Rocket Engine

**This article contains headings/subheadings that are linked to from other pages.** Please contact an administrator before altering any of the following headings/subheadings:
**Gnome Engine, Goblin Solid Motor, Ifrit Solid Booster, Phoenix Solid Booster, Pixie Engine, Mage Engine, Titan Engine, Apex Engine, Wyvern Engine, Dragon Engine, Spirit Engine**

*What does this mean?*
A **rocket engine** is an engine used on a rocket, designed to propel a rocket. They burn fuel to propel a rocket. In Juno: New Origins, there are nine default rocket engines. A default engine can be customized by going to Part Properties > Rocket Engine. At that point, there are multiple characteristics that can be changed, which can change the appearance and thrust of the engine.

## Performance

A rocket engine's performance is defined by multiple aspects, which are measured as **Thrust, Isp, Mass Flow, Nozzle Area Ratio, Exit Pressure, Air Pressure, Power Usage, and Spool Time.**

**Thrust** is measured in newtons (N), kilonewtons (kN), or meganewtons (MN). 9.81kN is needed to lift 1t mass.

**Isp** denotes efficiency, measured in seconds. A player would want a higher Isp, but sometimes, having a lower Isp would be acceptable, especially since a high Isp engine will often sacrifice other performance aspects like thrust.

**Mass Flow** dictates how much fuel is used, in grams/second (g/s), kilograms/second (kg/s), or megagrams/second (Mg/s).

**Exhaust Velocity** dictates the speed of which exhaust gases leave the nozzle.

The **Nozzle Area** **Ratio** is computed by the following formula: ***A = (3.14 \* n2)/(3.14 \* t2)**.* Let A be the Nozzle Area Ratio, n is the radius of nozzle exit, and t is the radius of the throat. It is the ratio of the size of the throat and the end of the nozzle. The higher this is, the more efficient the engine is in vacuum. The lower it is, the higher the sea level thrust and efficiency. Generally, a player would want a reasonable value for first-stage engines, while a value as high as possible for stages not involving any significant atmosphere (if any).

* Exhaust with Exit Pressure lower than Air Pressure
* Exhaust with Exit Pressure at or near Air Pressure
* Exhaust in a vacuum
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**Exit Pressure** (Pa, kPa, or MPa) is the pressure at which exhaust leaves the nozzle. A difference between Exit Pressure and Air Pressure results in performance loss and potential damages if more than 56kPa lower than Air Pressure. Differences also change the shape of the exhaust. Values lower than Ambient Pressure result in formations of shock diamonds and bouncing exhaust, while values greater than Air Pressure result in an outward expansion of the exhaust.

**Air Pressure** (Pa, kPa, or MPa) is the pressure at the selected altitude. A difference with Exit Pressure would degrade a rocket engine's performance.

**Power Usage** (W or kW) indicates how much electricity is being used by the engine. Negative values indicate power generation. Dividing Watt-hours by Watts will give how many hours that power can be used.

**Spool Time** (seconds) indicates how long it takes for a rocket engine to reach 100% throttle from startup. Usually, more powerful engines have a longer spool time.

## Engine Properties

### Engine Type

The engine type is the type of combustion system the engine uses, which greatly impact performance.

**Monopropellant** engines produce very low thrust, but lasts long, and has fast throttle response. These engines use monopropellant, the same fuel for RCS Nozzles.

**Nuclear Thermal** engines are very efficient, low/medium thrust engines. These engines use Liquid Hydrogen or Water. However, these engines have very slow throttle response.

**Liquid** engines are perhaps the most common engines. Their thrust and efficiency depends on the fuel and other properties. They are the most customizable engines, with selectable power cycles. These engines use kerolox, hydrolox, or methalox.

**Solid** engines are powerful, but inefficient, and once ignited, they burn at various set throttles until they run out of fuel. These engines use solid fuel, which is heavy.

### Fuel Grain (for solid engines only)

The fuel grain depicts the thrust outputs of a solid rocket engine.

The **Basic** fuel grain indicates that the engine throttles to 100% after burning a small portion of fuel, then fluctuates at around 90%, and finally throttles to 10% when fuel is almost drained.

An engine with the **Ramp Up** fuel grain operates at 40% power, then increases to 100% when there is 5% fuel left.

An engine with the **Steady** fuel grain operates at 100% power for the full duration.

An engine with the **Ramp Down** fuel grain starts at 100%, then reduces to 40% when there is 5% fuel left.

An engine with the **Impulse** fuel grain starts at 1000%, then throttles down to 10%, starting at 15% fuel and ending at 10% fuel. This damages the engine, partially for long duration burns.

### Power Cycle

*Main article: Power Cycle*
The power cycle is the method used for injecting the propellant. In actual gameplay, it simply changes how much fuel goes into the engine and adds pumps to the engine. The listed power cycles are more powerful in ascending order:

Pressure-Fed > Electric > Gas Generator I > Gas Generator II > Staged > Full Flow Staged

For Solid engines, the listed power cycles are more powerful in ascending order:

Amateur > Booster

### Chamber Pressure

Increasing the chamber pressure increases thrust and sea-level Isp, but increases mass and fuel consumption. It also makes the engine more sea-level efficient, which decreases Delta-V in a vacuum.

### Gimbal Range

The gimbal range determines how much the engine can move to help change attitude. This only applies to Nuclear Thermal and Liquid Engines.

### Fuel Type

*Main article: Fuel*

Selects different fuel types. Changing fuel can significantly alter performance.

In Liquid engines, kerolox, hydrolox, or methalox can be used, with lower efficiency but more energy density, high efficiency but lower energy density, or a compromise, respectively.

In Nuclear Thermal engines, liquid hydrogen or water can be used, either highly efficient but with low energy density or lower efficiency but higher energy density, respectively.

### Nozzle

There are many types of engine nozzles, and changing them can cause a difference in thrust. While some nozzle types are available for all combustion cycles, some are only available for some combustion cycles.

**Aerospike** is a unique nozzle style that looks like a rocket engine with the nozzle changed to a spike. It has no nozzle ratio, and increasing the nozzle length makes the spike longer. Aerospikes provide a relatively stable performance as the craft climbs through the atmosphere.

**Bell** is the basic and most common nozzle style. The nozzle is shaped like a bell, hence its name. It is used often. It's nozzle ratio can be changed between 1.69 and 148.84x.

**Cone** is a less common nozzle style. Very similar to Bell but the nozzle is not curved, with a nozzle ratio that can be changed from 1.59 and 117.51x.

**Bravo** is a unique nozzle style that runs on a closed gas generator cycle, allowing for more thrust and efficiency, with a changeable nozzle ratio between 9.00 and 36.00x.
**Delta** is another common nozzle style. It has a pipe that takes some of the combustion products and routes it to the nozzle. It likely represents a closed tap-off cycle, though tap-off cycles are open. It's nozzle ratio can be changed from 5.14 and 90.78x.

**Alpha** is a nozzle style that represents the RS-25 (Space Shuttle Main Engine). It has a nozzle ratio that can be changed between 13.96 and 54.76x. It is the most efficient nozzle.

**Omega** is a nozzle style that runs on a closed gas generator cycle, pumping a lot of gas-generator exhaust back into the engine which would otherwise be expelled, which is why is produces so much thrust. It represents the Rocketdyne F1, and it's nozzle ratio can be changed between 14.79 and 59.17x.

* The "Bell" nozzle
* "Cone" nozzle
* "Bravo" nozzle
* "Delta" nozzle
* "Alpha" nozzle
* "Omega" nozzle
* "Graphite" nozzle
* "Echo" nozzle
1/8**Graphite** is a nozzle style that has two nozzles, the outer, parallel nozzle, and the inner, cone-shaped nozzle, with a nozzle area ratio changeable between 5.06 and 20.25x.

**Echo** is a nozzle that has a nozzle ratio that can be changed between 3.76 - 15.03x. For solid engines, this nozzle is the most efficient and powerful.

### Nozzle Permissions

Certain nozzle styles can only be used on certain power cycles and engines.

 **Bell** | ✔
 **Cone** | ✔

### Nozzle Dimensions

Increasing the **Nozzle Throat Size** increases thrust at the cost of vacuum efficiency. It also adds some mass and price, and increases fuel consumption. Increasing the **Nozzle Length** increases efficiency (and somewhat increases thrust) in a vacuum, but can drastically reduce thrust and efficiency at sea-level, and adds mass and cost.

Typically, sea level optimized engines have high throat size and medium nozzle length, and vacuum optimized engines have low nozzle throat size and high nozzle length.

### Nozzle Area Ratio

The nozzle area ratio is how many times bigger the nozzle exit area is than the nozzle throat area. A rough formula to calculate area is A = 3.14 \* Radius². Higher nozzle ratios provide higher efficiency in a vacuum, but low efficiency at sea level. On the other hand, lower nozzle ratios provide higher sea level efficiency and less efficiency in a vacuum.

To change the nozzle ratio, change nozzle throat size and nozzle length, or select a different nozzle.

### Throttle Input

Changing this changes the slider used to change the engine's thrust.

## Real life

### Monopropellant

Real life monopropellant engines flow a strong reducing agent, usually hydrazine, over a catalyst, which heats and expands the agent, providing small thrust.

### Liquid

A fuel and oxidizer is pumped into a chamber and combusts. The exhaust is choked in a tight throat until it reaches supersonic speeds, before surging into an expanding nozzle to speed up.

### Nuclear Thermal

A liquid, without the oxidizer, is passed through a nuclear reactor. The reactor heat expands the liquid to hundreds of times it's volume, creating pressure in the combustion chamber, and then choking the exhaust and routing it into a widening nozzle to speed it up.

### Solid

A solid mix of fuel and oxidizer is placed into a tube, with a hole in the center. The material is ignited, and the exhaust is choked in a tight throat until it reaches supersonic speeds, before surging into an expanding nozzle to speed up. The style of the hole in the middle (the fuel grain) depicts the thrust curve, by changing the surface area of the propellant as the fuel burns. A simple cylinder hole will result in a ramping up thrust curve, while other grains exist, that may make the thrust steady or decrease over time.

## Default Rocket Engines

There are 11 default rocket engines that each demonstrate a certain type and power cycle. Each engine can be modified completely, for a thrust range from 1.33N to 75.2MN (sea level) or 17.4N to 77.6MN (vacuum), with corresponding Isp and Mass Flow.

* Gnome Engine
* Goblin Solid Motor
* Ifrit Solid Booster
* Phoenix Solid Booster
* Pixie Engine
* Mage Engine
* Titan Engine
* Apex Engine
* Wyvern Engine
* Dragon Engine
* Spirit Engine
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### Gnome Engine

The Gnome Engine demonstrates a Pressure-Fed liquid rocket engine, and has an extremely low overall thrust. It has a very poor sea level Isp which greatly improves in a vacuum. It uses kerolox.

### Goblin Solid Motor

The Goblin Solid Motor demonstrates an Amateur Solid rocket engine. It is a low thrust engine and has a low efficiency.

### Ifrit Solid Booster

The Ifrit Solid Booster demonstrates a Booster Solid rocket engine. It is a powerful engine with a low efficiency.

### Phoenix Solid Booster

The Phoenix Solid Booster demonstrates a Booster Solid rocket engine. It has very high thrust, but still suffers from poor efficiency (Isp).

### Pixie Engine

The Pixie Engine demonstrates a Electric Liquid rocket engine. It has a very fast throttle response, but still has low thrust, with a decent efficiency. It uses kerolox, but because it is electric, it needs electricity, which is stored in heavy batteries..

### Mage Engine

The Mage Engine demonstrates a Gas Generator I Liquid rocket engine. It has high thrust and mediocre Isp, at all altitudes. It uses kerolox.

### Titan Engine

The Titan Engine demonstrates a Staged Liquid rocket engine. It has high thrust and Isp, but is considerably more expensive. It uses hydrolox.

### Apex Engine

The Apex Engine demonstrates a Gas Generator II Liquid rocket engine. It has lower Isp, but very high thrust. It uses kerolox.

### Wyvern Engine

The Wyvern Engine demonstrates a Full-Flow Staged Liquid rocket engine. It has a very high thrust and Isp, but is extremely expensive per mass. It uses methalox.

### Dragon Engine

The Dragon Engine demonstrates a Nuclear Thermal rocket engine. It is a low-thrust, high efficiency engine, but has a very slow throttle response. It uses Liquid Hydrogen.

### Spirit Engine

The Spirit Engine demonstrates a Monopropellant rocket engine, one that uses the same fuel as RCS. It has a very low thrust accompanied with a medium efficiency. It is excellent for small probes.

## Special Properties

Rocket engines have special properties not attained by any other parts. These properties involve potential damage to the engine or other parts.

### Throttle Control

In accordance to the image on the right, the engine's max throttle can be increased, from 50...120% with the slider, or to any range by clicking on the percentage and typing a value which will be multiplied by 100 to get the throttle percentage. The throttle (or perhaps the slider the engine is throttled by) will remain at 0...1 or -1...1, but the throttling will be computed. However, running the engine at throttles greater than 100% will result in damage, which is faster as the throttle is far more higher than 100%.

### Heating Other Parts

Any parts where a certain percentage of them are in close proximity to a rocket engine's exhaust will rapidly heat to several thousand degrees Celsius, exceeding heat shield thresholds. Since this only occurs when more than a certain percentage is exposed to the exhaust, it is possible for a large part to be inside the exhaust but not heat up, especially since exhaust at sea level has a small area. This becomes increasingly possible in a vacuum, where the exhaust expands massively.

## Special Damages

Special from other parts, rocket engines can experience special damages that other parts cannot experience. All damages will progress until the part explodes or until the condition is removed.

### Overexpansion Damage

If Exit Pressure is more than 56kPa less than Air Pressure, the rocket engine will take damage when running, labeled "overexpansion damage." The greater the difference, the quicker the damage worsens.

### Overthrottling Damage

When throttling over 100%, either by throttle control or by using an Impulse Fuel Grain, the engine will experience damage. The rate of damage increases at throttles increasingly more than 100%.

## Trivia

* In the game, Solid fuel can be transmitted, and drain from top to bottom. In real life, Solid fuel is a rubber-like substance, and thus cannot be transferred. Also, Solid in real life burns from the center of the tank to the edges.
* Additionally, in real life, Solid engines produce thrust based on the length of the fuel tank, nozzle throat, and even the nozzle area, while making the fuel tank wider increases burn time. In the game, the thrust and burn time depend on simply the size of the fuel tank, along with the engine's grain and cycle. Again, the thrust depends on the surface area of the solid propellant, not the size of the fuel cylinder.
* The rocket engine and nozzles are all named after mythical or religious creatures, English vocabulary, or the Greek alphabet.