Liquid Propellant Primer - Revision history

Lwcamp: /* Fuels and Oxidizers and Exhaust oh my */

2026-02-19T06:27:51Z

Fuels and Oxidizers and Exhaust oh my

← Older revision		Revision as of 22:27, 18 February 2026
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	An absolute conservation-of-energy-dictated upper bound on the exhaust velocity (and therefore specific impulse) of a chemical rocket propellant combination can be given by this simple equation:		An absolute conservation-of-energy-dictated upper bound on the exhaust velocity (and therefore specific impulse) of a chemical rocket propellant combination can be given by this simple equation:

	<~~math alt~~=>~~V_{~~eMAX}=~~\sqrt{2E_s}~~</~~math~~>		<table style="margin-top:0.5em; margin-bottom:0.5em; text-align: center;" align="center" cellpadding="0" cellspacing="0" >
			<tr>
			<td nowrap="nowrap" align="right"> <i>V<sub>eMAX</sub></i> = <span style="font-size: 150%;">√</span>
			<td style="border-top:solid 1px black;" nowrap="nowrap">  2 <i>E<sub>s</sub></i>
			</table>

	* where <~~math alt=~~>~~V_{~~eMAX}</~~math~~> is the exhaust velocity.		* where <i>V<sub>eMAX</sub></i> is the exhaust velocity.
	* where <~~math alt=~~>~~E_s~~</~~math~~> is the specific energy of the propellant mixture.		* where <i>E<sub>s</sub></i> is the specific energy of the propellant mixture.

	However, this is far from the whole game. To make good use of the propellant’s specific energy, the exhaust’s average molar mass must be as low as possible. This is because, assuming perfect expansion, the equation for a rocket’s exhaust velocity is:		However, this is far from the whole game. To make good use of the propellant’s specific energy, the exhaust’s average molar mass must be as low as possible. This is because, assuming perfect expansion, the equation for a rocket’s exhaust velocity is:

	<~~math alt~~=>~~V_{~~e}=~~\sqrt{\frac{2k}{~~k-~~1} \cdot \frac{~~R~~}{M} \cdot E_{~~s~~} \cdot C_{~~p}}</~~math~~>		<table style="margin-top:0.5em; margin-bottom:0.5em; text-align: center;" align="center" cellpadding="0" cellspacing="0" >
			<tr>
			<td rowspan=2 nowrap="nowrap" align="right"> <i>V<sub>e</sub></i> =
			<td rowspan=2 nowrap="nowrap" align="right"> <span style="font-size: 250%;">√</span>
			<td style="border-top:solid 1px black;" nowrap="nowrap">  2 <i>k</i>
			<td rowspan=2 style="border-top:solid 1px black;" nowrap="nowrap">
			<td style="border-top:solid 1px black;" nowrap="nowrap"> <i>R</i>
			<td rowspan=2 style="border-top:solid 1px black;" nowrap="nowrap">   <i>E<sub>s</sub></i> <i>C<sub>p</sub></i>
			<tr>
			<td style="border-top:solid 1px black;"> <i>k</i> - 1
			<td style="border-top:solid 1px black;"> <i>M</i>
			</table>

	* Where <~~math alt=~~>k</~~math~~> is the heat capacity ratio.		* Where <i>k</i> is the heat capacity ratio.
	* Where <~~math alt=~~>R</~~math~~> is the ideal gas constant.		* Where <i>R</i> is the ideal gas constant.
	* Where <~~math alt=~~>M</~~math~~> is the exhaust’s average molar mass.		* Where <i>M</i> is the exhaust’s average molar mass.
	* Where <~~math alt=~~>~~C_p~~</~~math~~> is the exhaust’s specific heat.		* Where <i>C<sub>p</sub></i> is the exhaust’s specific heat.

	Thus, broadly speaking to achieve higher exhaust velocities, one must seek a propellant combination with very high specific energies while at the same time avoiding ones with exhaust average molar masses that are too high. Matters of specific heat and heat capacity ratio of the exhaust additionally ensure that the business will never be boring and there will be gnarly choices to make.		Thus, broadly speaking to achieve higher exhaust velocities, one must seek a propellant combination with very high specific energies while at the same time avoiding ones with exhaust average molar masses that are too high. Matters of specific heat and heat capacity ratio of the exhaust additionally ensure that the business will never be boring and there will be gnarly choices to make.

Tshhmon at 21:42, 23 April 2024

2024-04-23T21:42:27Z

← Older revision		Revision as of 13:42, 23 April 2024
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	* To Tshhmon for minor edits		* To Tshhmon for minor edits

	[[Category:Propulsion]]		[[Category:Propulsion]][[Category:Transportation]][[Category:Transportation & Infrastructure]][[Category:Planet-Space Launch and Entry]]

Tshhmon at 16:16, 7 April 2024

2024-04-07T16:16:01Z

← Older revision		Revision as of 08:16, 7 April 2024
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	* To Tshhmon for minor edits		* To Tshhmon for minor edits

	~~[[Category:Spacecraft & Infrastructure‏‎]]~~[[Category:Propulsion]]		[[Category:Propulsion]]

Tshhmon at 19:19, 4 April 2024

2024-04-04T19:19:00Z

← Older revision		Revision as of 11:19, 4 April 2024
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	* To Tshhmon for minor edits		* To Tshhmon for minor edits

	[[Category:Spacecraft & ~~Infrastructure~~]][[Category:Propulsion]]		[[Category:Spacecraft & Infrastructure‏‎]][[Category:Propulsion]]

Tshhmon at 19:13, 4 April 2024

2024-04-04T19:13:53Z

← Older revision		Revision as of 11:13, 4 April 2024
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	* To Tshhmon for minor edits		* To Tshhmon for minor edits

	[[Category:Spacecraft & Infrastructure]]		[[Category:Spacecraft & Infrastructure]][[Category:Propulsion]]

Tshhmon at 16:36, 9 October 2023

2023-10-09T16:36:37Z

← Older revision		Revision as of 08:36, 9 October 2023
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	* To Rocketguy for minor edits		* To Rocketguy for minor edits
	* To Tshhmon for minor edits		* To Tshhmon for minor edits

			[[Category:Spacecraft & Infrastructure]]

Tshhmon at 18:31, 20 March 2022

2022-03-20T18:31:17Z

← Older revision		Revision as of 10:31, 20 March 2022
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	Nitrous Oxide is essentially a rather Rube Goldberg-ish way of bringing Oxygen to the combustion chamber, but it allows you to do away with cryogenic storage, paying for that with a sizable performance hit, risk of explosions, and lower density. All of this means that nowadays Nitrous Oxide is used mostly for hobby purposes.		Nitrous Oxide is essentially a rather Rube Goldberg-ish way of bringing Oxygen to the combustion chamber, but it allows you to do away with cryogenic storage, paying for that with a sizable performance hit, risk of explosions, and lower density. All of this means that nowadays Nitrous Oxide is used mostly for hobby purposes.

	Hydrogen Peroxide is, in the words of John Clark of Ignition! fame, the eternal bridesmaid. ~~Dense~~, storable, and ~~suffering not~~ too badly in performance in comparison with straight liquid Oxygen, it is ~~however~~ also a rather touchy chemical, requiring great attention to materials choice when building the engine and tanks and great cleanliness when maintaining and refueling them, if one wants to store good old ~~HTP~~ without explosions.		Hydrogen Peroxide is, in the words of John Clark of Ignition! fame, the eternal bridesmaid. It has a lot to recommend it, as it's dense, storable at room temperatures, and doesn't suffer too badly in performance in comparison with straight liquid Oxygen. However, this isn't the stuff you'd buy at the drug store. You can't just go buy a bottle of disinfectant and hope to launch your moon rocket - drugstore hydrogen peroxide is typically only a few percent actually Hydrogen Peroxide, with the rest being plain old water. The limits available for solvent use through many chemical suppliers are still only 50-60% Hydrogen Peroxide, and thus almost half water. Moon rockets need much higher purity, north of 80 or 90%. At these levels, it's known as "High-Test Peroxide" (HTP), which can cause burns to skin, set things on fire, and decompose spontaneously given a catalyst (useful for hypergolic performance, but troublesome for cleaning). This HTP is thus also a rather touchy chemical, requiring great attention to materials choice when building the engine and tanks and great cleanliness when maintaining and refueling them if one wants to store good old Peroxide without explosions.

	Nitric Acid and Dinitrogen Tetroxide are the oxidizers of choice to be employed with Hydrazines: relatively dense, giving performance pretty close to liquid Oxygen and easily storable, they are however nearly as noxious, if not more than the Hydrazines themselves.		Nitric Acid and Dinitrogen Tetroxide are the oxidizers of choice to be employed with Hydrazines: relatively dense, giving performance pretty close to liquid Oxygen and easily storable, they are however nearly as noxious, if not more than the Hydrazines themselves.

Tshhmon at 21:16, 19 March 2022

2022-03-19T21:16:51Z

← Older revision		Revision as of 13:16, 19 March 2022
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	The first part of a fuel/oxidizer combination is, of course, the fuel. God is merciful, and He has given us a bountiful amount of cool things that can be burnt in a rocket motor: first of all, we have the Hydrogen of great promise, then the many Hydrocarbons, the always-enchanting Hydrazines as well as a wealth of minor contenders, such as Alcohols and so on and so forth.		The first part of a fuel/oxidizer combination is, of course, the fuel. God is merciful, and He has given us a bountiful amount of cool things that can be burnt in a rocket motor: first of all, we have the Hydrogen of great promise, then the many Hydrocarbons, the always-enchanting Hydrazines as well as a wealth of minor contenders, such as Alcohols and so on and so forth.

	It must be, however, noted that certain propellants and oxidizers are liquid at different temperature ranges, some are liquid at room temperature, some are liquid at 200 degrees below freezing and are referred to as soft cryogens, and the hard cryogens are even colder with some liquid only barely above absolute zero.		It must be, however, noted that certain propellants and oxidizers are liquid at different temperature ranges, some are liquid at room temperature, some are liquid at 200 degrees Celsius below freezing and are referred to as soft cryogens, and the hard cryogens are even colder with some liquid only barely above absolute zero.

	Hydrogen boasts a very high specific energy with basically anything that can burn it, as well as such a low molecular mass that even a slight excess of hydrogen in the propellant mix will ensure very low molecular mass in the exhaust, but it suffers from a pitifully low density that gravely hurts the impulse density of any combination using it. It is also a hard cryogen, requiring very low temperatures for storage. However, it also has a tremendous heat capacity, making it very useful for any engines which need cooling or for using some kind of transpiration heating. Hydrogen is a true primadonna - it can give you much, but it also asks much in return.		Hydrogen boasts a very high specific energy with basically anything that can burn it, as well as such a low molecular mass that even a slight excess of hydrogen in the propellant mix will ensure very low molecular mass in the exhaust, but it suffers from a pitifully low density that gravely hurts the impulse density of any combination using it. It is also a hard cryogen, requiring very low temperatures for storage. However, it also has a tremendous heat capacity, making it very useful for any engines which need cooling or for using some kind of transpiration heating. Hydrogen is a true primadonna - it can give you much, but it also asks much in return.

Tshhmon at 21:13, 19 March 2022

2022-03-19T21:13:46Z

← Older revision		Revision as of 13:13, 19 March 2022
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	{\| class="wikitable sortable"		{\| class="wikitable sortable"
	\|-		\|-
	! Fuel !! Oxidizer !! O/F <ref group="T">Optimal oxidizer to fuel ratio</ref> !! Specific Gravity <ref group="T">Ratio of substance density to water's density.</ref> !! Specific Impulse (Sea Level Adapted Nozzle) <ref group="T">In units of seconds.</ref> !! Density Impulse (Sea Level Adapted Nozzle) <ref group="T">In units of g-s/cm<sup>3</sup>.</ref> !! Specific Impulse (Vacuum Level Adapted Nozzle) <ref group="T">In units of seconds.</ref> !! Density Impulse (Vacuum Level Adapted Nozzle) <ref group="T">In units of g-s/cm<sup>3</sup>.</ref>		! Fuel !! Oxidizer !! O/F <ref group="T">Optimal oxidizer to fuel ratio</ref> !! Specific Gravity <ref group="T">Ratio of substance density to water's density.</ref> !! Specific Impulse (Sea Level Adapted Nozzle) <ref group="T">In units of seconds (Isp/g<sub>0</sub>).</ref> !! Density Impulse (Sea Level Adapted Nozzle) <ref group="T">In units of g-s/cm<sup>3</sup>.</ref> !! Specific Impulse (Vacuum Level Adapted Nozzle) <ref group="T">In units of seconds (Isp/g<sub>0</sub>).</ref> !! Density Impulse (Vacuum Level Adapted Nozzle) <ref group="T">In units of g-s/cm<sup>3</sup>.</ref>
	\|-		\|-
	\| Hydrogen \|\| Oxygen \|\| 4.8 \|\| 0.33 \|\| 422 \|\| 139 \|\| 482 \|\| 159		\| Hydrogen \|\| Oxygen \|\| 4.8 \|\| 0.33 \|\| 422 \|\| 139 \|\| 482 \|\| 159

Tshhmon at 21:06, 19 March 2022

2022-03-19T21:06:53Z

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	~~{{PageConstructionNotice}}~~

	When somebody online asks the question “what’s the best liquid rocket propellant mix”, you are bound to hear a preposterous amount of b*llshit flying as everybody tries to champion their favorite mix, refusing to heed the opposition’s remarks.		When somebody online asks the question “what’s the best liquid rocket propellant mix”, you are bound to hear a preposterous amount of b*llshit flying as everybody tries to champion their favorite mix, refusing to heed the opposition’s remarks.

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	\|-		\|-
	\| UDMH \|\| Dinitrogen Tetroxide \|\| 2.7 \|\| 1.18 \|\| 314 \|\| 370 \|\| 366 \|\| 431		\| UDMH \|\| Dinitrogen Tetroxide \|\| 2.7 \|\| 1.18 \|\| 314 \|\| 370 \|\| 366 \|\| 431
	\|}
	\|}		\|}
	<references group="T"/>		<references group="T"/>