They talk over and over about their engine, when there are plenty other hard problems they do not have solutions for. For instance light enough cryogenic fuel tanks don't exist. This was part of the reason the Venture Star / X-33 never met their weight budget.
Came here to say the same thing. And note that you're "burning" 400MW of energy in the heat exchanger (transferring it to the helium) at some point you have to re-cool that helium back to liquid) if you outgas the helium you are a net loss because helium has a limited supply. Doing it with hydrogen doesn't get nearly the thermal mass. So it is a really really cool engine, but it remains to be seen how practical it can be.
The second thing, consider the experience from the SR-71. To stay light and still be able to withstand the frictional heating of the air at mach 5 requires some pretty exotic materials. And matching thermal properties for materials at mach 5 vs sub mach. Covering the whole thing in thermal tiles adds weight which comes out of the payload.
To me, adding on the possibility that it could be some sort of Mach 5 super airliner, suggests that the practicalities of it as an SSTO platform are perhaps not nearly so compelling as once thought. Fortunately they were not talking about the multi-gigawatt ground lasers to boil hydrogen. That idea, while nominally physically possible, is impractical to imagine the rest of the world being "ok" with.
To me, adding on the possibility that it could be some sort of Mach 5 super airliner, suggests that the practicalities of it as an SSTO platform are perhaps not nearly so compelling as once thought
Ding!
Yeah, that was my reaction. If SpaceX get the reusable first stage tech running on Falcon 9, then they shave $40M off the cost of each launch (the first stage fuel and oxidant costs $300,000; the stage itself, including nine motors, costs around $40M). Even if each flight costs a couple of mill to refurbish and can only fly 10 times, they'll still have cut the $40M price to around $6-7M.
Meanwhile, here's Skylon, a decade late off the starting blocks. With the Sabre engine running on liquid hydrogen, which is a beast to handle, with valuable He as a consumable.
Frankly, the airliner option looks silly to me. The only real target market for a Mach 5 in-air vehicle is the military.
Liquid Oxygen is (surprise!) a powerful oxidisation agent whilst Hydrogen will cause early failure of the metal parts via hydrogen embrittlement. Liquid helium is inert and much less likely to cause problems I'd guess.
This seeems to use gaseous helium but other than that it seems to be close to the reason. The last thing you want is for any of those heat exchangers to break, as you are effectively mixing the components of rocket fuel.
The main reason though seems to be the temperatures involved. They are exposing hot air to near liquid helium (thanks to being cooled the the liquid hydrogen), thus cooling the air before it reaches the compressor etc that will make it suitable for the rocket engine proper.
At the same time they seem to be heating the hydrogen up. Easier to ignite a gas, or something close to it, than liquid. Just look at how your typical car engine works (never mind a fuel air explosive).
I thought the liquid helium is only for an engine testbed. Production Skylon engines will use liquid hydrogen and oxygen, effectively using their criogenic fuel as a heat sink.
That is a tank serviceable for a few hours which keeps itself cool on the pad with boil-off. An economic aircraft would have to stand thousands of hours with airframe stresses.
In short, cryogenic liquids, especially Hydrogen, is incredibly destructive to any container. It's pretty much the smallest molecule around and will wiggle into and make anything brittle over time.
Graphene may yet be a pipe dream. If we manage to manufacture it in large and perfectly uniform quantities, it's a wonder-material. If we don't - it could make for slightly better carbon fibre composites.
Honest question: Can someone explain why a Mach 5 air-breathing engine is important for reaching low-Earth orbit (~Mach 25)? I still don't understand after watching the video.
I recall from a video interview that Elon Musk is skeptical of the utility of air-breathing engines for economic access to LEO since, as he put it, the atmosphere is "thick as soup" and you basically want to get out of it ASAP before you get up to orbital velocity. Breathing air up to Mach 5 allows you to save some weight, but you have to bring enough liquid O2 for the rest of your acceleration from Mach 5 to Mach 25. So is the idea that this modest savings in weight is all that's needed for a single-stage to orbit?
Let's see. The sabre engine has a closed cycle specific impulse of 450, which gives an effective exhaust velocity of 4.4 km/s. LEO requires about 7.8 km/s velocity while mach 5 is 1.7 km/s. In one case you accelerate from 0 to 7.8 km/s an in the other from 1.7 to 7.8 (for 6.1 km/s delta-v). The first figure gives a mass ratio of 5.9 while the second one gives you ratio of 4.0. You save about 1/3 of the initial mass if you only have to burn on a closed cycle from Mach 5 upwards.
In reality gravity is a tough mistress so actual rockets launching from a pad require about 9.0 km/s delta-v because they lose a substantial amount due to the gravity losses of climbing up from Earth's gravity well. This is especially true for the first few km's of altitude where the thrust-to-mass ratio is quite low. Potentially, the skylon will eliminate most of those gravity losses by effectively launching from 26km altutude at mach 5 (i'm ignoring the relatively modest fuel consumption for reaching this state - the specific impulse is about 3500).
Assuming a delta-v of 9.0 km/s needed for a single stage rocket gives a mass ratio of 7.7. This means that the skylon should save about 48% of the initial mass required by a SSTO rocket launching from sea level.
Some background: This company's engine is based on its previous design, the SABRE engine. That engine is designed to reach an altitude at 28km, flying at Mach 5 (which works out to be about 1.5km/s at that altitude, ~15% of orbital velocity at LEO). After that the engine is designed to switch to a rocket mode, where it burns fuel and oxygen it brought, rather than taking it from the atmosphere.
Since the an air breathing engine is much more efficient (10x) than a traditional rocket engine, it allows the launch vehicle to conserve mass that would otherwise be used to overcome the high drag encountered in lower altitudes. You can roughly think of this as a rocket with all of its mass being launched at 28km, with a velocity of Mach 5 already.
"Getting out of the atmosphere" is a pretty vague statement. Atmospheric pressure and density is about 1-2% that of the ground. So drag at these altitudes will be less. However, some quick and dirty calculations by me makes this seem like a relatively small number compared to the amount of energy it takes to actually get to orbit. Quick search indicates the same as well[1].
However, a "thick" atmosphere is something that has to be dealt with. For example, rockets also need to overcome pressures that exists at lower altitudes. If you take a look at any rocket launch videos, you will notice that the exhaust at higher altitudes (typically about 2-3 minutes after launch) will expand outwards with respect to the rocket itself. This means that the flow out of the rocket engine is "underexpanded", making the engine less efficient than it would be otherwise. By starting your rocket engine at a higher altitude, you can improve the efficiencies by designing your rockets to lower atmospheric pressures (I don't know if this is done for the SABRE engine, but this is theoretically possible).
So based on these, the main motivation for this is probably for the mass savings and efficiency. Additionally, this engine presents itself with potentials for complete reusability, which is very attractive as most current launch cost is from the unrecoverable launch vehicle itself.
>So based on these, the main motivation for this is probably for the mass savings and efficiency.
They plan to invest all those savings into a single-stage-to-orbit launcher. It's appealing in a similar manner that SpaceX's stage-return strategy, but in a one-spacecraft package. High-mach aircraft will find other uses too besides orbital launchers.
As for saving a 'mere' 15% at launch that can be crucial from a payload perspective in the rocket equation.
Starting the rocket at 15% velocity is not the same as saving 15% of the energy though.
* you have slightly reduced gravity field
* you have a big win in reduced drag energy loss
* these wins have an exponential effect on your fuel requirements at sea level, since less weight in additional fuel again means less fuel to transport that fuel up there etc.
* On the other hand, energy is proportionate to velocity squared, so that gives you less than 15% savings if you assume no atmosphere.
Looking at the wiki[1] these SABRE engines (according to manufacturer specs) will have a 7-8 times higher fuel efficiency than typical rocket engines while in atmosphere (3500 seconds vs. 450 seconds specific impulse). Since this has been peer reviewed by ESA, I'm trusting these numbers. The ascend profile of these machines will look quite a bit different than rockets, so it's hard to compare the efficiencies, but the 'rocket starting at mach 5 horicontal velocity and out of atmosphere' comparison seems to be a good intuition to me, although we still have to reduce it by the amount of fuel that has been used in order to get to that point (which will not be insignificant, even though it's 7.7 lower). We also shouldn't forget that the hybrid system has a thrust-to-weight ratio of 14 compared to 150 of SpaceX' merlin engine, which means that it will take longer for such a system to reach the same speeds, reducing its efficiency gains. The reduced weight of the whole orbiter will make up some of that though, so unless we get more publicised informations on the whole ascend path along with simulated data it's hard to tell. All I know is: When using the simplified versions in KSP these hybrid engines make a lot of sense, i.e. it's the easiest way of getting an SSTO there ;-).
The atmosphere is thick as soup, and that's why lift and flight are possible. Conventional rockets are way too heavy to fly (in the literal sense); they just bring tons of fuel and ditch the containers in stages.
SSTO instead brings little enough fuel (weight) that it can fly itself through the soupy part, which is much more efficient (by like an order of magnitude), on top of not requiring oxidizer for that phase. The longer you can stretch out the aerodynamic phase (i.e. the faster and higher you engines breathe), the less oxidizer you need to bring.
If you crunch the numbers (which apparently they did), you get that the equations work out to needing an engine that will stretch the aerodynamic phase up to mach 5. If your plane can't do that, getting to orbital velocity requires you to bring too much fuel for your plane to fly in the first place, and you might as well build a conventional rocket.
You can make a SSTO even without airbreathing, this is how the VentureStar would have worked.
I think there are two benefits. One is that the oxygen takes up most of the fuel mass (e.g. the first stage of a Delta IV carries 173 tons of oxygen and 30 tons of hydrogen), so getting some from the atmosphere is a big weight saving.
The other is that even if the air-breathing part on works in lowest part of the atmosphere, the denser parts of the atmosphere costs a lot of fuel because the rocket has to go slow and incurs more gravity drag. For the Falcon 9, first-stage separation occurs at Mach 6, roughly the same speed that the Skylon would switch from air-breathing to liquid oxygen. But in fact, the first stage is the biggest part of the rocket.
The short answer is that Mach 5 may be only a fifth of Mach 25, but since the rocket equation is logarithmic, this speed gain actually matters a lot. Just enough to go to orbit in a single stage, anyway. According to ReactionEngines calculus, that is.
There is no new information here. It looks like sciencealert.com got its information from businessinsider.com's Dec 11, 2014 article which references a "new video" as its source:
The article title refers to a "plane engine" not a "plane." There is a huge difference: the passenger plane airframes everyone knows and flies on are made by totally separate companies (Airbus, Boeing) from the engines (Rolls Royce, GE, Pratt & Whitney).
A great engine will get you nowhere without a great airframe, and when you start talking about supersonic passenger service, you have to add politicians as well. For example, the Concorde briefly flew between London and Singapore but was stopped partly due to complaints from the Malaysian government. And of course the Concorde is long since retired, despite no comparable replacement existing; this has nothing to do with its engines not being good enough.
Also important to mention why Concordes got cancelled: even with cheap oil, there wasn't much demand for the plane. After a certain point, not much changes between a 4 hour flight and an 8 hour flight, especially if you have red-eye flights. Unless we figure out a way to have fast planes be cheaper to run than current models, then markets seem to show lack of interest in a Concorde-like
the Concorde used about triple the fuel per passenger of other aircraft of its era (like the 747). Even with relatively cheap oil, that added fairly substantial cost.
Additional factors that led to its demise:
- it was a very loud aircraft, and couldn't go supersonic over populated areas due to sonic booms. This led to significant limitations on viable routes.
- it didn't have the fuel capacity to make trans-pacific flights. This also led to limitations on viable routes.
- Concorde required specialized pilot training, specialized maintenance, and even a flight engineer (which more modern jets don't require).
- supersonic flight wears out airframes. I worked in a museum with Concorde G-BOAG, and could see several spots where the airframe had been patched in more substantial ways than our other similarly-aged or older airliners (Eisenhower's 707-based Air Force One and the prototype 727, 737, and 747's.) Airbus didn't want to keep maintaining the airframes given the condition they were already in.
The Concorde used about triple the fuel per passenger of other aircraft of its era.
As a rule of thumb, going supersonic triples fuel consumption. There's some research on improving that, and also reducing the sonic boom, but nothing flying yet. The aircraft geometries proposed are exotic. Angled flying wings, aircraft that transition from long/narrow to short/wide, and double inline wings have been considered.
Triple is an odd figure. I know that air resistance at low speeds is typically modeled as a power between 1 and 2. How do we wind up with the factor of 3?
> "The Soviets also had a design that supposedly incorporated stolen Concorde blueprints"
From what I understand, the Tu-144 didn't really incorporate anything of significance from Concorde -- the plans they had access to were more like sketches than fully fleshed out blueprints, and they only had usable details on a few minor components. The Tu-144 shares mostly superficial similarities with Concorde, with only a couple of similarities that might possibly have been the result of espionage.
The reason that not much changes between a 4 hour flight and an 8 hour flight is that the journey adds about 2 hours on each end, making it a 8 hour vs. 12 hour journey. (1 hour to the airport and 1 hour at the airport).
Urban transit systems and airport security/boarding procedures will need to get a lot more efficient before supersonic flight will be commercially viable.
I would love to only have to spend one hour at the airport. From my experience, that's minimum for a domestic flight. International and you're talking 2+ hours.
(Unless we're talking about Europe; in that case 15 minutes should usually suffice^; at least last time I was there, a few years back. It was great.)
I was talking about first class passengers who can go to the front of ticketing and security lines, since economy class passengers are not likely to be in the market for supersonic flights.
Even with priority check-in/boarding and TSA pre-check, it's still a good idea to show up at the airport ~2 hours before an international flight. You can probably cut it closer but, personally, the last thing I want before an international flight is to be stressed about cutting things close.
Correct, especially with the added time of getting to, from and through the airports.
Some materials about a short-lived Hindenburg airship (http://www.retronaut.com/2011/04/inside-the-hindenburg-in-co...) mention its travel time between Europe and North America to be 40+ hours. I wonder, if they were available today, would there be a demand for this type of travel? For me the choice between a seat in 8 hour flight or a comfortable cabin in 40 hour is often not in favour of speed.
You can get a very comfortable "nook" for an eight hour flight across the Atlantic by flying First where available. And frankly, for that sort of time window, recline-flat business is is perfectly fine. I can't imagine anyone other than a Zeppelin buff wanting to spend 2 days to make the same trip. There's a market. As with long distance train travel and (basically the one) ocean liner. But it's minimal.
There have been murmurs of this possibly having a renaissance. [1]
I read an article in a magazine sometime back as well, talking about the possibility of luxury flights like London -> New York, but boarding in a nice piece of park land instead of an airport. The flight would be much longer, but there would be essentially no jet lag, much nicer quarters to sleep in, five star restaurant food, etc. It certainly seemed to me there is potentially a niche market there somewhere.
I agree with this, and most of the reservations expressed, but just to note: he said Europe to Australia in 4 hours. That's... more like 22 hours now, right? That would be a very significant improvement.
50 years of commercial airline operations have been hard for niche aircrafts. I'm not bullish on the hopes of a LEO commercial plane in our lifespan. It's a tough equation:
- a LEO plane won't be developed without presales that warrant safe funding (ie. from flagship airlines)
- incumbents are all about kerosene and do not want to add a significantly different tech to their already complex operations.
- new comers will not get access to enough capital to secure the development of a disruptive engine technology
Airlines make cash by moving a diversified mix of people and cargo. I just don't see anyone moving to the (super volatile) premium-only market with a supersonic high fixed cost jet.
You can currently pay $5000 more per return ticket (if you shop around) to do it business class. Full lie-flat bed, decent service, leg room, seatback power. You arrive feeling reasonably fresh (modulo the jet lag) and you can work en-route. That's the competition, and it'll be upgraded to compete if a hypersonic option ever comes on the market within an order of magnitude of its price -- think in terms of the first class cabins and bathrooms with showers that Emirates already put on their A380s, then upgrade it to a compact en suite hotel room with in-flight internet.
This. The maximum possible time saving looks even less significant when considering that since the physics and maintenance requirements for transporting people in turbofan-engined aluminium tubes at 1000km/h at 30,000ft is pretty well understood after half a century, the A380 will have better dispatch reliability and be substantially less likely to send your VIPs to a fiery death.
I suspect this is one of those Big Challenges that keeps tempting aeronautical engineers, and they try to apply the pushing-the-envelope high tech of their era to it. The problem seems to be the economics. There aren't a lot of people who are willing to pay 10x prices (or whatever) for 3x reductions in travel time.
My thoughts too. The main reason is not the engines, but the frame itself. It's not very commercially viable spend days in a hangar, after few hours of flight, scanning for the micro faults.
Ok, these guys claim to have invented a great engine, really just the pre-cooler. Why are they trying to build a plane instead of licensing it to others? That seems to be several orders of magnitude more difficult and increases the likelihood of failure.
Does he know of anyone in the tightly-regulated airline industry who would be crazy enough to build a completely new airframe and engine around a currently-experimental precooler?
At that speed, I wonder what accidents will look like. I respect this technology as totally serious but in case of accident the NTSB may just delegate the investigation to a Darwin prize:
Is the plane sensitive to slight variations, like a few people walking in the alley? Since at Mach 5 we can expect a 5000K temperature for the dislocated parts [1], will the pieces completely burn and disintegrate before falling back to Mach 1, effectively acting as a MH370 at each accident, by design? Can we at least have a video of that on 9gag? Or will we have to search a 5,000-km radius if we have a 20minutes uncertainty for the accident window?
With LH2 and LOx as fuel-oxidizer, whatever are the consequences of speed, they'll be over a completely pulverized ex-airplane, not over entire parts.
Also, with that to speed, I wonder how big a runway this thing would need. It would probably operate on a completely separated airport, faw away from cities.
That sort of sentiment is why we can't have nice things. These thing stake time.
Remember, this isn't a kick-starter campaign. They're not trying to dupe other people out of money on some hair-brained scheme. This work has been reviewed by the British Interplanetary Society and the ESA. It's real science, tackling a real problem.
I'll make a bet with you now. In 15 years, the Sabre Engine will be carrying more payloads into space than anyone else, and for less money than anyone else can offer.
> They should have built a working model of something by now
They built and demonstrated a reduced scale engine for the ESA. If you think it's that easy just to build a spacecraft when you own a machine shop and a sheet metal company then you have a lot of reading ahead of you.
I get the impression that the passenger flight times are more marketing talk intended to frame the technical achievement in terms people can understand rather than an actual intended use case. Reaction Engines do have plans to use the engine on a passenger plane, but the Skylon is designed to take payloads up into space cheaply.
The A-380 costs about 414 million usd and can carry ~500 people. A flight from Europe to Sydney takes north of 22 hours (not counting stop-overs).
Off the top of my head, this new plane seems like a pretty good deal, doing Europe to Sydney in ~4 hours with 300+ passengers for a unit cost of 1.1 billion.
You cannot ignore fuel cost and investment to handle the new fuel. We'll see what happens, but the most compelling thing to me seems to be the fact that it burns hydrogen and not kerosene which seems to be more environmental friendly, but I'm no chemist.
Burning hydrogen releases water vapour, while burning fossil fuels release carbon dioxide and water vapour. So, you could call it indeed more environmental friendly if you consider carbon dioxide harmful.
However, hydrogen doesn't occur on Earth in a pure state (at least not in mineable quantities). There are no hydrogen wells.
Creating the hydrogen (for example, by electrolysis of water) is expensive and generally a net loss of energy. Maybe storing energy in hydrogen is feasible if it's generated with excess nuclear or solar electricity.
> burning fossil fuels release carbon dioxide and water vapour
Either I have completely misunderstood something, or fossil fuels release only CO2 and H2O under ideal conditions, which never happens. Add to the fact that any jet engine is unable to utilize any after treatment systems to remove harmful products. I am fully aware that hydrogen is rather expensive to produce, but I think it's possible that nuclear energy, or (as you mention) excess solar/wind power is several magnitudes cleaner than even the cleanest burning fossil fuel engine (including all the resources it takes to pump it up and refine it).
Not odd at all. If you scroll down there are links to other articles. The last one is a photo of the catacomb mummies. Your fead reader simply chose the wrong photo from those on the page.
So is the breakthrough just the pre-cooler, or the pre-cooler together with the scimitar engine?
It sounds like the innovation is the helium pre-cooler, but even at that it's hard for me to understand whether their advancements are in working with liquid helium for a coolant or in the airflow cooling (both)?
I don't do anything remotely like this in my day job.
Where does this liquid helium come from, and more importantly where does it go after it's been heated? Im guessing they release it into atmosphere, so is there enough helium to mine for this plane to go mainstream?
It says in the Wikipedia article that: "The key design feature for the Scimitar engines is the precooler, which is a heat exchanger that transfers the heat from the incoming air into the hydrogen fuel."
The helium is cooled by the hydrogen going to the engine.
You can't run the pre-cooler on the hydrogen directly, because the hydrogen infiltrates the welds (and everything else) causing hydrogen embrittlement, which will quickly tear the heat exchanger apart.
A hydrogen/oxygen engine emits only water. You can choose to get the hydrogen from water using from a non-carbon energy source, for a completely carbon emission free system.
