Hydrogen Thrusters

Hydrogen Thrusters were the second type of thruster to be added to Space Engineers. Instead of using power, they burn Hydrogen gas as fuel to propel a ship in the desired direction.

The industrial variants are reskins that you can build if you own the Heavy Industry Pack.

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Hydrogen thrusters large and small
Hydrogen thrusters need large hydrogen tanks and conveyors
Heavy Industry variant of hydrogen thrusters
You recognise Hydrogen thrusters by their yellow flame
An asteroid mining ship with hydrogen thrusters
A hybrid ion/hydrogen ship is power-efficient in space and can handle planetary lift-off and landing
Industrial Large Hydrogen Thruster (DLC)

Usage

For the hydrogen thrusters to work, remember to disable Stockpile on the Hydrogen tanks when undocking. Also make sure that the conveyor system is intact and has power to transport the hydrogen from the tanks to the thrusters -- the conveyor lights must be green.

Typically, you will use large thrusters in one direction for lift-off, and several small thrusters in all directions as reaction control system (RCS).

How to save fuel by using Control Groups

As with all thrusters it's recommended to create control groups, one for all RCS thrusters, and one for only the lift-off thrusters. This allows you to switch on/off different sets of thrusters when needed. For example, switch off the large thrusters while steering carefully close to asteroids in zero gravity, switch them on for takeoff, landing, and de/acceleration.

If you intend to fly horizontally in gravity, you must keep Inertia Dampers on, but they waste fuel by engaging your braking thrusters (the fore thrusters) needlessly. Create a control group for your braking thrusters so that you can coast forwards with Inertia Dampers switched on, and braking thrusters switched off. To brake, reenable the braking thruster group again!

How to save fuel by using Thruster Overrides

Accelerating ("holding W key") and pushing against the artificial speed limit wastes hydrogen. When you configure your Tool Bar, not only add on/off toggles for your thruster groups and Hydrogen Tank Stockpile, but also add Increase and Decrease Thrust Override controls for the lift-off thruster group. Instead of having to hold W key down the whole way, use these controls to increase and decrease constant thrust, like a throttle.

Lift off and accelerate up manually by holding W key.
Once you hit the maximum speed of 100 m/s, let go of the key and tap Increase Thrust Override about a dozen times until you maintain that speed.
When planetary gravity starts decreasing, carefully tap Decrease Thrust Override to back off the fuel usage while keeping your speed at 99 m/s.

At lower and lower gravity, less and less override is needed to maintain the escape velocity. In space, switch off the override and fly as usual using WASD keys (and Inertial Dampers).

Pros and cons

The unique advantage of Hydrogen Thrusters is their consistent acceleration and strength and that they work equally well in space and in atmosphere, which makes them ideal for shuttles.

Their disadvantage is that they must be conveyored to a large source of hydrogen. This forces you to add Hydrogen Tanks and (for anything but the smallest short-range shuttles) also O2/H2 Generators filled with Ice.

The extra mass, volume, and conveyor tubing that these peripherals add to a hydrogen-powered ship can diminish the returns. However, if you find the right balance, the above-average acceleration and versatility of the Hydrogen Thrusters makes up for the added complexity and volume.

As all thrusters, large hydrogen thrusters cause damage to blocks up to 6 blocks length in front of them, and small thrusters up to 3 blocks length.

Hybrid usage

Pure Hydrogen ships work well in space and in the atmosphere, but require large tanks. For shuttles, it’s feasible to take advantage of both the efficiency of the atmospheric (or ion) thrusters and the power-to-surface ratio of the hydrogen thrusters in one ship: Use atmospheric (or ion) thrusters as the "primary thrust", and treat the hydrogen thrusters as secondary "afterburner".

An Ion-Hydrogen hybrid shuttle operates mainly at orbital space stations and it sometimes takes passengers down to the near-by planet surface.
An Atmo-Hydrogen hybrid shuttle operates mainly at planetary stations and it sometimes takes passengers up to a near-by orbital space station.

This way, the secondary hydrogen thrusters would make do with a limited volume of fuel (with or without fuel regeneration via an onboard O2/H2 generator), used only for high-thrust-demand situations such as combat manoeuvres, escaping a planet's gravity well, or avoiding cratering on re-entry.

Fuel Cost

Fuel Cost
Hydrogen Thruster size	MN of thrust force	Hydrogen cost in L/s
Large-Grid Large Hydrogen Thruster	7.2 MN	4,820.05 L/s
Large-Grid Small Hydrogen Thruster	1.08 MN	803.34 L/s
Small-Grid Large Hydrogen Thruster	480 kN	385.60 L/s
Small-Grid Small Hydrogen Thruster	98.4 kN	80.33 L/s

The formula for the fuel cost is FuelPerSec = <MaxPowerConsumption> / (Fuel_EnergyDensity * <Efficiency>) where

<MaxPowerConsumption> is the maximum power consumption of the thruster
<Fuel_EnergyDensity> is a constant of value 0.001556
<Efficiency> is the thruster block's fuel efficiency value (which is currently 1 for all thrusters)

For reference, 1 Newton (unit abbreviation 'N') is defined as the unit of force necessary to accelerate 1 kg of mass at 1 m/s^2. A planet with 1 G has an acceleration from gravity of 9.81 m/s^2. Thus 9.81 N of force is required to counter the gravity of 1 kg of grid mass against 1 G of gravitational acceleration (and, of course, more is needed to actually go _up_ and carry cargo). 1 kN is 1000 N, and 1 MN is 1000 kN (1,000,000 N).

Thrust per Volume

thrust per volume
hydrogen thruster size	kN of thrust per L/s of hydrogen	kN of thrust per block (3x3x3)	thrust per thruster-face block area (3x3)
Large-Grid Large Hydrogen Thruster	1.4934 kN	266.67 kN	800.00 kN
Large-Grid Small Hydrogen Thruster	1.3440 kN	1,080.00 kN	1,080.00 kN
Small-Grid Large Hydrogen Thruster	1.2245 kN	17.78 kN	53.33 kN
Small-Grid Small Hydrogen Thruster	1.2246 kN	98.4 kN	94.4 kN

As you know, the block sizes for the small grid thrusters are 1/5th the size, in each dimension, as the large grid blocks. Thus a Small-Grid Large Thruster has an equivalent of 6,666.25 kN per large block (17.78 * 5^3), and 1,333.325 kN per large block thruster face. It is, however, typically impractical to attempt to use small-grid thrusters on a large grid.

The small thrusters in each grid size are noticeably superior in thrust per volume and thrust per thruster-face block surface. The latter is an important consideration because, while a large thruster requires 27 blocks of volume (3x3x3), it can typically only be replaced by 9 small thrusters due to thrusters damaging blocks in their thrust path. Both small thrusters are still superior in thrust-per-surface-area compared to the same-grid-size large thruster, however, which means that replacing a large thruster with even only 9 small thrusters will improve total thrust. Note, however, that the large grid small thruster is 10% less efficient on fuel than the large grid large thruster (the small grid ones are nearly equal to each other).

Material Costs

It's also worth considering that a small thruster is more expensive and weighs more for its volume than a large thruster.

In large grid, 1 large thruster's worth of materials is worth 6 small thrusters on Steel Plates, 3.75 small thrusters on Construction Components, 6.25 small thrusters worth of Metal Grids, and 5 small thrusters worth of Large Steel Tubes.
In small grid, the ratios for large to small on materials are 4.29 in Steel Plates, 2 in Construction Components, 5.5 on Metal Grids, and 5 on Large steel Tubes.

As a result, 9 small thrusters both cost more materials and weigh more (since block mass is the sum of the mass of the components required to build it) than a single large thruster. Combined with the fuel efficiency disadvantage, this means that in particular the large grid small hydrogen thruster is quite a bit less useful compared to the large grid large. In small grid, the direct fuel efficiencies are effectively equal, so the weight is the only difference on fuel efficiency.

Comparison to Other Thruster Types: Power

While hydrogen thrusters provide the best option in the game currently for thrust-per-volume and thrust-per-surface-area, as well as the only thrust option that works equally well in and out of atmospheres, both atmospheric and ion thrusters can be compared against other thrusters when their electricity is supplied by the Hydrogen Engine, which produces 10 kW of power per L/s of Hydrogen.

For ion thrusters, the thrust per MW of power varies from 128.57 for the large-grid Large to 72 for the small-grid Large and Small (they have equal efficiency).
For atmospheric thrusters, the thrust per MW varies from 385.71 for the large-grid Large to 160 for the small-grid Small.
At 1 MW = 100 L/s, hydrogen thrusters vary from 122.59 kN per MW equivalent (SG small), to 149.38 per MW equivalent (LG large).

This means that ion thrusters are around as efficient on H2 as hydrogen thrusters, and atmospheric thrusters up to 2.5 times as efficient -- but remember, their efficiency degrades with altitude!

Comparison to Other Thruster Types: Volume

Hydrogen thrusters themselves are quite a bit more space-efficient, in terms of thrust-per-block-volume and thrust-per-surface-area compared to the other two options, especially if one needs to combine both ion and atmospheric for a craft that can operate both in and out of atmosphere. However, hydrogen storage is substantially less space-efficient than electrical energy storage.

For example, a small-grid large hydrogen tank, which is 3x3 (27 blocks of volume), has a capacity of 160 kL of hydrogen. This is equivalent to 0.444 kWh of energy storage when run through the hydrogen engine (10 kW-seconds per litre), or 0.01646 kWh per block. Meanwhile, a small-grid large battery, which is 3x2x3 (18 blocks of volume) can store 1 MWh of power, or 55.5 kWh per block, 3375 times as much energy density as a tank of hydrogen. And note that that's comparing them using the hydrogen engine, hydrogen thrusters are 5 to 25 times less efficient, depending on the thruster alternative being considered. The lack of storage density means that most hydrogen-driven craft must also have an O2/H2 generator on board (or multiple), and the ice to fuel them, increasing mass and decreasing the net thrust advantage. This issue is compounded by the need to run conveyers to all of the thrusters, while electric thrusters merely have to be attached to the grid (how much of a block space cost this is depends on the layout of the craft and any other reasons, such as weapons, that one might need to be running conveyors throughout).

As an example, two small-grid large hydrogen tanks attached to a single large hydrogen thruster requires a total volume of 81 blocks (3x3x9). The tanks have a total capacity of 320 kL, which can fuel the single large hydrogen thruster for 830 seconds of thrust, or ~13.83 minutes. Two small-grid large batteries and a single small-grid large atmospheric thruster, which occupies the same volume of 3x3x9, have a total battery capacity of 2 MWh, or 7200 MW-seconds of energy. That can fuel the single large atmospheric thruster for 3000 seconds, or 50 minutes, while providing 20% more thrust (576 kN from the large atmospheric thruster, versus 480 kN from the large hydrogen).

To minimize the extra internal volume required to run conveyors for the hydrogen thrusters, a common arrangement is to mount large hydrogen thrusters directly onto large hydrogen tanks, though one still needs a method of filling those tanks (either on-board, using an O2/H2 generator, or from an external ship/base via a Connector).

Comparison to Other Thruster Types: Material Costs

Lastly, it should be noted that Ion Thrusters require Thruster Components, which require Platinum to make. Under default settings, Platinum cannot be found on planets, only on moons or asteroids. As such, players that start on, or are otherwise marooned on, a planet must use hydrogen thrusters to reach and navigate in space until they locate and return with enough platinum ore to make thruster components to build ion thrusters.

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