Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

British aerospace company claims biggest engine advance since the jet

uk.reuters.com

Post reply on HN
Print view View original

101–109 of 109 posts

Re: British aerospace company claims biggest engine advance since the jet

#101

Post by lmm »

bathat wrote: post #90
EthanHeilman wrote: post #73

Earlier quoted context omitted.

According to this link ( http://www.g2mil.com/high.htm ), air friction is not completely trivial. Additionally rocket engines are more efficient in less dense atmosphere "because the thinner air allows a better plume". They estimate that launching from 30,000 feet provides 9.3% greater thrust. Note that much of the energy spent for orbital flight is not spend getting height, but spent getting speed. Efficiency is gre…

>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. 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…

>Unfortunately, kinetic energy scales as speed squared, so 20% of your orbital speed represents less than 5% of your orbital kinetic energy.

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.

Re: British aerospace company claims biggest engine advance since the jet

#102

Post by primitur »

anigbrowl wrote: post #92
loceng wrote: post #19

Earlier quoted context omitted.

My first thought was a metamaterial.

Something like this, I suspect: http://www.sharklet.com/technology/

I think you are right, its probably some metamaterial like shark skin, where the surface of the exchanger itself is designed to prevent frost crystallization from occurring .. i.e. the metamaterial is geared towards the properties of water at the stage where it goes from fluidic to crystal .. I forget the term .. so the structure of the surface of the exchanger is formed such as to demote crystallization long enough for the pressure forces to remove the liquid hydrogen ..

Re: British aerospace company claims biggest engine advance since the jet

#103

Post by btilly »

Gravityloss wrote: post #100
btilly wrote: post #30

Earlier quoted context omitted.

But you don't reduce the cost of going into orbit by 10%. Not even in the absurdly best case. 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…

A tank's weight is proportional to volume, not surface area, assuming equal pressure and material strength. 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.

A tank's weight is proportional to volume, not surface area, assuming equal pressure and material strength.

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.

Re: British aerospace company claims biggest engine advance since the jet

#104

Post by hkhenson »

Someone asked on another list for an explanation of the press release. This is my try.

Hypersonic engines are up against hard physics. The ram air heats so much in the inlet that it's hard for combustion to add much energy to make it go faster out the back.

The idea behind the SABRE engines is to cool the ram air before it is compressed. The heat exchanger to do this is what the press release is all about. With not much more than a ton of mass, it sucks 400 MW of heat out of the incoming air, dropping the temperature from 1500 C to -150 C in a few inches of heat exchanger that looks much like fabric because the tubes are so tiny.

The engine cycle also uses the temperature difference between the ram air and the LH2 to run the compressor. It takes close to 2/5th of the energy from burning hydrogen to liquefy it. The engines recover much of this by running a helium turbine on the temperature difference between the ram air and the liquid hydrogen flow to the engines. The turbine powers the compressor stage that raises the pressure of the -150 C air to rocket chamber pressure.

The design is extremely clever thermodynamics which also avoids most of the metallurgical problems of high temperature. Fabricating the air to helium heat exchanger was a very hard task. They have miles of tiny tubing, tens of thousands of brazed joints and they don't leak!

Using these engines and breathing air, the vehicle reaches 26 km and about a quarter of the velocity to orbit giving an equivalent exhaust velocity (back calculate from hydrogen consumption) of 9 km/s. That's twice as good as the space shuttle main engines. It is expected to go into orbit with 15 tons of payload out of 300 or 5% even though the rest of the acceleration is on internal oxygen that only gives 4.5 km/s exhaust velocity.

Leaving out the oxygen and using big propulsion lasers to heat hydrogen reaction mass, such a vehicle would get 25% of takeoff mass to LEO, reducing the already low cost by a factor of 5. That's enough to change the economics of power satellites from being too expensive to consider to a cost substantially less expensive than any fossil fuel.

But try explaining any of this in a press release.

Re: British aerospace company claims biggest engine advance since the jet

#105

Post by tsotha »

westicle wrote: post #94
tsotha wrote: post #76

I don't see where this technology "fits". When you're going to orbit, 90% of the flight takes place outside the atmosphere, so an air-breathing engine doesn't do much for you. For commercial sub-orbital hops, they'll have to bring the cost down considerably to compete with existing air service. Sure, we'd all love to be able to go from NYC to Frankfurt in an hour. But how many people will pay ten thousand dollars for…

Given that people already spend multiples of ten thousand dollars to fly it in greater comfort at "normal" speeds, I suspect there is already a significant market.

The numbers are too small to justify a separate design. This is why Concorde failed.

Re: British aerospace company claims biggest engine advance since the jet

#106

Post by Gravityloss »

btilly wrote: post #103
Gravityloss wrote: post #100

Earlier quoted context omitted.

A tank's weight is proportional to volume, not surface area, assuming equal pressure and material strength. 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.

A tank's weight is proportional to volume, not surface area, assuming equal pressure and material strength. 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 flig…

Nope, the shell. In a bigger tank (similar material and pressure) the shell has to be thicker. It's mathematically trivial.

Re: British aerospace company claims biggest engine advance since the jet

#107

Post by bathat »

lmm wrote: post #101
bathat wrote: post #90

Earlier quoted context omitted.

>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. 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…

>Unfortunately, kinetic energy scales as speed squared, so 20% of your orbital speed represents less than 5% of your orbital kinetic energy. 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.

>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 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.

Re: British aerospace company claims biggest engine advance since the jet

#109

Post by EthanHeilman »

bathat wrote: post #107
lmm wrote: post #101

Earlier quoted context omitted.

>Unfortunately, kinetic energy scales as speed squared, so 20% of your orbital speed represents less than 5% of your orbital kinetic energy. 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.

>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 f…

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

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?

Post reply on HN