Isn't that our biggest challenge right now? I like how dismissive you are of it, like we have somehow overcome that and are now on our way to Mars any day now.

If someone was able to reduce the cost of going into orbit by as little as 10% that would be a MASSIVE achievement. Getting out of the atmosphere is still a massively costly, logistical, and dangerous challenge.

Let us take Elon at his word that increasing the size of the booster stage by 5-10% can replace these engines entirely, and assume that it actually is 10%. The booster stage is mostly fuel, which varies as volume, but the costs are all in the metal part, which scales as area. So this size increase costs you at most 6.6%. It actually costs less because the complicated rocket thrusters are left unchanged. So let's say 5%.

But we're not reducing the cost of the whole rocket by 5%, just the booster stage. If we assume that the booster stage is, say, 70% of the rocket we're only saving a maximum of 3.5% in reductions on the existing rocket components.

But we haven't yet factored in the cost of the new component. Which is, after all, very much like the existing engines except more complicated. In a Falcon 9 we have 9 rockets, so each one is about 11% of the cost of the total. If we strap on 3 of these new things, which are much like rockets except more complicated because of the whole hybrid thing, they each can cost a maximum of about 1.2% of the total cost to stay within our budget.

So unless these things, which are more complicated than rockets, cost you 1/10th as much, you don't get any cost saving at all. It is difficult to see how they can be this cheap.

Meanwhile, back in reality, Elon has plans to reduce the cost to orbit by another factor of 10 in a similar time frame to when these might actually become commercially available. (He's already reduced launch costs by a factor of 10.)

A large part of how he's managed to reduce costs so much is that he does this kind of common sense analysis in his sleep, and uses it to ignore everything he has to ignore, and zero in on what he needs to pay attention to. The numbers on this technology simply don't add up for space.

(But they don't have to. A practical technology to double the speed of jets by a factor of 2 will be extremely interesting to various militaries. Plus the potential savings for the existing airline industry - which is a much, much bigger market than space right now - means that they are going to have no shortage of potential customers. Elon is simply not one of them.)

Also, you often can't just simply stretch tanks - you need to increase thrust. Otherwise your payload drops because of lower T/W and more gravity losses in early flight.

The weight of a full tank, yes.

The weight of an empty tank, no. An empty tank is mostly a shell, and the size of that shell corresponds to area.

Also, you often can't just simply stretch tanks - you need to increase thrust. Otherwise your payload drops because of lower T/W and more gravity losses in early flight.

I am assuming that Elon Musk's 5-10% estimate takes things like this into account.

Incidentally "early flight" in this case is very early. At the ground, oxygen levels are a bit over 20%. But as you go up, oxygen drops off faster than nitrogen, so oxygen intake falls off slower than drag. At some point you'll gain nothing. I do not know what that point is, but the oxygen/nitrogen level is part of why it is most efficient for commercial airlines to fly at around 9 km high. So it is really just a few km that you get a potential benefit. But your top speed at that moment is a pretty small fraction of what you need to get to orbit.

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.

You're also underestimating how hard rockets work while still in the atmosphere. For example, the shuttles SRB work entirely within the troposphere and stratosphere. They're about a million pounds of propellant each, and together they make up 70% of the shuttles lift off weight. If you eliminated the need for the oxidizer in the SRBs, you'd save nearly half the entire weight of the shuttle. Because of the non-linearity of the rocket equation, saving weight produces compounding advantage, so this would be huge.

The key goal of an orbital launch vehicle is generating the necessary speed for orbit (over 8,000 m/s, around mach 25). The difficulty of reaching the altitude of low Earth orbit is inconsequential in comparison. A rocket has the advantage that it can do its accelerating wherever it's more convenient, so the typical flight profile is first up and then over, because it's a hell of a lot easier to accelerate and travel at high speeds above most of the atmosphere. For example, the Falcon 9 reaches an altitude of 5km before it even goes supersonic, and will reach an altitude of 30km within the first 2 minutes of launch.

An airbreathing engine however needs to stick around in dense enough atmosphere for its engines to work. And if a vehicle relies on a significant amount of airbreathing then it needs to spend a significant amount of time in that denser atmosphere. And that means that it needs to do more of its accelerating in denser air, which means that it will encounter higher aerodynamic forces, higher drag, more heat issues, a higher max-Q, etc. Those sorts of forces tend to be the "long poles" that aerospace vehicles are designed around, it dictates everything from the materials used to the type of construction to the service life of the vehicle's frame, etc. This is something that positively cannot be avoided for an airbreathing vehicle.

Sure, the SRBs generate a ton of thrust on the Shuttle, but they also help push the Shuttle quickly to higher altitudes and lower air pressure. Before the Shuttle hits mach 2.5 (of 25) it is already at an altitude where atmospheric pressure is 1% of sea level.

As I said before, mass isn't the big driver of cost in orbital launch vehicles, cost comes from complexity which comes from operational complexity (flight profile, staging, etc.) and design complexity (engines, control systems, handling, etc.) A vehicle which saves fuel but increases operational complexity is not a cheap vehicle. Fuel costs around $1,000 a tonne, whereas an engine can easily cost $10,000 / kg.

The biggest win that a vehicle like Skylon would have initially is that it might make it easier to make reusable launchers. If that's the case then even an expensive launcher which can be reused only a handful of times might still be useful in reducing overall launch costs. But if an entirely rocket based vehicle can be made to be reusable then it's very unlikely to have better overall economics or operating characteristics, for all of the reasons I've listed previously.

The key point seems to be the complexity penalty of adding airbreathing to the engine vs the weight savings of less reaction mass. If we're comparing reusable apples to apples, this is really the value proposition. I'm clearly more optimistic on this point.

Also any engine that uses ram effect becomes more efficient at higher speeds. The SR71 uses less fuel per unit of distance the faster it goes, which is a bit counterintuitive. How big a benefit this is for space launch I can't really guesstimate but it's probably minor.

Reaction mass savings means more than just oxidizer material cost though. It ripples through the whole design. There aren't many times when the mass fraction of a rocket is working for you instead of against you.

I think we skipped over that a horizontal takeoff requires a lot less launch infrastructure. But being smart with rockets and launching from a barge in the ocean can equalize things.

As a summary, I think you and Elon may be right about Skylon for space launch. Mass produced rockets can get pretty cheap, and SpaceX does aspire to full reusability.

But space launch is only one of the two applications of a design like Skylon. Nothing SpaceX develops will be used for terrestrial transport. You aren't going to take a rocket to visit your family for the holidays, so Skylon may find a market there.

Skylon also could be used as a WhiteKnight style carrier for a more traditional second stage, which might still be interesting for space launch, but I'm pessimistic on this point because I think if the numbers worked the air force would already be using such systems instead of Deltas.

Skylon can also hedge that their high flow flash chiller is useful in other applications, and apparently they've developed an interesting high temperature composite material.

So on the whole I think it's interesting to watch what happens to them, even if it's not a sure bet.

Jet engines are just really really fast propellers. They don't need to carry their own reaction mass like a rocket. This gaves them a completely different efficiency profile (no reaction mass, means less weight, which means less fuel, which means less weight, rinse/repeat to the limit). This is the reason that 747s use jet engines rather than rockets. Being able to use both technologies fluidly within their optimal atmospheric envelopes would be a major breakthrough and would completely change access to space.

tl;dr Skylon can use a small, efficient jet engine to get high and fast enough that air resistance stops mattering and while slowly pivoting to a conventional rocket (which is optimal when the air gets thin).

Note that much of the energy spent for orbital flight is not spend getting height, but spent getting speed. Efficiency is greatly increased if you can reach high speeds without having to carry the reaction mass to reach those speeds.

LEO requires about 7.8 km/s, Skylon's jet engines can go around 1.7 km/s. You are reaching 20% of your orbital velocity without reaction mass.

http://en.wikipedia.org/wiki/SABRE_(rocket_engine)#Advantage...

The big savings, like spacex's grasshopper project, is of course a reusable space vehicle. The advantage to spacex's approach is that Skylon is a single stage to orbit space plane (you don't need to land a bunch of stages using retro-rockets).

If I was placing a bet, I would bet on spacex since they have a proven track record (rockets are easy, organizing/funding rocket companies is hard). Skylon is a great idea though and it is the general direction that aircraft engine design is headed (a peak at the future).

Unfortunately, kinetic energy scales as speed squared, so 20% of your orbital speed represents less than 5% of your orbital kinetic energy. To put this in perspective, the difference in gravitational potential energy between LEO and the earth's surface represents about 15% of your total on-orbit energy.

Now, it's true that an air-breathing engine doesn't need to carry reaction mass (and, sometimes, oxidizer), but the air engines add considerable complexity (which is always a bad thing) as well as weight (because you still need to carry a conventional rocket to finish orbit insertion). So what you need to do is ask how the weight penalty of the air engine compares to the weight penalty of carrying extra fuel in a conventional rocket (bearing in mind, of course, that there a pernicious positive feedback loops when scaling a booster).

Fortunately this is counterbalanced by the Oberth effect. Getting to 20% of your orbital velocity requires expending 20% of your rocket's delta-V. And since delta-V is logarithmic in your propellant mass (rocket equation) that could easily translate to needing half as much fuel.

Well, that doesn't really address my point, which was that you need to compare the weight of the hybrid engine to the weight of the extra fuel. The first problem is that an air-breathing engine is going to be something like 3 to 5% of the initial mass, and you have to carry it with you to orbit[1]. The second problem is how the fuel scales:

v_hybrid = 0.8 * v_conventional

Assume both have similar engines:

ln(m_hybrid-initial/ m_hybrid-final) = ln((m_conventonal-initial / m_conventional-final)^0.8)

m_hybrid-intial = (m_conventional-intial / m_conventional-final)^0.8 * m_hybrid-final

let delta equal the expression in parenthesis

m_h-i = delta^0.8 * (m_payload + m_engine) = delta^0.8 * (m_payload + m_h-i * 0.05)

m_h-i * (1 - 0.05 * delta^0.8) = delta^0.8 * m_payload

So the fuel load in a hybrid is going to be:

m_final = delta^(1-0.2) / (1 - 0.05delta^(1-0.2)) m_payload,

The factor in the denominator is what really kills you, and the hybrid is only going to give you a net benefit for deltas less than about 15. So, not only is there not a factor of 2 fuel savings, there isn't any fuel savings at all! Even if you assume an engine weight of only 3% of initial mass, the benefit is only for delta < 35, which is better than just about all actual (as opposed to paper) launchers. By the way, the Shuttle had a delta of about 85-90 for LEO precisely because its designers made the decision to bring wings (which we neglected above) along for the ride to orbit. That also contributed to the 1 in 50 accident rate of that launch system.

And none of this addresses the fact that you are optimizing the f*ck out of one of the least expensive components of launch cost by introducing all sorts of unnecessary complexity.

[1] Ok, I suppose you don't, but then you have to have some way of recovering it, and that adds an enormous amount of complexity to the system.

As I understand it, construction of the rocket is the most expensive part of a launch system. The point of skylon is to create a reusable single stage to orbit space plane. Shouldn't skylon's reusability make it "optimize the fck" out of one of the most expensive components?

His objection isn't based on technology though. It's based on physically enveloping the problem and comparing the two cases.

His assumptions seem to be: LOX will remain cheap. The atmosphere will still have 21% oxygen. Your technology can't separate the N2 from the O2 before ingestion.

Elon says that the braking effect of ingesting that nitrogen vastly overwhelms the small benefit of having a smaller O2 tank. That's it.

This argument doesn't rely on the air-breathing stuff being more complicated or expensive or heavy (though it will be). Technology might fix those problems, but that simple kinematic fact is enough.

The planned SABRE engine can act as a rocket as well closing the intake and using stored liquid oxygen so it would be capable of operating without an atmosphere and oxygen and landing on Mars.