Rocket engines are far more power intense than jet engines. Jet engines operate at over the melting point of their components for years thanks to clever design.
The fuel is quite cheap compared to the rocket that burns it.
A 777 engine is actually far more fuel efficient than a rocket engine. Take a look at specific impulse [0] for different engine types. A rocket is optimised to be very light and to use its own oxygen supply. A normal jet engine can be heavier and use the air as an oxidizer. Much more efficient for power production in a CCGT.
An air-breathing jet engine doesn't need to carry oxidizer, which in a rocket is most of the propellant weight. It also has access to unlimited reaction mass, so it can be much more energy-efficient in producing thrust (it is more efficient to produce thrust by accelerating a lot of mass by a little, than by accelerating a little mass by a lot, but a rocket can't take advantage of this because it would need to carry all that extra mass. A plane can use ambient air for this purpose)
This all adds up to a plane needing to carry many times less mass to gain the same altitude and speed as a rocket, at least within relatively dense atmosphere.
Rocket engines are actually very efficient, especially in vacuum. Very high expansion ratios can be achieved that converts almost all the heat into jet kinetic energy.
Here's why Elon Musk is more likely to be right: because fuel is cheap.
The cost of fuel and the cost of fuel tanks is an insignificant part of the cost of an orbital launch, around the 1% level. The major drivers of cost are overall system complexity and manufacturing cost of the engines. And here's the big problem for a Skylon spaceplane, rockets are fairly simple systems whereas hypersonic airbreathing engines are extraordinarily complex and difficult. And if you can manage reusability on your launcher then the ordinary rocket engine wins hands down.
The reason why the jet engine won out over the propeller in civil aviation is not because of the higher thrust or better performance of the jet, it's because of lower operational costs. A jet powered aircraft requires less maintenance per passenger-mile than a propeller driven aircraft does. Partly this is because, despite the design complexities involved, a jet engine is actually a much simpler system.
The idea of not having to haul up a full load of oxidizer on an orbital launcher is a tempting one, but it doesn't come easy. One of the big advantages of a rocket is that it can push up above the bulk of the atmosphere when it's still traveling fairly slowly and do most of its accelerating in a near-vacuum. This reduces aerodynamic drag, aerodynamic heating, and dynamic pressure forces. All of which are some of the most pernicious problems to deal with in a launch vehicle. No few launch vehicles have been lost just as they reach "max-Q" (the moment of maximum dynamic pressure), and for an air breathing launcher it would likely be forced to fly through even more severe aerodynamic regimes than most rockets for significantly longer periods of time. This is hard on the vehicle design, hard on airframe longevity, hard on the thermal protection systems, and hard on the whole vehicle in general.
So on the one hand you have a vehicle which requires significantly more robust engineering and significantly more complex engines and overall design while probably having a shorter total service life. And is perhaps some significant factor riskier to fly in general. And on the other hand you have dead simple basically 60 year old engineering that is just put together sensibly, flown within a familiar flight envelope in a way that minimizes risk and iteratively improved to continuously shave off operating costs. It's a pretty safe bet which one is more likely to actually lead to lower launch costs.
Rocket engines are actually extraordinarily efficient. They're the most efficient heat engines we have. More than 90% of the thermal energy produced in the thrust chamber is converted to kinetic energy of the exhaust jet (especially in high expansion ratio engines). The fraction of chemical energy of the propellants of a launch vehicle that ends up in the kinetic energy of the final stage is surprisingly high (well, it was to me, initially.)
Rocket engines can produce gigawatts as jet power. Kerosene and liquid oxygen is a very dense energy source. A jet engine is slightly bigger as it needs to pump air which is not very dense, but it's still much less complex than a power plant that has to generate electricity.
Rocket engines are extremely efficient. Especially in vacuum, they can convert almost all the heat into kinetic energy of the exhaust jet. They're the most efficient heat engines we have.
Hydrogen yields higher efficiency (specific impulse) with lower thrust. Larger-molecule fuels, such as kerosene, have higher thrust and lower efficiency. When in the atmosphere, the rocket engines are spending a lot of energy counteracting gravity, so higher thrust is important. Once in orbit, efficiency is far more important (indeed, some deep space engines such as ion engines produce only millinewtons of force, but are very efficient.)
A jet engine can't have such power density because it uses gaseous air which is about 1000 times less dense than liquid oxygen and only contains 20% oxygen.
Everything follows from that.
The pumps are not challenging temperature wise since they pump cold liquids.
The turbine is challenging, but the temperature can be limited by varying the ratio of propellants in the preburner (very lean or very rich means lower temperature). If you use lean, then it's a very oxidizing environment. If you go very rich, there's soot (if you use fuels with carbon).
And the chamber is not so challenging because there is so much cool liquid available for cooling.
You can boil water with a candle and a paper cup.
A high performance jet engine is a harder problem than a medium performance rocket engine.
The energy output of a jumbo jet is four orders of magnitude less then that of a rocket. A jet engine is equivalent to a controlled fire. A rocket is equivalent to a controlled nuclear explosion.
There's almost as much energy stored in a BFR as there is in the nuclear bomb dropped on Hiroshima. This energy is released over 3 minutes.
> but why doesn't that also affect jet aeroplanes?
Jet engines pull in air and expel it out the back, creating thrust. The energy to do so comes from fuel, but almost all of the reaction mass is air.
Rockets don't have this luxury; they must bring all the reaction mass with them. This causes a big problem of diminishing returns. Adding more fuel means you can burn longer, but also makes the rocket heavier so it doesn't accelerate as much with the same thrust.
The result is that the fuel required goes up exponentially with the desired delta-v, as expressed by the rocket equation .
It's not about energy, it's about momentum and energy/weight ratio. Due to the energy density of chemical fuels, the vast majority of a space rocket's fuel is actually used to carry the _rest_ of the rocket's fuel to the altitude at which it will be burned. I'm not good with rocket physics, but I'm pretty sure that gaining a few kilometers of altitude and 1000 km/h of kinetic energy will have a large impact on the total amount of fuel/propellant required to reach orbit. Is someone in here good enough with rocket physics to comment on this?
Of course, it would be even better if they could go to business-jet altitudes or above (15000 meters instead of the 9000 quoted in the article), and I am guessing from the number and size of the engines that this is what they are actually aiming at. But this is just idle speculation on my part.
Many smaller engines is cheaper than one big one as you can streamline manufacturing. Also large engines develop combustion instability much easier. They're also harder to cool because of the square-cube law (more heat, less surface area).
It also gives you engine out capability and also the ability to effectively throttle the rocket in a much more fine grained way by shutting off engines.
The fuel is quite cheap compared to the rocket that burns it.
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