Ship Thrusters and Fuel Consumption

I decided to write this article in response to a comment by Robert on my previous ‘Ship Lists’ article. He brought up the fact that just calculating the specific physical properties of a ship doesn’t tell the whole story of space flight without first considering fuel and payload. You can read the original comment here.

Quite honestly I glossed over the fuel and payload section in the previous article because I knew it would be a problem and I hadn’t quite decided yet how to handle the issues in-game. The question is of course “how realistic will the game be in terms of fuel and fuel consumption”? and “will be by fun/entertaining for the player to fill up the cargo hold full of fuel just to be able to leave the atmosphere of a planet”?

The three approaches which are most obvious at this stage are:

1) Simulate Realistic Physics and Fuel Consumption : This is certainly an appealing approach and a lot of people would perhaps admire such a simulation for its realism, but will definately be off-putting for casual gamers. Furthermore, accurately simulating spacecraft using real physics models is extremely complex and involve many factors; all of which aren’t really feasible or a benefit to simulate. This would also introduce issues of ship design. As an example, the thrusters required for huge spacecraft to launch would themselves be huge; and would have to appear to scale on the ship. Add to this the need to place thrusters all around the ship for rotating and ‘strafing’ etc. and the ships will no longer look slick and cool (yes cool) but like big masses of ugly efficient-functionality.

A few other less-than-fun issues introduced with this system are ‘what do you do if the player runs out of fuel?’ As always there are a few ways of handling this, but short of actually waiting for someone to come and pick you up would break the realism we were striving to achive in the first place. This is a concern.

2) Forgo Fuel Consumption: This approach was adopted in the Elite series whereby conventional thrusters didn’t require fuel for acceleration; and this definiately had its merits. For one, the player only needed to concern his/herself with purchasing fuel for hyperjumps to use the jump drive and there was nothing overly complicated with taking off and leaving a planet. Just ‘launch’, rotate the ship upwards and fire! As you can read in Robert’s previous comments, this eliminates the issue of taking off within a the gravational area of a planet.

3) Use a mixture of the two above. : Now I’m all for realism and I’ve read a lot of comments which go both ways on just how real to simulate things like this, but I’m just not convinced that simulating real space flight dynamics will be fun for the player. Also, to pass up the issue of fuel will remove some of the element of ‘danger’ and planning I think has the potential to be enjoyable. So at the minute I plan on using a mixture of the above, but perhaps a little differently than tradional space sim games.

I am thinking about including three different engines types for flight, each of which will be mountable in one ship thruster slow/position. Here are the three types (not including Jump-Drives):

ION/Plasma Trusters

ION thrusters are used in real life for long distance space flight. They are typically favoured because of the relatively low fuel consumption when compared to traditional chemical engines. This improvement in fuel consumption is because electic thrusters typically offer much higher specific impulse (this represents the change in momentum per unit amount of fuel).

There are a few draw backs with ION thrusters though, which when implemented in the game will lucky add a little diversity. The first draw back is that they don’t provide as much thrust compared to chemical thrusters due to practical power source contraints. The second draw back is that they only work in environments that are void of ionized particles, so if you decide to equip just ION thrusters on your ship, you wont be able to land on planets with atmospheres.

There are three kinds of ION thruster in real life (Electromagnetic, Electrothermal/Plasma and Electrostatic). There will be diferent ION trusters available within the game but the differences will be purely cosmetic (different thruster description, different coloured beam), with all thrusters performing with the same characteristics:

Pros :
Low fuel consumption (read: no-fuel consumption)

Cons :
Cannot fly within atmosheres
Low thrust (force) output

Summary :
A good option for beginning players as the there is no need to worry about fuel consumption and/or running out. Hopefully more experienced players will want to eventually opt for more power chemical thrusters for greater acceration.

Chemical Rocket Thrusters

Currently all spacecraft use chemical rockets for launch. Chemical thrusters require a large amount of fuel to operate, but provide consideratly more thrust than ION thrusters. In the context of the game, chemical thrusters will be required to navigate within the atmosphere of a planet, and will indeed require fuel to do so, although the fuel consumption may not be realistic:

Pros :
Provides much more power compared to the ION thruster.
Functions in atmospheric conditions (i.e. planets with atmospheres).

Cons :
Requires fuel.
Would be plausible that the player can run-out of fuel and thus would need rescuing.

Summary :
A good choice for players who want faster ships but requires a little more journey planning and management of fuel.

Hybrid Thrusters
Whilst perhaps not currently technically possible I would like to include some kind of hybrid technology to make the game more sympathetic to new players who perhaps don’t want the hassile of outfitting different ships and just want to ‘shoot things’. The hybrid engines will non consume fuel in outerspace, and will require significantly less fuel when within a gravatational area. This will come at the expense of having significantly lower thrust outputs then the chemical or ION/plasma thrusters.

Pros :
Offers both the benefit of power provided by the chemical engine and the ease of mind of a fuel-less alternative for emergencies.
No restricts on environment conditions

Cons :
Provides less acceleration than either the electric or chemical engines of the same class.

To Sum Up

So hopefully with the details mentioned above it should be a little clearer to see what I intend to do. If you decide you want to outfit one ship for space dog-fighting, then you may want to use chemical rocket engines and refuel often for the advantage of higher accelertion. If you are a trader, then perhaps ION thrusters would be beneficial considering you needn’t worry about fuel on long journeys. The last question which I haven’t address yet is the actual consumption of the chemical engines, however this will be fun-factor based so its a little too early to say.


  1. #1 by Chris Harshman on May 17, 2010 - 3:26 pm

    There is a distinct difference between Plasma and Ion engines, mainly true Ion engines cannot work in an atmosphere, but Plasma engines can.

    A Plasma torch is an example of that, but you can increase the scale, also the modern plasma engines that are currently in development for a mars mission can provide theorically enough thrust to launch off a planet like earth.

    Also, for fuels, there is no reason that nuclear reactors or fission reactors cannot be used, and even now with modern nuclear reactors the technology exists to recharge to 100% depleted fuel rods, so by having 2 sets of fuel rods you can continue indefinatly.

    Also, this is a SciFi Game, so there is no reason you cannot make up numbers/facts or anything you want, espically if you base it in theroical fact, sure there will be people who complain, but the potential market (audience) will be that much bigger.

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