (1.22G)

Fuel Use at Full Burn: 17.955 litres per hour

Engine Size: 25 HS Engine HTK: 12

Cost: 75 Crew: 2

Materials Required: 18.75x Duranium 56.25x Gallicite

Development Cost for Project: 750RP

The Fuel Efficiency is calculated as base 8, x0.75 for engine size, x0.1995 for engine power modifier, which equals 1.197. Fuel use in litres per hour is therefore 15 MN x 13.5151 = 17.955. So this engine produces more thrust than the above one and only uses a tenth of the fuel. However, it is 5x larger so the base acceleration is much lower. Even so, you will actually get more Delta-V for the same fuel from this engine than the one above - it will just take longer to do it. More on Delta-V in the ship design section.

Note that there is no accounting for exhaust velocity in the engine design. This is a key element in the design of real rocket engines. It has a huge effect on fuel efficiency and will affect the acceleration provided by the engine once the speed of the rocket approaches that of the engine's exhaust velocity. However, I have to draw a line somewhere between realism and fun and in the case of exhaust velocity I decided that having a simpler fuel efficiency rating for the engine that could easily be understood by players would be preferable to players having to understand Tsiolkovsky's rocket equation and associated material. I think the current mechanics of engine design allow for a lot of freedom, and provide the players with the feel of a Newtonian game without having to get into serious math.

(

http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation)

**Ship design**The ship below is a Daring class area defence cruiser from my NATO vs Soviet Union campaign, converted to use the new engine mechanics.

**Daring class Area Defence Cruiser ** 6,873 tons standard 10,700 tons full load 682 Crew 1643.6 BP

Armour 5-43 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 6 PPV 15

Maint Life 4.98 Years MSP 576 AFR 152% IFR 2.1% 1YR 39 5YR 580 Max Repair 252 MSP

Active Signature 214 Thermal Signature 1500 EM Signature 1680/0

Magazine 831

Newtonian Magnetic Confinement Fusion Drive (12) Total Power 150 MN Fuel Use 1200 litres per hour Exp 5%

Full Load Acceleration 14.02 mp/s (1.43G) Hourly Acceleration 50.47 km/s Daily Acceleration 1211.22 km/s

Standard Acceleration 21.83 mp/s (2.23G) Hourly Acceleration 78.57 km/s Daily Acceleration 1885.77 km/s

Fuel Capacity 1,750,000 Litres Full Burn Duration 60.8 days Delta-V Budget (Full Load) 80,366 km/s

Mk5 Guided Missile Launching System (15) Missile Size 1 Rate of Fire 10

SN/SPG-31 Anti-Missile Fire Control (3) Range 35.3m km Resolution 1

SN/SPN-27 Navigation Sensor (1) GPS 1680 Range 30.4m km Resolution 60

SN/SPS-30 Missile Detection Sensor (1) GPS 252 Range 35.3m km Resolution 1

The key changes are as follows:

**Standard vs Full Load:** The standard mass of the ship is without any fuel, cargo, ordnance, colonists or parasites. Full load obviously assumes everything that could be loaded is loaded. In Aurora FTL, the current mass of the ship is tracked very carefully, so as you use fuel or fire missiles, the mass of the ship will be slightly reduced and your maximum acceleration rate will increase. The full load is also the volume of the ship, which will be used for FTL purposes. Volume is not affected by current mass and will always be equal to the full load mass.

**No Maximum Speed:** Maximum speed has been removed from the summary. Ships no longer have a maximum speed as it is based on acceleration rates and available fuel. In Aurora a ship had to continue using fuel to maintain velocity. In Aurora FTL, once you have velocity you can turn off the engines and you will continue at the same speed. However, you will need to use the engines to slow down again.

**Active, Thermal, EM Signatures:** These now have their own line on the summary. Many players didn't know what the old abbreviations meant and space on the top line was an issue with the additional of the standard load tonnage.

**Engine Line:** The engine line now shows the total thrust of the engines, compared to the old individual power rating from Aurora. Fuel use is the combined fuel use for all engines, in this case 1200 litres per hour. Bear in mind that most ships will likely spend a considerable amount of movement time with engines off. They can accelerate to a desired velocity, disengage engines and then engage the engines again to decelerate when required.

**Full Load Acceleration:** The 14.02 mp/s (1.43G) section indicates that at full load, this ship has an acceleration rate of just over fourteen meters per second, which is equivalent to 1.43G for the crew. In an hour the ship can accelerate to a fifty kilometers per second and in a day to 1200 km/s. Bear in mind it will take just as long to decelerate. This utterly changes the tactical situation from Aurora. For example, if you charge into an unknown system you cannot simply reverse course if you detect an alien ship. You will have to either decelerate first and then reverse course, maybe increase speed and try to charge past, or attempt to change course, which is going to be extremely difficult unless you are going relatively slowly in comparison to your acceleration rate. I haven't done the final course change calculations yet but I can already see that you are probably going to have a pick a destination very carefully.

I haven't decided yet what to do with regard to gravity but I am leaning toward technobabble which states the existing trans-newtonian materials from Aurora are anti-grav materials in Aurora FTL, allowing you to build ships with an anti-gravity field. This solves the gravity issue not just in playability terms, as otherwise a lot of early ships wouldn't be able to leave Earth

, but also in performance terms as otherwise I would have to be checking the gravitational effect on every ship for every significant body in the system on every sub-pulse. Yuck! The disadvantage of ignoring gravity is that you can't do maneuvers to pick up speed from planets, although I can probably build something into the technobabble along those lines (switching off the anti-grav, etc). I will also probably add aerobraking maneuvers as well. Still a lot of work, which is why it will be months before anyone see this.

**Standard Acceleration:** This provides the same as above, with the assumption the ship is not carrying anything and is using its last dregs of fuel. This line and the one above provide a range of acceleration rates between which the ship will be operating. On the Ship window, the Ship summary substitutes this line with Current Acceleration, with the values based on the current ship mass.

**Fuel Capacity:** The first section is as before, with the total fuel capacity. This will likely be far greater in Aurora FTL than in Aurora. Next is the Full Burn Duration, which is the total time the ship could run its engines at full power using the total fuel capacity. Finally, the Delta-V Budget. There is no such thing as range based on fuel in Aurora FTL because you have effectively unlimited range. Instead you have a Delta-V budget, which is the total amount of velocity change you can achieve with your current fuel. The Daring has a Delta-V Budget of approximately 80,000 km/s, which means it could accelerate to 40,000 km/s and then decelerate down to zero, or it could accelerate to 4000 km/s and back to zero ten times, or carry out any combination of acceleration and deceleration that adds up to 80,000 km/s. This budget is calculated using a minute by minute calculation of the full burn duration, taking into account the slowly decreasing mass of fuel over the 60.8 day period.

Delta-V is the key value when looking at the ship design and the use to which you intend to put it. A system defence ship which is intended to fly toward an intruder, engage it and return home to its fuel source will only need enough fuel to accelerate to a desired speed, decelerate to zero and then carry out the reverse in order to return home. The outward leg will presumably require a longer period of acceleration as velocity is more important. A Delta-V budget of 15,000 km/s would allow an acceleration to 5000 km/s, the same deceleration to zero and then a return home leg with an acceleration to 2500 km/s. A long range geological survey ship would probably accelerate and decelerate many times. It will need a very fuel efficient engine and a very large delta-V budget.

As a further example, here is an early game freighter. This ship is designed with much more fuel efficient engines. The Delta-V budget is 30,000 km/s as that is deemed sufficient to carry out four accelerations and decelerations with an final intended speed of around 3000 km/s, while still allowing some reserve. This is because the FTL system in Aurora FTL, which I will go into in a future post, will often result in you entering your destination system with the wrong heading. It may be possible to correct course if the deviation is minor, or it may be necessary to decelerate to rest and accelerate again.

**Atlas class Freighter** 8,350 tons standard 34,100 tons full load 26 Crew 528.6 BP

Armour 1-93 Shields 0-0 Sensors 1/1/0/0 Damage Control Rating 1 PPV 0

MSP 10 Max Repair 50 MSP

Active Signature 682 Thermal Signature 400 EM Signature 0/0

Cargo 25000

Commercial Ion Engine (4) Total Power 40 MN Fuel Use 105.5 litres per hour Exp 4%

Full Load Acceleration 1.17 mp/s (0.12G) Hourly Acceleration 4.22 km/s Daily Acceleration 101.35 km/s

Standard Acceleration 4.79 mp/s (0.49G) Hourly Acceleration 17.25 km/s Daily Acceleration 413.89 km/s

Fuel Capacity 750,000 Litres Full Burn Duration 296.2 days Delta-V Budget (Full Load) 30,362 km/s

I realise this alternative Aurora will not be for everyone and it will be a niche area of a niche game

. Tactical planning is going to be critical, combat will be totally different, expansion will be slow and there will be a little more micromanagment than Aurora, although I will try and reduce that as much as possible. For example, in the fleet orders window you can include a maximum speed in an order so a fleet will accelerate to that speed, then switch off engines. Engines will be re-engaged when the fleet decides it needs to start slowing down. I'll post updates as I add new features and probably some mini-AARs from testing. I am not promising a release date

as it depends on outside factors but hopefully there will be something before Xmas.

Steve