39
$\begingroup$

I have been talking with a friend of mine and the subject turned to modern rockets versus Apollo era rockets and fuel efficiency. My friend stated that today’s rockets use half the fuel of the rockets of those days, and can travel equal or further the distance. This didn’t seem right to me, but I do not understand what factors would go into comparing a rockets fuel efficiency ( He keeps citing better impulse as the key factor, and a change in fuel types ).

So I would like to know the following:

  1. Is this is true that today’s rockets use half the fuel they used to?
  2. What metrics would one compare to determine the fuel efficiency and how to compare them?

My apologies if this seems somewhat of a silly question, but after a lot of reading ( tbh a lot of it went over my head ) and googling I could not find a satisfactory answer.

Thank you.

$\endgroup$
14
  • 11
    $\begingroup$ I think you're missing an "h" in that title. But no, your friend is completely wrong. Efficiency has improved a few percent, not doubled. The second stage of the Saturn V had ISP of 421s, the best test models now can reach 451s(7% better). What has changed is new technologies of propulsion being used, like ion engines etc. $\endgroup$ Nov 11, 2021 at 5:02
  • $\begingroup$ Ahah yes, I was indeed missing an “ h “. I edited the title thank you. And thank you for your answer. $\endgroup$
    – BrandroidV
    Nov 11, 2021 at 5:19
  • 1
    $\begingroup$ Agree your friend is wrong about the efficiency gain, but also worth noting that you don't need any efficiency gain at all to go the same distance with less fuel - you just have to take a smaller payload (although cutting fuel by half is a stretch). $\endgroup$ Nov 11, 2021 at 17:52
  • 3
    $\begingroup$ It doesn't take that much improvement to get 50% fuel savings, due to the rocket equation exponential. $\endgroup$ Nov 11, 2021 at 22:25
  • 2
    $\begingroup$ @Panzercrisis: Even with the same fuel you can have more or less exhaust velocity and therefore ISP. And don’t forget engine design. For example open cycle engines just dump the turbopump exhaust while full-flow staged combustion cycle engines are almost perfect in extracting all energy. $\endgroup$
    – Michael
    Nov 12, 2021 at 7:51

6 Answers 6

58
$\begingroup$
  1. If this is true that today's rockets use half the fuel they used to?

No. One might imagine that 60+ years of development must have produced large gains, but chemical rocket performance is fundamentally limited by the amount of energy in the chemical fuels, and the 1960s engines were already getting at least 2/3 of the maximum theoretically possible performance (see comparison table below).

Now, ion thruster technology has advanced greatly, but those can't be used to reach orbit.

  1. What metrics would one compare to determine the fuel efficiency and how to compare them?

The usual primary metric is specific impulse.

Effective exhaust velocity

But specific impulse is a somewhat unintuitive quantity to understand, so let's start with effective exhaust velocity, which is the average speed of an exhaust particle (in the backward direction). For example, the Rocketdyne F-1 engines used in the first stage of the Saturn V (the Apollo rocket) have an effective exhaust velocity of 2.58 km/s at sea level.

What does 2.58 km/s mean in terms of rocket performance? It means if you build a rocket whose weight is about 63% fuel, and you fire the engine in deep space until the fuel runs out, the rocket will now be going 2.58 km/s faster in whatever direction it was pointing:

How v_e translates into delta v

What if the fuel is not 63% of the weight? Use the Tsiolkovsky rocket equation:

$$\Delta v = v_e \mathrm{ln} \frac{m_0}{m_f}$$

where $\Delta v$ is how much your velocity changes, $v_e$ is the effective exhaust velocity, $m_0$ is the initial mass of rocket plus fuel, and $m_f$ is the final mass of the empty rocket. I started with 63% $= \left( \frac{e-1}{e} \right)$ because then $\frac{m_0}{m_f}$ is $e$, whose natural log is 1, meaning $\Delta v = v_e$.

Notice that it doesn't matter how long the burn takes, nor the thrust of the engine, the final change in velocity is the same. That's the magic of the rocket equation!

So, what is change in velocity, $\Delta v$, good for? In the solar system there are two main uses for $\Delta v$: launching from the surface to achieve orbit, and transferring from one orbit to another. The article Delta-v budget has some examples, but the most relevant to Apollo is the $\Delta v$ to get into low Earth orbit from a sea level launch, which is (very roughly) around 10 km/s. That breaks down as about 8 km/s of required velocity to stay in orbit (any slower and you'll come back down) and 2 km/s spent lifting the rocket against gravity and pushing through the air on the way up.

The bottom line is, for any given mission, you need a certain amount of $\Delta v$. And while you can get more $\Delta v$ by increasing the proportion of fuel, that gets diminishing returns very quickly due to the natural log in the rocket equation. On the other hand, any increase in $v_e$ translates directly to a proportional increase in $\Delta v$, which means more mission without sacrificing payload.

Comparisons

So let's take a quick comparison of $v_e$ for the F-1 and the SpaceX Merlin engine. This is a relatively fair comparison because both burn RP-1 (refined kerosene) and liquid oxygen in a gas-generator cycle. These characteristics are good for a first stage due to high energy density per unit volume and high thrust, although other fuels have better $v_e$.

  F-1              2.58 km/s (sea level)
  Merlin           2.77 km/s (sea level)
  F-1              2.98 km/s (vacuum)     65% of max
  Merlin           3.05 km/s (vacuum)     66% of max
  Theoretical max  4.61 km/s (vacuum)

The theoretical maximum is based on the total chemical energy in the fuel.

I speculate that the better $v_e$ for the Merlin has more to do with its smaller size, thus making it easier to achieve stable, efficient combustion, than with technology improvements aimed at performance.

Specific impulse

Finally then, what is specific impulse? It's obtained from $v_e$ by dividing by the gravitational acceleration on Earth:

$$ I_{sp} = \frac{v_e}{g} $$

where $g$ is usually standard gravity, or about $9.81 \frac{m}{s^2}$. The resulting quantity has units of seconds. For example, for the F-1 at sea level, $I_{sp} = 263 s$.

What is the physical significance of $I_{sp}$? Well, consider our rocket from before with 63% fuel by mass. Suppose we start the rocket while it is sitting on the pad, let it just barely lift off, then hover just off the pad until it runs out of fuel (this assumes we can arbitrarily throttle the engine without affecting its performance, which is not realistic, but ignore that). $I_{sp}$ is how long it will hover. That is because, for every second of hovering, we consume 9.81 m/s of $\Delta v$ in order to overcome gravitational acceleration accumulated during that second. After $I_{sp}$ seconds, all of our $\Delta v$ is gone.

Fuel types

The question mentioned fuel types. This answer is already too long, but I'll just briefly mention that different fuel types do have different performance characteristics, but they also come with other tradeoffs, and which is best is highly dependent on the mission objectives. For example, the Saturn V used RP-1/LOX in its first stage for high thrust and energy density per volume, but LH2/LOX in its second and third stages for better energy density per unit mass and $v_e$, while the Apollo spacecraft (command/service and lunar modules) used hypergolics for reliability and storability.

$\endgroup$
5
  • $\begingroup$ Kudos, this is a great answer! $\endgroup$
    – Alex B
    Nov 11, 2021 at 21:45
  • $\begingroup$ Thank you for your well thought out explanation, it really helped me have a better understanding of how specific impulse comes into play. $\endgroup$
    – BrandroidV
    Nov 12, 2021 at 0:42
  • $\begingroup$ Fuel is also extremely complex. There are most likely more dense and more powerful fuel available, but they have really bad tradeoffs like being extremely toxic, creates a lot of toxic byproducts, and so forth, and all these turns into massive $$$ to deal with and process/store/transport, not to mention increased risk of killing people and environmental catastrophe if an accident happens. $\endgroup$
    – Nelson
    Nov 12, 2021 at 4:38
  • 2
    $\begingroup$ To be complete, thrust does matter greatly when getting off the planet. For one you need a thrust to weight ratio grater than 1, which is where ion drives fail. Higher trust is better because you spend less time fighting gravity; if you want to look it up the term for this is "gravity losses". That said, wikipedia gives typical gravity loss at 2km/s in addition to the 8km/s needed to escape Earth gravity, so with infinite thrust you'd only gain like 20% efficiency. $\endgroup$
    – csiz
    Nov 13, 2021 at 1:45
  • $\begingroup$ @csiz: There's also a tradeoff in terms of needing sturdier rockets to handle higher G loads, increasing the non-fuel part of the rocket mass. Not to mention payload considerations, especially for crewed flight, of substantially higher accelerations. $\endgroup$ Nov 13, 2021 at 8:43
54
$\begingroup$

My friend stated that today’s rockets use half the fuel of the rockets of those days, and can travel equal or further the distance.

Your friend doesn't know what they are talking about. That is nonsense. That's not saying it right. It is utter nonsense.

What SpaceX and other new space companies have done is to focus on massively reducing cost rather than slightly improving performance. If reducing cost by a lot means reducing performance by a bit, so be it. If reducing cost by a whole lot means not having a supply chain that involves all 50 states and the District of Columbia, so be it. Old space companies liked those ridiculous supply chains because it made Congress happy. If reducing cost by a whole, whole lot makes Congress a bit less happy, so be it.

What SpaceX has done is to adapt late 20th century / early 21st century concepts to the process of designing, building, and flying rockets. Their engines are no more efficient than were the rockets of the 1960s. Their design and development processes, manufacturing processes, and operational processes are extremely less costly than were those of 1960s era space flight companies.

$\endgroup$
18
  • 19
    $\begingroup$ Ultimately, SpaceX has taken advantage of the fact that it's not hampered by legacy military/industrial complex rules written by lobbyists and rubber-stamped by Congress. $\endgroup$ Nov 11, 2021 at 15:09
  • 24
    $\begingroup$ Or a bit more provocative, SpaceX is showing why capitalism beats communism in the space race. $\endgroup$
    – MSalters
    Nov 11, 2021 at 15:13
  • 5
    $\begingroup$ @MSalters given that the actual work done during the space race was performed by private, capitalist, profit-seeking enterprises, I fail to see how you can brand rent-seeking behavior as a "failure of communism". In reality, it is proving that capitalism works phenomenally when paired with gov't capture. $\endgroup$ Nov 11, 2021 at 22:43
  • 14
    $\begingroup$ @MSalters it proves that a coordinated effort for a particular goal is more effective than a committee whose primary motivation is not the completion of the goal. Given that the soviets put a man in space first with an economy 1/3 the size of the US I don't think Capitalism itself is automatically the winner here. $\endgroup$
    – Turksarama
    Nov 11, 2021 at 22:51
  • 30
    $\begingroup$ @MSalters SpaceX would not be where it is if it wasn't for the large investments NASA made in SpaceX. NASA invested in SpaceX before it had made a single successful flight, continued investing as it showed success, and then invested even more without SpaceX having shown an ability to carry humans into space. SpaceX is not a shining example of capitalism. It is a shining example of a public-private partnership done right. $\endgroup$ Nov 11, 2021 at 23:35
10
$\begingroup$

David Hammen's answer is right, but I would also add that none of today's rockets have the payload capacity of the Saturn V. That's why they use half the fuel, because they are not launching humans to the moon.(With all the support equipment that requires.) Starship Heavy and New Glen are tomorrow's rockets, and they are every bit as large or larger as Saturn V. And they will use even more fuel than Apollo did. But by reusing the booster and other parts, as well as other efficiencies, they can substantially reduce price.

$\endgroup$
3
  • 3
    $\begingroup$ The OP talked about a particular performance index: fuel efficiency (specific impulse) think you are mixing in a number performance indexes: thrust and cost. This is akin to comparing cars using weight capacity and/or cost when the subject of discussion is mileage. $\endgroup$
    – Ng Ph
    Nov 11, 2021 at 17:11
  • 4
    $\begingroup$ @NgPh OP never said "specific impulse" OP said, "half the fuel...[to go]...equal or further the distance." That demonstrates total naiveté with regard to the costs of space exploration. Nobody who knows what they're talking about ever asks how many miles per gallon a launch system gets. Brianorca's answer (somewhat) and David Hammen's answer (moreso) talk about the important costs. $\endgroup$ Nov 13, 2021 at 2:42
  • 1
    $\begingroup$ @Solomon Slow, the OP also expressed the following: "I do not understand what factors would go into comparing a rockets fuel efficiency (He keeps citing better impulse as the key factor, and a change in fuel types )". I don't understand your "absolute naiveté" nor David Hammen's "utter nonsense". There are many other ways to enlighten someone that progress in business efficiency is more prominent than in fuel efficiency. Besides, the OP didn't claim that the OP cited friend faithfully. And IMO there is nothing wrong about asking questions on subjects you do not master. $\endgroup$
    – Ng Ph
    Nov 13, 2021 at 11:20
9
$\begingroup$

The J-2 engine of the Saturn V 2nd and 3rd stage had two successors, the J-2X and the HG-3 engines. All three engines used LH2/LOX and were designed for vacuum.

The Shuttle engine RS-25 used the same propellants and there is vacuum data too.

The RL-10B-2 is still used for the Delta III and IV rockets. LH2/LOX is used and there is a specific impulse for vacuum.

So we may compare the specific impulses and the percentage of the theoretical maximum which is 532.5 s for LH2/LOX:

type  impulse              dev. start   first flight
J-2   421   s    79.06 %   June  1960   Feb   1966
J-2X  448   s    84.13 %   July  2007
HG-3  451   s    84.69 %
RS-25 452.3 s    84.94 %   about 1970   April 1981
RL-10 465.5 s    87.42 %                      1998

So there is an improvement. But the HG-3 was cancelled and never flew.

$\endgroup$
1
  • $\begingroup$ The RS-25 doesn't really fit in this comparison, because it didn't have a truely vacuum-optimised nozzle, else it would beat the others by quite a bit thanks to its staged combustion cycle. $\endgroup$ Nov 13, 2021 at 0:19
5
$\begingroup$

Specific impulse is indeed the main way that rocket engine efficiency is compared, as a rocket has to lift its own fuel.

There's a table on the wiki page, but it also includes lots of non-rocket engines, and doesn't include any SpaceX engines. So here's a few I've collected for you:

Saturn5 1st stage: 263s
Saturn5 2nd and 3rd stage: 421s
Space Shuttle RS-25: 453s
Raptor: 365–380s

Higher numbers are better, so no, it hasn't improved much.

$\endgroup$
9
  • 9
    $\begingroup$ You should not compare apples with oranges. Saturn V 1st stage engine was using RP1/LOX, the 2nd and 3rd stage engine used LH2/LOX. The Shuttle engine used LH2/LOX too. The raptor engine uses LCH4/LOX. You can't tell from this numbers if there was an improvement at all for the Raptor. The Shuttle engine was an improvement compared with Saturn V 2nd and 3rd. $\endgroup$
    – Uwe
    Nov 11, 2021 at 15:30
  • 1
    $\begingroup$ Wouldn’t it make more sense to compare payload weight vs. overall weight? Or payload vs. fuel weight? $\endgroup$
    – Michael
    Nov 11, 2021 at 16:35
  • 4
    $\begingroup$ Also, the RS-25 was 453s in vacuum. At launch on the pad its ISP is only 366s and it's paired there with solid boosters making 60% of the vehicle launch thrust at only 242s ISP. $\endgroup$
    – J...
    Nov 11, 2021 at 17:03
  • 2
    $\begingroup$ Saturn V 1st stage specific impulse is given for sea level, 2nd and 3rd stage engines for vacuum. $\endgroup$
    – Uwe
    Nov 11, 2021 at 18:36
  • 2
    $\begingroup$ @Uwe the question was whether fuel efficiency in rockets has increased overall since Apollo, not whether it improved for RP1 specifically(it has btw, the RD180 gets 310s). If SpaceX has mastered using a better fuel and can get 360+s in Raptor, that counts. And going from 260s->360s. That's a 39% improvement, which is very solid. For reference, commercial jet engines improved their efficiency by 45% over the same period: theicct.org/publications/… $\endgroup$
    – Eugene
    Nov 11, 2021 at 23:58
4
$\begingroup$
  1. Is this is true that today’s rockets use half the fuel they used to?

No, but cars, trucks trains and airplanes do. And for everyone but those on the rocket, that's a much bigger net gain, because we use all those a heck of a lot more than we use rockets.

The simple fact is there isn't a lot of gain to be made in rockets. It's like if you have a common "80%" gas furnace, you're not going to double its fuel efficiency. Besides, inefficiency causes heat, and that heat would need to be ejected somehow, or else equipment will overheat. On a rocket in space, there is nowhere for heat to go except out the nozzle (or other exhaust pipe).

On that point, there is one notable gain in how they power the fuel and oxidizer pumps. Historically, the pumps were powered by a gas generator which consumed fuel and oxidizer, and threw its hot exhaust overboard. Even worse, stochiometric (perfect ratio) combustion would make the turbine too hot, so they add considerable mass of extra fuel just to cool the airstream down! All utterly wasted - a) lost heat energy, b) loss of reaction mass since rockets work by ejecting mass (their exhaust) at very high velocity, and c) loss of fuel value from that "cooling" fuel.

enter image description here

Rolling coal: The black smoke is the very rich "gas generator" exhaust. source

Meanwhile, cold/liquified fuel and oxidizer need to be turn from liquid to vapor in order to burn in the engine. This latent heat of vaporization was "stolen" from engine thrust. (fuel is pre-heated in the engine nozzle jacket, but nothing pre-heated oxidizer.)

This isn't anywhere near 50%, but it's the biggest opportunity for efficiency gains available.

Today, the staged combustion cycle has been mainstreamed. This is where some fuel is combined with all the oxidizer (or vice versa), used to power the turbo pump, and fed into the main rocket engine. Thus the formerly discarded exhaust heat is providing some latent heat of vaporization to the oxidizer (fuel).

The trade-off is you need much better metallurgy tech in the turbine metals (that's why gas generators used excess fuel instead of oxidizer; the latter is much more corrosive). But the pump seals do not need to be perfect - small internal leaks along the rotating shaft between the raw oxidizer pump and the oxidizer-rich turbine cause no trouble at all except for a microscopic loss of pump efficiency.

However it still needs a perfect seal between oxidizer and fuel! SpaceX gets rid of that, with a complete second set of burners and turbines that run fuel-rich, and use that to pump the fuel. Again, minor leaks between fuel-rich exhaust and raw fuel are no great concern.

So to answer your question, going to staged combustion has been an efficiency bump for rocket engines, yes - though certainly not anything like 50%.

and a change in fuel types

Energy per mass of fuel is not the same from fuel to fuel.

Indeed, liquid hydrogen is "top of the heap". But you don't see a lot of designs using it. That's because it's fairly difficult to work with, so they willingly pay the efficiency cost to use a more manageable fuel like RP-1 (kerosene).

$\endgroup$
2
  • $\begingroup$ " an obscure Soviet technology" The 1970s era Space Shuttle Main Engine was a staged combustion engine. Maybe you're really talking about O2 rich staged combustion? Or full flow staged combustion? The SSME was neither of those. $\endgroup$ Nov 13, 2021 at 23:04
  • 1
    $\begingroup$ @OrganicMarble Good point, removed. $\endgroup$ Nov 13, 2021 at 23:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.