The article seem legit and it reads well but … it’s making some seriously big claims - is the writer and the company getting carried away a bit?
The Sabre has been in development for a long time without producing any real results. I haven’t heard of anything technically wrong with the concept, it just doesn’t seem like getting it to work in a real aircraft, or spacecraft is going to be very easy.
Here’s the wiki on the Sabre.
It seems the issue was funding. I guess, now the key tech has apparently been proven by the ESA, finally Government is on board for a full-size prototype.
Thanks for your response.
Personally I love the elegance of the idea. Use oxygen in the air for as long as possible then, using the same unit, switch to an onboard supply to orbit. I also like the use of heat exchangers to bring one gas up to working temperature by using it to bring another gas down to working temperature.
It also looks like one bad mother so that helps.
Their Mach 5 airliner is even more badass.
Also no bad thing the designer is Bond, Alan Bond.
Even if the SABRE engine itself has been demonstrated (and exactly how they would demonstrate its function in the hypersonic and transition from scramjet to rocket propulsion without an actual flying article) there are still many technical hurdles to making the proposed Skylon spaceplane configuration practicable, including the thin-film ceramic skin backed by an active cooling system, composite cryogenic propellant tankage, control at hypersonic speeds, et cetera. The Skylon itself is an enormous vehicle that would not conceivably be able to land at normal commercial runway lengths. And to assert the payload capacity without even flying a proof-of-concept article to give some insight into practical scalability is bald-ass speculation at best. There is just no empirical basis to even assess how much capacity it could carry with any degree of certainty, notwithstanding the development schedule to turn a novel and unproven technology into an operational vehicle in the span of eight years.
Spaceplanes seem intuitively a better route to a reusable space launch vehicle because they land (and in this case, take off) like an aircraft, and certainly the theoretical advantage of using oxidizer from the atmosphere for the first few tens of thousands of feet would seem advantageous. But this comes with all of the extra mass required to provide wings or a lifting body, which detracts directly from payload mass (because you have to carry it all the way into orbit) and requires compromises in structural strength and internal layout, all of which is only of use for the last few minutes of reentry. One of the major problems with the Lockheed X-33 development was the extremely lightweight and high stiffness tanks that had to withstand cryogenic temperatures. In comparison to the dome terminated cylindrical tanks used on conventional staged rockets and the STS “Shuttle”, in which ullage is readily controlled by thrust or (in the case of smaller tanks) flexible bladders, the more complex tankage workable for an aircraft type application is problematic when it comes to controlling propellant location and requires greater strength for pressurized propellants.
Given the enormous difficulties encountered in past efforts to use hydrogen for airplane-type applications (which has extremely low viscosity, has to be maintained at cryogenic temperatures, high boiloff, and very low energy density even in liquid state) such issues cannot just be waved away as “engineering details”; they are key pieces of technology which would be required to make a spaceplane viable, not to mention the issues with handling the large volumes of liquid hydrogen which would be required to permit rapid “commercial airplane”-like turnarounds. And the advantages of hydrogen–the high specific energy and low molecular mass of the resulting combustion products–are less critical in the early stages of ascent in the troposphere, which is why essentially all currently operating multistage rockets (except for the forthcoming American Space Launch System which is based upon Shuttle technology) use hydrocarbon fuel and LOX oxidizer or the storable hypergolics (typically UDMH and nitrogen tetroxide) for lower stages and reserve the bulky hydrogen for upper stage or transtage applications which operate outside in vacuum.
A “£60m investment” is not nearly enough funding to go from a working proof-of-concept engine (if Reaction Engines has even that much) to a complete working orbital vehicle of novel configuration and operation, notwithstanding doing so on such a short timescale. But this is nothing new. Dig back through articles of Popular Science and you’ll find exactly the same claims made in the early 'Nineties, save that the vehicle was called HOTOL (HOrizontal Take Off and Landing) and the engine was called Swallow. The American Strategic Defense Initiative Office had considerable interest in this for the “Star Wars” missile defense program for a couple of years before reviews by independent boards (which included people who were otherwise enthusiastic about the possibility of SSTO vehicles like the Lockhead SSX, McDonnell Douglas Delta Clipper, and Phoenix, concluded after parameter studies that the HOTOL was just not viable without miraculous improvements in material, thermal protection, and flight control technologies.
Stranger
Fair comments, the technological hurdles to overcome are huge indeed but I’d suggest that what appear to “miraculous improvements” to us now have a habit of becoming reality. The problems to be overcome are matters of engineering and materials but they are amenable to solutions. Sure enough, if no-one tries, no one will succeed.
The development and application of new technologies do not “have a habit of becoming reality” in the fashion that the passive language would suggest, but instead are the result of tens or hundreds of thousands of hours of research, developement, testing, manufacturing and application process development and often no small amount of trial and error for even the simplest technologies to go from conception to technical maturity. Dismissing these deveopments as just “matters of engineering and materials” is akin to Brook’s Law (“Nine women can’t make a baby in one month,”); that is, regardless of the amount of effort or creativity you throw at a problem, if it poses physical constraints and conditions which exceed existing capabilities (as the thermal protection system proposed for the Skylon does) you can’t just multiply the effort required to design a system by some power factor and assume it will be achievable within a defined schedule.
The prime example of this is controlled nuclear fusion; we’ve been “twenty five years away” from a working fusion reactor for the last sixty years, not because we lack funding or creativity, but because the more we’ve learned about plasma physics and the better computer simulation tools have come the more we’ve learned about how much harder the problem is than originally understood. Other areas of engineering are the same; if you are remaining within the capabilities of existing materials and construction methods, building a bridge or a pump is “just” a matter of finding a way to assemble the design elements into a working solution, but if you are dealing with conditions which exceed prior experience and material capabilities, you find that the problem doesn’t just scale up in difficulty but in even your ability to understand the problems up front. Overcoming these issues isn’t just a matter of throwing more effort into it; it relies on innovations and discoveries that occur at intervals that are difficult to predict or plan, and often come from places that are least expected.
The SABRE engine is but one component of a working spaceplane like the Skylon, and even if it is fully functional as described there are other technologies that have to be shown as not only technically workable but actually feasible to apply in a reliable and cost-effective fashion. The infamously problematic thermal tiles on the Shuttle Orbiter are a cautionary tale of this; although they perform amazingly well from an insulation standpoint, they were so delicate and difficult and labor intensive to fabricate, install, and maintain that they delayed the launch of the STS for over three years, required a substantial portion of the operating budget to maintain, and ultimately played a large role in preventing the STS from operating at the predicted rate and operating costs which were originally intended. They worked as insulation, but were not workable in the sense of being a functional improvement over previous methods (blunt-arsed capsules with a generous heat shield which for reentry protection) and have not and will not lead to greater improvements.
Regardless, the £60 million investment is not enough to go from basic demonstration of a propulsion system to a fully functional high performance HTHL SSTO vehicle, and the timeline stated in the article to implement this technology is patently absurd. There is the assumption that cutting edge technologies. complete reusability, and absolute minimum “dead” weight are required to attain routine orbital space access, but the reality is that accepting somewhat lower performance and an overall larger vehicle to payload mass results in greater reliability and lower overall costs. When you invest your efforts in improving the state of the art–the so-called “bleeding edge”–you tend to spend a lot of effort tied up in dealing with tiny but critical technical development issues rather than focusing on achieving the optimax solution based upon cost and reliability.
Stranger
The advantage of air breathing isn’t just using atmospheric oxygen- it’s using the atmosphere as reaction mass. A LH/LOX rocket engine has a specific impulse of around 450, whereas an air breathing engine at Mach 5 can be expected to have a specific impulse of 1500 or higher. That translates into an enormous savings of fuel as well as oxidizer, especially because the first stage of a rocket is necessarily much heavier than the upper stages- so much so that people have proposed launching rockets on rails up the side of mountains to avoid having to rely on pure rocket thrust for the first few thousand feet. And the high-volume LH2 tankage is actually a design feature- it’s expected to give the Skylon a higher surface area to weight ratio on reentry.
The problems you mention are significant, but if a hypersonic dual-mode engine can actually be made to work, it changes the concept from science fiction to a near-term engineering challenge.
Stranger, you are responding (rather patronisingly I might say) in more detail and with more seriousness than is necessary, considering the nature of the throwaway comments and musings on the glamour of spaceflight.
I realise you can’t know (and I won’t tell you) what business I’m in but rest assured that I’m fully aware of the cost and timescales required for even very minor innovations in cutting edge technology. I said the problems are matters of engineering and materials but that is not dismissing them as minor. I also work daily with technology that only a couple of decades ago was considered impossible and yet is now fairly mundane. It does happen.
I don’t underestimate how difficult Skylon may prove to be, nor how unlikely it may be for such a system to be economically viable.
The fact remains though that progress is being made and innovative technology is being produced. Even if the ultimate end goal proves out of reach I suspect these innovations will find applications in other unanticipated areas.
Your posts read as a council of despair…“it’s too hard, don’t try.” That isn’t your true position is it?
For what it’s worth, I read Stranger’s posts as a caution: “Someone who’s claimed to have solved the hardest part of the hard problems, without providing functional evidence, is trying to sell you something.”
And if this is what he means, I concur.
I’ll believe they’ve solved the problem when they have a complete functional prototype SABRE engine, complete with a working heat exchanger for the compression-stage supercooler capable of the 1 GW-per-cubic meter thermal transfer rate they’re claiming, and light enough to preserve a reasonable system weight, and durable enough to survive reuse. Preferably demonstrated in a regime of multiple manned or unmanned flight tests.
I would wager something to do with submarines.
At low altitude flight, specific impulse is less compelling than raw thrust per gross vehicle mass; it is actually a better to trade minimum mass and high propellant efficiency (which is essentially what specfic impulse represents) for carrying more propellant initially (and more tankage which is dispensed with relatively early in flight) and achieving higher speed quickly to minimized losses due to gravity drag (the losses due to the downward vector of thrust to keep the booster from falling back down before it achieves orbital speeds). For example, although the RS-25 Space Shuttle Main Engines were the highest thrust per weight cryogenic engines ever built with an impressive vacuum I[sub]sp[/sub] = 452 s and a sea level I[sub]sp[/sub] - 363 s, each of the two RSRM boosters provided more than twice as much thrust as all three SSMEs combined despite having a plodding sea level I[sub]sp[/sub] of 242 s (268 s in vacuum). During their ~125 seconds of action time the SRBs each provide about 575 million lb-s of impulse, while the SSMEs together provide about 780 million lb-s of thrust over a 510 s firing time. By an propellant efficiency metric the SRBs suck, but without them the Shuttle could never make it to orbit without adding at least the equivalent of six more SSMEs and twice again as much propellants. (The comparison isn’t quite fair because the SSMEs are opimized for vacuum operation while the SRBs were designed to provide the early high thrust to get the STS moving, but it does illustrate the point that fuel efficiency not as important a consideration as often advertised for low speed in-atmosphere operation.)
Fuel is cheap, and the mass of the lower stage (especially discardable boosters) detracts only a fraction compared to payload mass, so for each kilo of downstage mass you may be losing a few tens of grams of payload mass, which is easily made up by giving the stage a little more fuel capacity (up to the point that you have problems scaling or mounting more engines or difficulty handling the size of the vehicle). Adding mass to the orbital vehicle, however, detracts essentially 1:1 to payload mass, so for every kilogram of additional mass you carry in wings, thermal proteciton systems, engine mass for capacity that is greater than needed to achieve orbit, et cetera you lose a kilogram of theoretical payload. Fuel is an almost negligable part of the costs of space launch (and hydrocarbon fuels are much easier to handle in large volumes than cryogenic hydrogen) and the trade between additional downstage weight, lower performance, and lower cost versus high performance at a high cost point and the necessity of carrying additional structural mass into orbit inevitably favors the former in terms of payload mass to orbit.
And as discovered with the Columbia disaster (and acknowledged all the way back to the original STS Test and Evaluation program) the more complex the geometry and the greater length of leading edge which have to be protected, the more a reentry structure is prone to damage and unexpected shock heating interactions. There are an entire host of reasons that despite all of the concepts you’ve seen of horizontal takeoff and landing space vehicles you’ve seen, none have ever come close to flying.
It isn’t my intention to imply that, and I don’t think my words reflect that position. However, if you make claims about how solving one part of the problem makes everything fall in line in short order without consideration to the other challenges (as the article does) then it presents and reinforces the notion that success will be forthcoming in short order, and when it is not (typically the case) it results in a pullback in funding, claims by critics that space access is (and will always be) to expensive, and generally retards progress that would be achieved with more realistic goals and expectations.
Certainly a steady investment in technology innovation and development is a good thing, and not just for space applications, but depending on the lastest gee-wiz technology to solve all of the problems with what is already a difficult to implement design generally ends up compounding the problems rather than allieviating them, as exemplified by the Shuttle thermal tiles example. Developing a sustainable space access architecture shouldn’t rely upon exotic materials and edge-of-performance-margin systems any more than you would construct a functional building out of carbon nanotube composites, and for exactly the same reasons. If you actually want to get to space (and especially if you want to do so cheaply enough to take large mass payloads regularly) the focus should be less on the method that looks cool or promises rapid turnaround with no demonstrated proof-of-concept, but a method that doesn’t distress the state of the art and uses proven technology and solutions.
Stranger
The OP’s link is to an article that is based on information obviously provided by the public relations department at Reaction Engines Ltd. I saw something about the funding months ago, so it’s not brand new information. Their engineers might have some reasonable goal in mind for the Sabre engine that could be accomplished within a reasonable time frame.
Stranger,
“Fuel is an almost negligible part of the costs of space launch”
Can you give us an idea of how the costs break down?
Liquid oxygen (used as the oxidizer for most LH[sub]2[/sub] and RP-1 fuels) is around US$0.10 per lbm in bulk.
Liquid hydrogen is somewhere around US$10 per lbm in bulk (although most users will produce LH[SUB]2[/SBU] by steam reforming on-site due to the sheer amount of fuel required and the difficulty in transportation.
RP-1/RG-1 (basically high grade aviation kerosene) runs about US$1.70 per lbm last I checked.
I’m not going to address hypergolics because I don’t have any references on current cost and I know that back when they were being produced in bulk the costs were going through the roof primarily due to the costs of handling and environmental remediation. I’m also not addressing solid propellants as their costs are driven low production volumes and availability of constituents, plus that a significant part of motor manufacture is the loading of propellant, so the breakdown of costs is different than a liquid propellant engine.
LH[SUB]2[/SUB]/LOX tends to run at about a mix ratio of 5:1 to 6:1, while RP-1/LOX generally runs about 2.3:1. The amount of propellant needed per pound delivered to orbit (includes spacecraft, payload, and upper stage depending on type of mission) is dependent upon vehicle configuration, but for conventional multistage launch vehicles it comes in at roughly about 12:1 for LH[SUB]2[/SUB]/LOX and 20:1 for RP-1/LOX. (Hydrogen should offer better performance, but the amount of additional tankage required to contain it exercises a significant mass ratio penalty, and most concepts that use liquid hydrogen like the X-33 also assume exotic lightweight tanks or else try to maximize volumetric efficiency in a broad configuration like the Boeing “Double Bubble” booster or the Chrysler SERV.) So with RP-1 we’re looking at about US$25 per lbm to orbit and even the more pricey LH[SUB]2[/SUB] comes out to around US$100 per lbm to orbit. Given that launch costs (for Western liquid propellant vehicles) come out somewhere between US$10k to US$13k per pound of payload (not including upper stages, fairings, and other non-payload parasitic mass) to low Earth orbit, you can see that even with venting losses and spare propellant to support launch delays, the costs are much less than 1% of total cost of a launch.
Trying to minimize propellant mass or lower stage mass at the expense of greater complexity or smaller margins (which add cost) is a poor bargain, at least for mass payloads. Single Stage To Orbit vehicles (which are marginally feasible at low payload mass fractions) may be suitable for personnel transport and small, high value cargo, but trying to scale a vehicle to a size sufficient to lift heavier cargos does not make them more cost-effective, especially spaceplane type vehicles which have a substantial weight penalty for reentry capability that even using air as a propellant fluid on ascent does not mitigate.
Stranger
As would I, though I don’t think that is exactly the case here even though a full working engine is some way away.
Stranger, I note your clarifications on where your concerns lie, I agree with you and I hope my position has been clarified as well.
AK84 -no, I wish it were that exciting. I may have made myself seem far more mysterious than I deserve but my coyness is just due to commercial sensitivity (I consult) and I hate giving too much away on t’internet.
I seem to recall you said once that you spent some time in New London. Thats why I thought, big submarines base that is.
Ah well.
Stranger, why are Russian space travel costs so much less than those of the US, despite having similar levels of technical sophistication.