What is the best rocket engine?

Rocket engines - one of the heights of technological progress. Working at the limit of materials, hundreds of atmospheres, thousands of degrees and hundreds of tons of traction - this can not but admire. But a lot of different engines, which ones are the best? Whose engineers will climb the podium? Finally, the time has come to answer this question with all frankness.

Unfortunately, the appearance of the engine can not be said how great it is. We have to dig in the boring numbers of the characteristics of each engine. But there are many, which one to choose?

More powerful

Well, probably, the more powerful the engine, the better it is? More rocket, more payload, space exploration starts moving faster, isn't it? But if we look at the leader in this area, we will have some disappointment. The largest thrust from all engines, 1,400 tons, at the side accelerator of the Space Shuttle.


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Despite all the power, it is difficult to call solid fuel boosters a symbol of technical progress, because constructively they are just a steel (or composite, but it does not matter) cylinder with fuel. Secondly, these boosters became extinct with the shuttles in 2011, which undermines the impression of their success. Yes, those who follow the news about the new American super-heavy rocket SLS will tell me that new solid-fuel boosters are being developed for it, the thrust of which will be 1600 tons, but, first, this rocket will fly soon, not earlier than the end of 2018 . And secondly, the concept of “we take more segments with fuel so that the thrust is even more” is an extensive way of development, if you wish, you can put even more segments and get even more traction, the limit here has not yet been reached, and it is unnoticeable that led to technical excellence.

The second place is held by the domestic liquid engine RD-171M - 793 tons.


Four combustion chambers are one engine. And man for scale

It would seem - here it is, our hero. But, if this is the best engine, where is its success? Okay, the rocket "Energy" died under the rubble of the collapsed Soviet Union, and "Zenith" finished the policy of relations between Russia and Ukraine. But why the United States does not buy this wonderful engine from us, but half the size of the RD-180? Why does the RD-180, which began as a “half” of the RD-170, now produces more than half of the RD-170 thrust - as much as 416 tons? Strange. Unclear.

The third and fourth places are taken by engines with rockets that no longer fly. UA1207 solid fuel (714 tons), standing on Titan IV, and the star of the lunar program engine F-1 (679 tons) for some reason did not help to live up to today’s outstanding performance in power. Maybe some other parameter is more important?

More efficient

What indicator determines the efficiency of the engine? If a rocket engine burns fuel to accelerate a rocket, then the more efficiently it does, the less fuel we need to spend in order to reach the orbit / Moon / Mars / Alpha Centauri. In ballistics, there is a special parameter for evaluating such efficiency - the specific impulse.

The specific impulse shows how many seconds the engine can develop a thrust of 1 Newton on one kilogram of fuel.

Record holders are, at best, in the middle of the list if they are sorted by specific impulse, and F-1s with solid-fuel boosters are deep in the tail. It would seem that here it is, the most important characteristic. But look at the leaders of the list. With the indicator of 9620 seconds in the first place is a little-known electrojet engine HiPEP


This is not a fire in the microwave, but a real rocket engine. True, the microwave he still has a very distant relative ...

The HiPEP engine was developed for a closed probe project for studying the moons of Jupiter, and work on it was stopped in 2005. On tests of the prototype engine, as the official NASA report says , it developed a specific impulse of 9620 seconds, consuming 40 kW of energy.

The second and third places are taken by the not yet flown electric VASIMR engines (5000 seconds) and NEXT (4100 seconds), which showed their characteristics on test benches. And the engines flying in space (for example, the series of domestic engines SPD from the Fakel Design Bureau) have indicators of up to 3000 seconds.


Engines Series SPD. Who said “cool backlit speakers”?

Why have these engines not yet ousted the rest? The answer is simple, if we look at their other parameters. The thrust of electrojet engines is measured, alas, in grams, and they cannot work at all in the atmosphere. Therefore, it will not be possible to assemble an ultra-efficient launch vehicle on such engines. And in space, they require kilowatts of energy, which not all satellites can afford. Therefore, electric propulsion engines are mainly used only at interplanetary stations and geostationary communication satellites.

Well, well, the reader will say, we will reject electrojet engines. Who will be the record for the specific impulse among chemical engines?

With an indicator of 462 seconds, the leaders among chemical engines will be the domestic KVD1 and the American RL-10. And if KVD1 flew only six times in the composition of the Indian GSLV rocket, then the RL-10 is a successful and respected engine for the upper stages and upper stages, which has been working perfectly for many years. In theory, it is possible to assemble a carrier rocket entirely from such engines, but the thrust of one engine at 11 tons means that they will have to be put on the first and second stages in dozens, and there are no willing people to do so.

Is it possible to combine large traction and high specific impulse? Chemical engines have rested against the laws of our world (well, hydrogen with oxygen with a specific impulse does not burn more than ~ 460, physics forbids). There were atomic engine projects ( one , two ), but this has not gone further than projects. But, in general, if humanity can cross a high thrust with a high specific impulse, it will make space more accessible. Are there any other indicators by which you can evaluate the engine?

Tense

The rocket engine ejects mass (combustion products or working fluid), creating thrust. The higher the pressure in the combustion chamber, the greater the thrust and, mainly in the atmosphere, the specific impulse. An engine with a higher pressure in the combustion chamber will be more efficient than an engine with a low pressure on the same fuel. And if we sort the list of engines according to the pressure in the combustion chamber, the pedestal will be occupied by Russia / USSR - in our design school they tried their best to make efficient engines with high parameters. The first three places are taken by the family of oxygen-kerosene engines based on the RD-170: RD-191 (259 atm), RD-180 (258 atm), RD-171M (246 atm).


Combustion chamber RD-180 in the museum. Pay attention to the number of studs holding the lid of the combustion chamber, and the distance between them. You can clearly see how hard it is to keep aspiring to tear off the cover 258 atmospheres of pressure

The fourth place was occupied by the Soviet RD-0120 (216 atm), which holds primacy among hydrogen-oxygen engines and flew twice on the Energia launch vehicle. Fifth place, too, with our engine - RD-264 on the fuel pair asymmetrical dimethyl hydrazine / nitrogen tetroxide on the Dnepr LV operates with a pressure of 207 atm. And only in sixth place will be the American engine of the Space Shuttle RS-25 with two hundred and three atmospheres.

More reliable

No matter how promising the engine is, if it explodes a second time, there is little benefit from it. More recently, for example, Orbital was forced to abandon the use of the NK-33 engines that had been stored for decades with very high performance, because the accident on the test bench and the spectacularly beautiful night explosion of the Antares PH engine questioned the appropriateness of using these engines further. Now Antares will be transplanted to the Russian RD-181.


Large photo on the link

The reverse is also true - an engine that does not differ in outstanding values ​​of thrust or specific impulse, but is reliable, will be popular. The longer the history of the use of the engine, the more statistics, and the more bugs it managed to catch on the accidents that have already happened. The RD-107/108 engines on the Soyuz are based on the same engines that launched the first satellite and Gagarin, and, despite the upgrades, have fairly low parameters today. But the highest reliability in many ways pays for it.

More accessible

An engine that you cannot build or buy has no value for you. This parameter is not expressed in numbers, but it does not become less important. Private companies often can not buy ready-made engines at a high price, and are forced to make their own, albeit simpler. Despite the fact that they do not shine characteristics, these are the best engines for their developers. For example, the pressure in the combustion chamber of the Spacex engine Merlin-1D is only 95 atmospheres, the milestone that Soviet engineers crossed in the 1960s, and the United States in the 1980s. But Musk can make these engines at his production facilities and receive at cost in the right quantities, dozens per year, and that's cool.


Engine Merlin-1D. Exhaust from the gas generator as the "Atlas" sixty years ago, but it is available

TWR

Since we are talking about the Spacelines 'Merlines', one cannot but mention the characteristic that the SpaceX PR and SpaceX fans have in every way - thrust-to-weight ratio. Thrust-to-weight ratio (also specific thrust or TWR) is the ratio of engine thrust to its weight. According to this parameter, the Merlin engines are far ahead of them, they have over 150. On the SpaceX website they write that this makes the engine "the most efficient ever built," and this information is spread by PR people and fans across other resources. In English Wikipedia, there was even a quiet war, when this parameter was pushed wherever possible, which led to the fact that this column was completely removed in the engine comparison table . Alas, in such a statement much more PR than the truth. In pure form, the thrust-to-weight ratio of the engine can only be obtained on the stand, and when starting a real rocket, the engines will be less than a percent of its mass, and the difference in the mass of the engines will not affect anything. Despite the fact that an engine with a high TWR will be more technological than a low engine, it is rather a measure of technical simplicity and engine tension. For example, the F-1 (94) engine surpasses the RD-180 (78) in the parameter of thrust-capacity, but the specific impulse and pressure in the combustion chamber F-1 will be noticeably inferior. And to raise the thrust on the pedestal as the most important characteristic for a rocket engine, at least naive.

Price

This parameter is largely related to availability. If you make the engine yourself, then the cost can be quite calculated. If you buy, then this parameter will be specified explicitly. Unfortunately, this parameter does not build a beautiful table, because the cost price is known only to manufacturers, and the cost of selling the engine is also not always published. Time also influences the price; if in 2009 the RD-180 was estimated at $ 9 million, now it is estimated at $ 11-15 million.

Conclusion

As you have probably guessed, the introduction was written somewhat provocatively (sorry). In fact, rocket engines do not have one parameter by which they can be built and clearly say which one is the best. If you try to derive the best engine formula, you get something like the following:

The best rocket engine is such an engine that you can produce / buy , while it will have a load in the required range (not too big or small) and will be so effective ( specific impulse, pressure in the combustion chamber ) that its price will not become too heavy for you.

Is it boring But the closest to the truth.

And, in conclusion, a small hit parade of engines that I personally consider to be the best:


The RD-170/180/190 family . If you are from Russia or you can buy Russian engines and you need powerful engines on the first stage, then the RD-170/180/190 family will be an excellent option. Efficient, with high performance and excellent reliability statistics, these engines are at the forefront of technological progress.


Be-3 and RocketMotorTwo . The engines of private companies engaged in suborbital tourism will be in space for just a few minutes, but this does not prevent to admire the beauty of the used technical solutions. The BE-3 hydrogen engine, restarted and throttled in a wide range, with a load of up to 50 tons and an original circuit with an open phase transition, developed by a relatively small team is cool. As for the RocketMotorTwo, with all the skepticism in relation to Branson and SpaceShipTwo, I can not help but admire the beauty and simplicity of the scheme of a hybrid engine with solid fuel and gaseous oxidizer.

F-1 and J-2 In the 1960s, these were the most powerful engines in their classes. And it is impossible not to love the engines that gave us such beauty:



RD-107/108 . Paradoxically? Low parameters? Only 90 tons of traction? 60 atmospheres in the chamber? Drive turbopump from hydrogen peroxide, which is outdated by 70 years? It is all unimportant if the engine has the highest reliability, and at cost approaches “big silly carrier” . Yes, of course, someday his time will pass, but these engines will live at least ten years more, and it looks like they will set a record for longevity. It will not work to find a more successful engine with a more glorious history.

Used sources

• The material is largely based on this summary table from the English wiki, they are trying to give a link to each number and keep the material relevant.
• Full picture of the KDPV with copyrights, which had to be cut off when cropping - here .

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