Reaction Engines brings us a step closer to Mach5 commercial air travel | Hacker News Reader

Reaction Engines brings us a step closer to Mach5 commercial air travel

thestar.com

48 pointskareemm62 comments

We're not going to have Mach 5 commercial air travel for the same reason we don't have Mach 2 or 3 commercial air travel. The economics just don't work.

That reason is much more boring than the technical obstacles, which are formidable, fun to try to solve, and kind of beside the point. In that sense, this problem resembles a lot of software engineering.

But just to keep this comment from being too depressing, here's one technical gripe: consider that liquid hydrogen is an extremely bulky fuel, even if you don't include the necessary insulation. Notice how nearly the entire volume of the satellite launcher in the diagram consists of hydrogen tank. There's not a lot of room in there for passenger lounges and drink trolleys.

> The economics just don't work.

Given that you don't know how much this will cost to operate, you can't possibly know that, right?

At some point, i think i read a claim that an airliner based on this technology would be able to supply Europe - Australasia trips for about the same price as a business class ticket on a conventional airliner on that route. Passengers currently buy a fair few of those tickets; some might choose to do the trip in two hours instead of twenty-four in exchange for a smaller seat and the loss of a couple of meals.

SpaceX is predicting $2000/kg to low earth orbit.[1] Reaction Engines are predicting an order of magnitude cheaper than current tech[2], so let's be charitable and say that this is $200/kg. (Although the Wikipedia article mentions $650/kg).

Consider that a Skylon launch is for a non pressurised satellite and capable of lifting 15,000 kg, which supposedly could be for 30 passengers in a passenger module [3].

That would meant $100,000/passenger to anywhere on earth. Cheap if you want to go to orbit, less so for a trip to Japan maybe.

[1] http://en.m.wikipedia.org/wiki/Falcon_Heavy#Pricing_and_deve... [2] http://www.reactionengines.co.uk/space_skylon.html [3] http://en.m.wikipedia.org/wiki/Skylon_(spacecraft)#Specifica...

Why do you assume intercontinental flight will use the same configuration as the orbiter? First of all, the orbiter still needs lots of oxygen on board, even though they can save around half of it with their hybrid jet/rocket engine. Furthermore it needs heavy heatshields. Both things that are not needed (or to a much lesser extent) in an intercontinental craft. The saved weight for oxygen alone should allow you to go from 30 to 200 passengers, giving you another order of magnitude - and suddenly you're at first class ticket prices.

You need some way to keep the passengers from being roasted alive, and you need to engineer it to commercial air safety standards (redundant systems, emergency exits, little sign that says 'no smoking'). By the end of the process you'll be pining for the days of having to just lug around a little oxygen.

We know how much the Concorde cost to operate: a zillion shittons (metric) [http://www.concordesst.com/retire/faq_r.html] . And that was with proven technology and all the sunk costs behind them. I think that makes a sensible floor for this project, unless you are willing to argue that researching and deploying this completely new technology would somehow cost less.

Is there really a tremendous amount of demand on the Australia / Europe segment? If London/NYC wasn't viable for the Concorde (probably the most popular international airline segment), I can't imagine the demand being higher on routes to Sydney.

If the fixed costs of a flight are high, it will likely be even more expensive than the Concorde on trans-Atlantic routes.

Concorde reportedly made BA about £750M.

I'm not sure whether that figure takes into account the development costs - but the reason that it was retired was due to Airbus withdrawing support.

http://www.concordesst.com/retire/faq_r.html

The CEO of Airbus said in the Financial Times that "The costs of operating Concorde, and in particular maintenance and support, have become such that operations are unrealistic for any operator."

The reason BA booked a profit is because of massive de facto government subsidies for Concorde, for reasons of national prestige.

As far as I remember, stopping Concorde had more to do with poor sales after the Air France crash rather than anything inherent in cost efficency.

parasitic drag increases with the square of velocity. these engines wont change that.

They do. Because they can operate in much thinner air.

They claim about the same fuel expense for long distance flight as today's craft. The reason is that they fly much higher where air resistance is very low. Fuel volume is a good point though - it probably means that a commercial airliner would need to be made bigger than now in order to be cost effective?

At the end of the day, if they can deliver a craft that carries ~200 people ~8 times faster to long distances than a normal airliner for a similar fuel expense as today, with a turnaround time similar to today, you'll have something that generates money ~5 times faster, which should please even the bean counters.

But obviously, there are many ifs in that equation, so we just have to wait and see. I just wouldn't be too dismissive right from the start, as always with unproven but promising technology.

To go higher with tons of weight requires even more fuel. This is a _very_ common rocket science problem and is the reason why almost all space vehicles have stages that drop off.

To go high, you need more fuel, to get more fuel up you need more fuel... etc etc.

"we just go higher with less air resistance" is a bullshit marketing thing and not really showing the math.

Engines designed to be supersonic are also _very_ shitty at subsonic speeds. Sure, cooling 1000 degree air for use in a turbo fan is great. You're not gonna get _any_ benefit doing it at ground level.

You don't need the fuel to go high.[1] You need it to go fast. Orbital velocity is Mach 25, not the puny Mach 5 that they talk about for transporting passengers here.

[1] See Virgin Galactic's 'spacecraft'. You see lots of fuel tanks? No? That's because they only do a little 'hop' to 100km or so, not accelerating sideways.

The fuel required to accelerate from Mach 5 in near vaccum to Mach 25 is a fraction of what you need to accelerate the vehicle + fuel combo to Mach 5 in air. Remember how the space shuttle had those massive rockets on the side, that fell off way before it was close to horizontal? You need the fuel to get high enough to go fast.

The basics of your argument are true, but the air breathing capabilities change the magnitude of the effect quite a bit. Saving the oxygen weight in the 'first stage' helps a lot. One oxygen atom has a molar mass 15 times larger than one hydrogen atom. Burning O and H to H2O thus means you have 2/17 of the mass laying in the hydrogen, 15/17 in the oxygen. It's an order of magnitude savings.

Again, look at the Virgin Galactic Spacecraft as an example. The reason why they can go so high without much of anything, is that their first stage is an air breather.

Furthermore, the fuel that you need to bring to go from Mach 5 to Mach 25 is what's responsible for a large size increase of your first stage - to have more fuel to burn at a high altitude / high velocity requires even more fuel at the ground.

Overall, you have to look at an airbreather quite differently than conventional rockets and you have to look at suborbital flights differently than orbital - you can't really approximate it from the space shuttle without going into the details.

This is actually incorrect. The space shuttle had massive launch engines because it needed the thrust to get off the ground and into space. But from the words coming out of SpaceX, about 80% of a rocket's fuel is actually expended accelerating to orbital velocity once you're in space.

But, once you're no longer needing to fight gravity so much, it's a lot more efficient to use lower thrust engines for the job, provided you've got the time.

In defense of parent (although I'm speaking against my own initial argument right now): I think there are two ways to look at it. One is to look at the final energy state in orbit, where kinetic energy is much higher than potential energy. One is to look at how much fuel energy is spent for each delta-T for potential and kinetic energy, and integrating that over time. Intuitively I expect that to give a different result, e.g. most fuel spent for increasing potential energy. The difference[1] comes from the potential energy that gets stored in the remaining fuel that will be converted into kinetic energy later at a lower cost. So, there is an argument to be made for both sides, however one has to be careful in how to apply this knowledge to a new technology like air breathing rocket motors - it has a very different economy.

[1] edit: There is a second factor that will also change that result compared to the first: air drag. I.e. some fuel energy will be lost in heating up the surrounding air.

> Notice how nearly the entire volume of the satellite launcher in the diagram consists of hydrogen tank. There's not a lot of room in there for passenger lounges and drink trolleys.

The passenger variant presumably wouldn't need as much fuel as the one designed to launch satellites into orbit.

Why not? You have to slow down and land with the full passenger load, rather than launching the payload into space[1]. And you need to engineer the vehicle to very conservative airline safety standards.

[1] Although launching the passengers into space is an attractive option, given the kinds of people who could afford tickets on this thing.

You need a massive multi-stage rocket that is 90% fuel (+ reactant etc.) by mass to get a couple of tons of satellite into orbit 160km up; yet a fraction of that fuel mass can take a plane and hundreds of passengers thousands of kilometres at a stretch, but only 1 or 2km high.

In other words, going up is hard.

The trick is, and it is a trick, to "instantly cool the air". Think about that for a moment. Generally the way most 'air breathing' engines work is that they take air, heat it (which expands it (thanks Boyle!)) and toss it backwards which imparts a forced in the forward direction (thanks Newton!). The difference in temperature between the inlet and exhaust temperature of a jet engine is a component of its efficiency. (see http://en.wikipedia.org/wiki/Jet_engine#Energy_efficiency for some math :-)

But basically if you're suddenly cooling air it gets smaller, which creates a lower pressure, and was used effectively in steam pumps to create the vacuum needed to lift water out of mines.

So the Reaction Engines concept takes really hot air that is compressed because it is being pushed out of the way by the air frame, then super cools it which causes it to become much smaller, and then dumps that into the engine. Which then heats it up again and pushed out.

If I understand the theory correctly, it takes heat from the air, removes it and puts it into liquid hydrogen (which then becomes vaporous(sp?) hydrogen) and then burns that hydrogen with residual oxygen to put the heat back into the air plus what ever is extra from the hydrogen. What bugs me about it is that it seems to double count the energy in the air rather than single count it. But I'm interested in seeing one of their machines in action. If someone knows whether or not there is a violation of thermodynamics in there I'd like to understand that.

That said a mach 5 vehicle with a range of 12,000 miles and a payload capacity of 15 tons has much more interesting uses than carrying passengers I think. Would make a heck of a cruise missile.

No one is using the energy in the air for anything. It is not an important term. You're reading the idea as though somehow the energy from the air is used for something - its not.

The issue with hypersonic flight is that it's impossible to sustain a flamefront (combustion) of supersonic air. So you have to slow down the air before you feed it into your engine, or you flame out. But you've got high velocity air suddenly being compressed - which means you're going to superheat it.

That's also a problem, because the superheated air will pre-burn or even detonate fuel in the wrong part of your engine. Or just melt your air-intakes. So you need to cool it - and you need to cool it really fast because you need a lot of air to make your high-speed engine go fast.

The "cooling it fast" is the trick. Dumping the heat into the liquid hydrogen isn't - you do that because you need to get rid of that heat quickly, and the best way to do that is to take some of that cryogenic fuel you have so much of, heat it up and then immediately eject it from vehicle.

>That said a mach 5 vehicle with a range of 12,000 miles and a payload capacity of 15 tons has much more interesting uses than carrying passengers I think. Would make a heck of a cruise missile.

yes and no. No - early detected and easily destroyed even by previous generation of anti-missile systems. This is basically why Valkyrie and the likes were scrambled 50 years ago. Yes - quick deployment against countries still not having any meaningful anti-missile defense :)

Also, quick jumps above 60mi (space international border) would probably allow to legally fly modern day U-2 style missions over some relatively narrow territories like over Crimea, Cuba, North Korea, etc...

The working hypothesis at the moment is that that type of suborbital hypersonic aircraft is what's being built by the US at Area 51 these days. Certainly the focus of public research funding has had that theme to it.

well, judging by nobody hearing the sonic booms - somebody living in CA would definitely hear it when it would be going into Pacific, i think X-15 and Branson are the closest takes on it (both being almost the same actually, and military can realistically use such approach today, though no country has reported such flyovers and considering that it can be easily spotted if used...).

It's funny how often advanced technology has, as its limiting factor, the ability to efficiently cool air. It's kind of promising, I suppose, since to cool air is remove energy from a system and hence has a theoretical negative net cost of energy.

In a previous post (a little over a year ago) about the same engine I posted a link to a video with a Q&A with Elon Musk where he was a bit pessimistic about the idea.

I'm not a rocket scientist so I wouldn't know the first thing about it, but if anyone is interested in his (then) opinion it's after 48m55s in the video here: http://www.oxfordmartin.ox.ac.uk/videos/view/211

Elon Musk isn't the expert in this area. Practically no one except Reaction Engine employees and some guys at Area 51 are - in the entire world. It's certainly not an easy thing to do, but since they actually do seem to have the heat exchanger figured out then that's kind of a big deal.

Burying the lead is the whole commercial air travel angle though. What they're building is a fully reusable, cheap to operate SSTO to low earth orbit.

ESA did a review of Skylon and the Sabre engine and said "...the SKYLON vehicle can be realised given today's current technology and successful engine development."

http://www.reactionengines.co.uk/space_skylon.html

Of course, there needs to be a successful engine development project. Which needs another billion or two to complete according to the CEO/MD of Reaction Engine. (He was a bit more specific than that when I heard him say it, but it is in the ballpark.)

the thermodynamic economics of ram/scram jets at the Mach5+ speeds is almost killed compare to the rocket engines by the fact that oxygen is only 20% of the incoming air, while the whole incoming air needs to be processed through the engine, ie. compressed/(may be cooled interstage)/heated/expelled. Various unavoidable losses on this 80% (nitrogen) of the incoming air eat deeply into end-result efficiency of such engine when compared to a rocket engine.

Sounds a little odd. I'm not really an expert on this area or anything, but it sounds plausible that if they are able to cool the incoming air stream to that temperature, then they might be able to make the speeds they're claiming. But I have to wonder, not only how they made a heat exchanger capable of cooling the incoming air that fast, but what are they doing with those megawatts of heat that they're taking out of the air? They claimed air temperatures coming out of the heat exchanger of -150, so where are they getting a cooling fluid colder than that to do exchange with?

The incredibly huge tank of liquid hydrogen not pictured in the diagram. The same one that makes the idea completely unfeasible for commercial transport.

I'm assuming this can't be used for flying over land due to sonic boom?

Depends which land and — most likely — depends who builds it.

It's not like the US forbid overland supersonic flight for their military planes (and overland commercial supersonic flight was only banned as the US's sole SST program was cancelled)

During 54 years of living in 5 different metropolitan areas of the U.S., I've never heard a sonic boom. (I know what one sounds like because they were frequent during a vacation I took in West Germany a long time ago.)

The Blue Angels, for example, are not authorized to exceed the speed of sound: http://www.blueangels.navy.mil/show/faq.aspx

I lived near colorado springs in the early 80s, as a child, and I can remember on two occasions being (figuratively) knocked out of bed by a sonic boom.

Presumably from the Air Force Academy. Only happened those two times the entire time we lived there - presumably someone got in trouble for it...

The US military is typically only allowed to go supersonic over land in the US in certain special areas.

So they can cool the engine, but what about the heat on the rest of the plane?

That's not the problem. The problem is that hot inlet air means that once you burn your fuel, the air post combustion stage is then too hot and melts things.

It's a huge problem, unless you don't mind a flying oven. In the SR-71, the crew had to wear special cooling suits (presumably these will be available in first class).

What I mean is that from a material standpoint, the engine turbine temperature tends to be the design limiting factor.

Yes, there's going to be hot spots on leading edges due to stagnation for example, but you're not burning fuel there to intentionally jack the temperature up.

Any pressure suit requires cooling, because the human body produces about 100 watts of thermal power.

meh, insulated cabin. No big deal. just shitloads more weight and/or space (nasa aero gels would probably be great here).

Aka this thing is never going to see commercial flight.

The SR-71 Blackbird cooled itself through a combination of radiative cooling (hence the black), and using its fuel (that was about to be burned) as a heat sink. Some combination of these techniques (and others) would probably be used.

Here the fuel is already being used as a heat sink to cool the inlet air. Liquid H2 doesn't have that big a heat capacity, you can't use it cool the fuselage and the cabin air and your cocktail and still expect it to keep the engine from melting.

the video is 2 years old

I've read about this before, and I do think it sounds pretty neat, but a small part of me hopes it doesn't work for putting satellites into space, only because the technology doesn't help us land on other planets.

I disagree. Familiarity with microgravity and the vacuum of space in orbit will make developments in interstellar travel that much cheaper.

I think we're pretty familiar with vacuum at this point. The obstacle to practical interstellar travel is 99.99999% propulsion, which you don't learn a lot about by endlessly falling in a circle.

And the obstacle to propulsion is the inability to lift large payloads cheaply enough into orbit to start with.

The problem is we can't build big things in LEO. At the moment its that our entire LEO-access infrastructure is woefully underdeveloped.

Cool. Love getting down voted for having an opinion & starting the conversation.

The first few miles are the most expensive.

I saw a very good presentation by the CEO of Reaction Engine the other day, where he went through where they are at with the engine (Sabre) and with the vehicle (Skylon).

He showed how Skylon could lift an interplanetary vehicle in pieces, just like the International Space station was built from modules. The Skylon/Sabre tech will certainly help us reach other planets, as the first 100 km to orbit is the really expensive part, and Skylon/Sabre addresses that well I think. They need some more development money though.

I didn't say reach, I said land.

Nitpick about the title[0], but it's a kind of thing that really irks me. It's "Reaction Engines", not "Reaction engines". It's a proper name of a private limited company, not a class of engines.

[0] - at the time of writing this, the title was: "Reaction engines bring us a step closer to Mach5 commercial air travel".

I'm the OP. I'm 90% sure I submitted it as "Reaction Engines" - I was surprised it was "Reaction engines" when I visited the site an hour after submitting. I would've edited it when I revisited the site, but wasn't able to by that point.

But, then, it's also entirely possible I submitted it incorrectly.

No, that was our mistake. Sorry—fixed now.

Based on the grammar following "Reaction engines," it's clear that the OP simply thought the new engine was actually called a "reaction engine"

(I'm sure the author would not write "Google bring us closer to autonomous cars")

The Star is a British paper: UK usage would be Reaction Engines bring, and yes the author would write "Google bring...".

Well, the article clearly states that Reaction Engines is a rocket company, so it follows that OP must have not read the article very carefully, if at all.

Reply on news.ycombinator.com