The Reaction Engines Skylon/SABRE Master Thread (6)

[edzieba]

Please do post an image or video of a rocket running off compressed air and fuel if there is any example, even a lab experiment - running means more than a second or two.

Never flown a bottle rocket? Squirt a little Butane into an empty plastic bottle, allow it to flash to vapour and mix with the air while you screw on the cap (with a hole for the nozzle), then poke in the igniter leads through said hole and provide a spark.
Gas/Gas Butane/Air autogenous pressure-fed cycle engine, possible one of the most test-flown engine cycles on the planet!

[john smith 19]

Did you deliberately cut out from your quote what I responded to? It was merely a response for 'Star One' who seemed to suggested that just because they are now breaking ground on a facility that the Sabre engine is moving beyond the theoretical. Not yet. It may, or it may fail miserably for many reasons. (not all being technical)

Sounds like concern trolling to me. All that "Just because they have, doesn't mean they will go anywhere further."

Isn't that your usual complaint?

[adrianwyard]

The STOIC Expansion-deflection nozzle tests used H₂-Air for the air-breathing part of the nozzle. Remember SABRE 4 no longer uses a single thrust chamber and nozzle for the air-breathing and rocket modes.

A quick search found the following:

https://www.researchgate.net/publication/305442675_Testing_air-breathing_rocket_engines

[RanulfC]

Avron wrote:

In theory one needs to look at the physics - engine instability has been a massive problem - one only needs to look at Saturn F1 - keeping the flame-front attached to the injector is not trivial and that's with pure fuel and oxygen. Compressed air is far from been pure oxygen - good luck keeping the engine stable, I have yet to see in all history a rocket breathing engine - no a scramjet is a jet, not a rocket - lots of scramjets - no rocket engines.

The F1 is a VASTLY different and at the TIME complex engine that had issues due to its size not the propellants it used. It also has a high pressure system which was the cause of much of the instabilities. Similar issues happened with the SSME, again due to the high pressure nature of the engine. Considering the SABRE combustion chamber won't have a high pressure design nor does it need one why would you assume an issue that is unlikely to happen?

Point of fact? FEW SCRamjets, fewer still actually work, none of which applies to SABRE.

And you wrote:

Please do post an image or video of a rocket running off compressed air and fuel if there is any example, even a lab experiment - running means more than a second or two.

Keep in mind that just because YOU don't happen to know of any doesn't mean it hasn't been done.
As above REL tested an engine and they have been run in the past both in the US and overseas. (The RL10 was tested on compressed air and LH2 at one point but the documentation, like most of the alternate propellants is tough to come by) How much time do you have? Some links to get you started:
http://www.astronautix.com/p/propellants.html
https://history.nasa.gov/sp4232-part1.pdf (The sad part here is the subcontractor for the LACE work actually reported to the main contractors that they could and did compress and inject "deep cooled" oxygen made from air with a standard turbopump but because it wasn't 'liquid' at injection the information was ignored... in the 1950s!)
https://en.wikipedia.org/wiki/Liquid_air_cycle_engine (Not directly but it links some of the testing done with "liquid air" engines)
https://en.wikipedia.org/wiki/Air_turborocket
https://en.wikipedia.org/wiki/ATREX

Note for a nice video you can check out the "Space Access ramjet test" which shows an air-turbo rocket using LH2 and LOX AND Air. (Yes they used AIR in the ejector rocket as well as the bypass ramjet) Oh and note that neither the Air Force Laboratory or ESA/DLR noted "LH2/Air" rocket combustion as an 'issue' (either minor or major) so you MIGHT want to do some research before you claim lack-of-knowledge as being "proof" of something

Pete:

Reaction Engines plan offers:
The pre-cooler for the theoretical engine,

An engine no one has yet to claim will not work as advertised, in fact the opposite is the general consensus. It's not like we didn't already test similar engines, (no NOT SCRamjets they in fact don't work all that well so far as testing shows) in the 50s and find they worked as advertised to the specified speeds and altitudes. We also know for a fact that pre-cooling works and works quite well IF you can get the pre-cooler to work. (Which REL has shown to work and no one is saying it doesn't)

for the theoretical hypersonic airplane,

No actually it's a 'spacecraft' not an 'airplane' there are going to be large differences as the two are not, (and have never been) interchangeable platforms.

Yes in fact, "mach-5" is the acknowledged 'boundary' of hypersonic flight and since we've tested literally thousands of airframes to that speed and a bit beyond using 1950s materials and technology I'm not really sure what the point you're trying to make here is. Since it won't spend a whole lot of time at that speed before going to full rocket mode, (again American tested and showed this was a possible and working engine system if not the exact same design in the late 1950s) for climb into space I don't see why the need to point this out. EVERY launch vehicle is a 'hypersonic airframe' at some point both going up AND coming back down.

that can theoretically be optimized to the point where it can deliver small-payload to LEO.]

Uhm you DO know what we're discussing here right? 33,000lbs/17mt is VERY hard to call 'small' don't you think? A Falcon-9 only delivers a "bit" more at 50,000lbs/22mt and has to use two (2) stages to do so, one of which is still expendable at this time.

That's a lot of THEORY, and very little machinery, despite working on the concept since 1958

1982 actually unless you're going to include the US Aerospaceplane effort at which point you need to keep in mind THAT started in 1949. You seem to be conflating 'theory' with "it-hasn't-flown-as-designed" yet so it's all 'theory' which considering several MAJOR aerospace organizations have confirmed that while they don't believe the Skylon concept as it currently stands CAN actually do Single-Stage-To-Orbit still agree that the MAJORITY of the technology and specifically the propulsion system CAN AND WILL operate as planned through the suggested flight regime!

So we're back to the airframe being 'theory' but keep in mind that REL has always said the airframe isn't 'fixed' and is for reference until such a time as an actual airframe design and construction company is willing to step forward and do the work. Also consider that several airframe companies around the world have done vast amounts of work on hypersonic airframes that could mount the SABRE engines so the actually 'theory' of even the airframe is more reality contingent on financing than anything else.

Frankly it's quite amazing that the amount of people who think "Skylon" has no chance yet miss the more amazing fact that the people who actually work in the aerospace industry don't have an issue because they understand and acknowledge that the SABRE works which is really the most important fact of the matter. REL will keep pushing "Skylon" and SSTO but really, (and this is what every study and expert has said but no one is listening) even if one has to 'settle' for a TSTO vehicle the SABRE engine as a viable propulsion is THE breakthrough and the most exciting thing to come along in decades.

Randy

[ChrisWilson68]

Did you deliberately cut out from your quote what I responded to? It was merely a response for 'Star One' who seemed to suggested that just because they are now breaking ground on a facility that the Sabre engine is moving beyond the theoretical. Not yet. It may, or it may fail miserably for many reasons. (not all being technical)

Sounds like concern trolling to me. All that "Just because they have, doesn't mean they will go anywhere further."

No, he didn't say "Just because they have, doesn't mean they will go anywhere further."  He said they haven't yet.

[john smith 19]

Did you deliberately cut out from your quote what I responded to? It was merely a response for 'Star One' who seemed to suggested that just because they are now breaking ground on a facility that the Sabre engine is moving beyond the theoretical. Not yet. It may, or it may fail miserably for many reasons. (not all being technical)

Sounds like concern trolling to me. All that "Just because they have, doesn't mean they will go anywhere further."

No, he didn't say "Just because they have, doesn't mean they will go anywhere further."  He said they haven't yet.

The literal quote was "Putting the cart before the horse."

I find it fascinating that when SX amazing people talk about the schedules and wheather or not something is possible  they always assume SX can do what it says it can, when it says it can.

Booster recovery. Despite long Grasshopper test series still took 5 landings to get barge recovery worked out and realize you needed grid fins to do so.
US recovery.  Not going to happen despite 2011 video showing their (at the time) plan (missing a few key details) plan.
FH Announced IOC 2013. Flew 2018.

So far on 2 major projects SX took a lot longer than expected and on the third they've given up. So they will probably deliver a result eventually.

In contrast REL (when fully funded) have delivered what they promised, when they promised.

So I expect that when they finish their test site they will deliver what they say they can deliver and they will move on to the full engine. Because designing a jet engine is a task that's been done (literally) 1000s of times and the cycle REL uses de-couples high speed rotating machinery from the very high temperatures (and Oxygen rich environment) of the combustion chamber, radically easing the design problem.

Most people who
a)Know enough about how novel what SX is planning to do
and
b)Aren't SX amazing people
don't expect them to deliver on anywhere close to their announced schedule. It's not just that they are "unknown unknowns" it's that they have  to be flight tested with the actual full size hardware to be resolved. SABRESkylon simply designed out those issues to begin with.

Hopefully this time they've found an architecture they can make work, but then they thought that about F9 US recovery before, didn't they?

[chipguy]

So far on 2 major projects SX took a lot longer than expected and on the third they've given up. So they will probably deliver a result eventually.

In contrast REL (when fully funded) have delivered what they promised, when they promised.

SpaceX has launched 50 payloads to orbit or beyond while REL has made
a sub-component of a sub-scale engine. Comparing the two in terms of
delivering launch solutions is a complete non sequitur (my polite version).

What I find interesting is in an era of stupid money flowing into space
start-ups like no tomorrow is that REL's "universally beloved and vetted"
approach has attracted no attention outside military dreams for faster
planes.

[john smith 19]

SpaceX has launched 50 payloads to orbit or beyond while REL has made
a sub-component of a sub-scale engine. Comparing the two in terms of
delivering launch solutions is a complete non sequitur (my polite version).

Building yet another VTO TSTO ELV is indeed nothing like SABRESkylon.

Recovering a VTO stage from M6 (mostly in a vacuum) is also nothing like SABRESkylon

Being driven by a single visionary with an overall goal that sets every decision made is also nothing like REL.

REL have had very limited funding and focused on the parts that have never been done before, because those are the parts that make or break the idea.

What I find interesting is in an era of stupid money flowing into space
start-ups like no tomorrow is that REL's "universally beloved and vetted"
approach has attracted no attention outside military dreams for faster
planes.

That you know of.

REL are both pretty reticent about funding and have to work very hard to get it.  They've been completely up front when they said "It's the size of an A380. It's going to cost like an A380."
And TBH BFS is also about the size of an A380. A VTOL A380.  Yet no one thinks that's going to have any problems with it.  I look forward to the flight test programme with interest (and that's me being very polite).

However as SX is a private company making significant profits it can afford to avoid discussion of how much BFR/BFS will cost to develop.

I'd love to see the REL investor pitch. I suspect it could be capable of improvement but I also note that it's kept them running since 1989. There are very few companies left in this business that age that are a) Not massive defense conglomerates and b) Making significant progress on executing their design

[docmordrid]

>
However as SX is a private company making significant profits it can afford to avoid discussion of how much BFR/BFS will cost to develop.
>

I'm guessing you've not watched either IAC presentation, or followed their online comments?

[john smith 19]

>
However as SX is a private company making significant profits it can afford to avoid discussion of how much BFR/BFS will cost to develop.
>

I'm guessing you've not watched either IAC presentation, or followed their online comments?

And you'd be wrong. The question is wheather or not I believe them.

In fact, looking again at the presentations I'm still trying to find a number for the development cost of the BFR/BFS, not the mfg cost.

[Each is the size of a A380/787. 
Those took about $12Bn to develop, from companies that have at least 40 (or in Boeing's case more like 90) years of doing so.

Do you have a number for SX's development cost?

[Elmar Moelzer]

And you'd be wrong. The question is wheather or not I believe them.

In fact, looking again at the presentations I'm still trying to find a number for the development cost of the BFR/BFS, not the mfg cost.

[Each is the size of a A380/787. 
Those took about $12Bn to develop, from companies that have at least 40 (or in Boeing's case more like 90) years of doing so.

Do you have a number for SX's development cost?

SpaceX has demonstrated to be a lot more efficient than traditional aerospace companies when it comes to development, but I am still sure it is several billion.
With all profits going to BFS development in the coming years, they should _just_ have enough to make it happen. Once their satellite constellation comes online, things will look completely different anyway in terms of company funds.

[envy887]

>
However as SX is a private company making significant profits it can afford to avoid discussion of how much BFR/BFS will cost to develop.
>

I'm guessing you've not watched either IAC presentation, or followed their online comments?

And you'd be wrong. The question is wheather or not I believe them.

In fact, looking again at the presentations I'm still trying to find a number for the development cost of the BFR/BFS, not the mfg cost.

[Each is the size of a A380/787. 
Those took about $12Bn to develop, from companies that have at least 40 (or in Boeing's case more like 90) years of doing so.

Do you have a number for SX's development cost?

2016 IAC estimate was $10B dev cost for the large ITS. Both the Saturn V and Shuttle, the most comparable projects, cost around $40B in initial dev costs, adjusted for inflation. Since SpaceX's cost structures are less than 1/4th of NASA's, evidenced by Falcon, FH, and Dragon, the $10B is not entirely unfounded.

REL doesn't have history of their own costs for actual flight vehicles to draw on for estimates (and probably wouldn't even be building flight vehicles). I'm not aware of any remotely similar project ( that actually flew) to Skylon, which is a much larger advancement in the state-of-art than BFR/BFS. The closest is probably X-33/Venturestar, which burned $1.2B without flying the subscale tech demo.

[Elmar Moelzer]

2016 IAC estimate was $10B dev cost for the large ITS.

I think it is safe to assume that development of the smaller BFS will be less than that?

[su27k]

>
However as SX is a private company making significant profits it can afford to avoid discussion of how much BFR/BFS will cost to develop.
>

I'm guessing you've not watched either IAC presentation, or followed their online comments?

And you'd be wrong. The question is wheather or not I believe them.

In fact, looking again at the presentations I'm still trying to find a number for the development cost of the BFR/BFS, not the mfg cost.

[Each is the size of a A380/787. 
Those took about $12Bn to develop, from companies that have at least 40 (or in Boeing's case more like 90) years of doing so.

Do you have a number for SX's development cost?

If we're using aircraft development cost as estimator, then it's worth pointing out that Falcon 9 is similar in weight to a 737 or A320. A quick look at Wikipedia shows the latest A320 upgrade A320neo costs $1.3 billion to develop, the latest 737 upgrade 737MAX's development cost is even higher. Comparing these to what we know about Falcon 9's development cost: $400M initially + $1B reusability upgrade, it looks like SpaceX's development cost for a totally new launch vehicle with never seen before technology is equivalent to Airbus/Boeing doing an upgrade on their existing airframe.

[CameronD]

If we're using aircraft development cost as estimator, then it's worth pointing out that Falcon 9 is similar in weight to a 737 or A320. A quick look at Wikipedia shows the latest A320 upgrade A320neo costs $1.3 billion to develop, the latest 737 upgrade 737MAX's development cost is even higher. Comparing these to what we know about Falcon 9's development cost: $400M initially + $1B reusability upgrade, it looks like SpaceX's development cost for a totally new launch vehicle with never seen before technology is equivalent to Airbus/Boeing doing an upgrade on their existing airframe.

Is that a surprise??  It shouldn't be.  One look at photos of SpaceX's factory floor shows very little difference in design and manufacturing techniques.  It's not "never before seen technology"... it's just a novel way of putting aerospace-rated components together... and flying them.

[john smith 19]

Is that a surprise??  It shouldn't be.  One look at photos of SpaceX's factory floor shows very little difference in design and manufacturing techniques.  It's not "never before seen technology"... it's just a novel way of putting aerospace-rated components together... and flying them.

True

Until stage recovery was made to work most of SX's novelty was not in what it did but how it did it.
(compared with common aerospace industry practice).  IE Mostly building in house, not mandating aerospace suppliers exclusively.

But. A LOX/RP1 fueled TSTO with Aluminum tanks and gas generator cycle engines? That's about as low risk a design as it gets. The existing knowledge base in all those areas was huge.

SpaceX has demonstrated to be a lot more efficient than traditional aerospace companies when it comes to development, but I am still sure it is several billion.

Let me suggest that statement is highly debatable.

What SX proved (but had been long suspected) was that the US standard NAFCOM cost model (derived mostly from cost plus government contracts) is grossly distorted relative to project where companies have to put up their own money and don't have to meet Govt regulations FAR 22 procurement rules.

But where FAR22 (like Dragon 2) then they seem to run at about the pace (and cost) of Boeing.

Jess Sponable made a similar observation his ability to deliver the DC-X (M3, 4xRL10 engines, LO2/LH2 propellant) for c$63m in the early 90's, by following commercial procurement rules, not government.

2016 IAC estimate was $10B dev cost for the large ITS. Both the Saturn V and Shuttle, the most comparable projects, cost around $40B in initial dev costs, adjusted for inflation. Since SpaceX's cost structures are less than 1/4th of NASA's, evidenced by Falcon, FH, and Dragon, the $10B is not entirely unfounded.

I could believe that price for a single stage of this size. But a) the BFR and BFS designs are now quite significantly diverged, and the test programme significantly more complex than for an ELV (or even a semi RLV), given the re-mating involved.

REL doesn't have history of their own costs for actual flight vehicles to draw on for estimates (and probably wouldn't even be building flight vehicles).

Actually REL have been very careful to do cost modelling. IIRC they use Dieter Koells cost modling approach.

That's why they expect the whole SABRESkylon/SUS programme to be $12Bn.

Of course if that same cost model said F1/F9 to first flight came out as $2Bn perhaps we should revise SABRESkylon's price down a bit as well?

I'm not aware of any remotely similar project ( that actually flew) to Skylon, which is a much larger advancement in the state-of-art than BFR/BFS.

One of those statements that sounds plausible but is highly debatable given developments so far.

Fuel. Methane is the propellant de jour but no ones actually built an LV (full orbital) that uses it yet. Hundreds of LH2/LO2 stages launched since the early 1960's, including all SAturn V US's, Centaurs and Shuttles.

Engine. Already de-scoped once. Goal of 1000s of re-uses. I don't doubt the number of hours on the airframe can be increased from the SSME. I have serious doubts they will get anywhere near that in the first generation.

Tanks. Rocket Labs have demonstrated 1.5m CFRP LOX tanks in an ELV during full launch conditions.
No reuse. No demonstration of Methane. Not just SoA, also Start of the Art.

Structure. Tanks are aeroshell. Aeroshell is tanks. SOP for VTO LV's but the big question mark is BFS. Like Shuttle it has to be strong in two axes and resist both aero and thermo forces (which is partly why NAA split it off into an ET).
There is no history for large CFRP structures with the range of cyclic stresses, over the range of temperatures, over the flight number. None. You're talking something the size of a wind turbine blade, accelerating up to M23, then back again, filled with LOX and liquid Methane.

TPS. That seems to be a ground up design (insofar as we know anything about it at all). Ceramic tiles apparently. BTW from the NASA Shuttle TPS DB refurb costs for tiles were $12 000/ Sq. m, while the same for blankets were $3000 /Sq.m.  Given the size of BFS (I'll assume BFR can get the job done with PICAX) they will have to do better, otherwise that refurb bill is going to be quite high.

Then we get onto the landing and mating process, and the on orbit refueling. Again no history of doing this and no way to avoid doing it.

The closest is probably X-33/Venturestar, which burned $1.2B without flying the subscale tech demo.

If only that were true, at least in the funding 
With that kind of money REL would have built their test bed engine by now and a flight test vehicle to demonstrate transition from air breathing to full rocket mode.

As has been pointed out repeatedly X33 was essentially LM's  strategy to preserve it's ELV business, against the threat of an SSTO actually working. It succeeded brilliantly in doing that. They played the NASA procurement process and loaded it up with all sorts of unnecessary junk leading edge technology and NASA took the bait hook, line and sinker.

No doubt the staff on the ground were keen enough to succeed but to outsiders (in the US and elsewhere) who studied the project it was clear senior LM management designed it to fail.

And guess what. It did. 

[su27k]

Is that a surprise??  It shouldn't be.  One look at photos of SpaceX's factory floor shows very little difference in design and manufacturing techniques.  It's not "never before seen technology"... it's just a novel way of putting aerospace-rated components together... and flying them.

True

Until stage recovery was made to work most of SX's novelty was not in what it did but how it did it.
(compared with common aerospace industry practice).  IE Mostly building in house, not mandating aerospace suppliers exclusively.

But. A LOX/RP1 fueled TSTO with Aluminum tanks and gas generator cycle engines? That's about as low risk a design as it gets. The existing knowledge base in all those areas was huge.

Note stage recovery R&D cost is included in the estimate. If you skipped the $1B cost of reusability research, then the low risk design of F9 v1.0 only costs $400M to develop, less than 1/3 of the cost to develop a comparable airliner.

After they went to v1.1 then v1.2, it's no longer low risk design or novel way of putting aerospace-rated components together, it's something else all together. It's the only Kerosene TSTO that can launch significant mass to GTO, no one else comes even close.

[envy887]

Skylon also needs fuel, engines, structures, and TPS that not only have never been flown, they haven't even gotten part component level ground testing. It requires much more advancement in SOA than BFR does, and somewhat more than BFS.

BFS will use an ablative, almost certainly pica-x. BFR will likely use metallic TPS. Both will be flight proven on Falcon/Dragon. BFR will work reasonably well even if SpaceX grossly misses many of their performance targets. I'm pretty sure that's not the case for any SSTO concept.

[john smith 19]

Note stage recovery R&D cost is included in the estimate. If you skipped the $1B cost of reusability research, then the low risk design of F9 v1.0 only costs $400M to develop, less than 1/3 of the cost to develop a comparable airliner.

However since an airliner has to carry untrained passengers it's hard to say how much of that cost is down to having wings and how much to testing it well enough to ensuring it's safety is at a level that modern airline passengers will accept.
BTW SABRESkylons development cost estimate includes a full test programme

After they went to v1.1 then v1.2, it's no longer low risk design or novel way of putting aerospace-rated components together, it's something else all together. It's the only Kerosene TSTO that can launch significant mass to GTO, no one else comes even close.

Perhaps you should further qualify that last sentence with "that's been designed in the last 20 years in its weight class without the backing of a government."
All parameters of which make a substantial difference to how advanced it is.
IOW In a 1 horse race the 1 horse is, by definition, the lead horse.

[john smith 19]

Skylon also needs fuel, engines, structures, and TPS that not only have never been flown, they haven't even gotten part component level ground testing. It requires much more advancement in SOA than BFR does, and somewhat more than BFS.

Except BFR is not a rocket, is it? It's a stage.
We think it will work but even then you're not even 1/2 way to orbit.

I'll repeat once again that SABRESkylon is a High risk/High cost/High reward programme.
Yet in fact BFR/BFS will be in the same cost range and comparable risk levels without similar rewards for anyone but SX.

The REL design has its share of issues that need to resolved, but the difference is most of the SABRESkylon design can be debugged before construction, on the ground, because the theory to design all the parts, and the trajectory it will fly already exists

Truss structure design may no longer on the curriculum for Aerospace engineering courses but they are still a standard design technique for many other areas. Likewise its takeoff and climb are well within common state of practice. "Zoom climb" at 45deg is fairly well know in military circles. Shuttle, X37b and Buran  have all demonstrated the reentry flight path and both X37b and Buran demonstrated fully automated runway landing.

In contrast BFR/BFS operating mode is unique. There is literally no vehicle anywhere, or anywhen that's operated like this. 
It's a 787 sized tail sitter,  a class of aircraft not seen anywhere since the lat 1950's and was single seat even then.
But it's a tail sitter that has to come down on another stage.

BFS will use an ablative, almost certainly pica-x. BFR will likely use metallic TPS.

Are we surea about that? I tried to find the job ad for a TPS that emphasized metallic TPS but I couldn't.
The one I did see talked a lot about the underlying technology of rigid ceramic tiles, and implied this is going from first principles.  Lots of experience of various diagnostic hardware, up to SEM's and XRD's. You don't see those on a factory floor and you don't need them if the mfg process is pretty much locked down.

Both will be flight proven on Falcon/Dragon.

Really? AFAIK Dragon  2 will also be PICAX. IIRC most (all?) development work on PICA X has been to make it cheaper to mfg, not to improve it's operating properties.

BFR will work reasonably well even if SpaceX grossly misses many of their performance targets.

Again, BFR is not a LV, it's a stage of a launch vehicle.  If LEO is "Halfway to anywhere" then BFR is "Not quite half way to being halfway to anywhere."

BFR with a payload fairing is essentially an expendable SSTO. It can't survive full reentry from orbit and it will take a very large payload hit to get there in the first place. The whole LV is about 1100t, so maybe 11 tonnes to LEO? 1% is the usual expected payload for VTO rocket powered SSTO. That's not even an F9 and launching an expendable BFR as an SSTO LV will be quite expensive.

The trouble is getting it a bit wrong on BFR will have serious knock on effects to BFS, given it's not a straight cylinder, it's now got wings as well, and the mass trade on payload to structure is now 1:1.

I'm pretty sure that's not the case for any SSTO concept.

And it's the thinking that sort of verbal short hand engenders that means you need to spell out exactly your assumptions.
Because I think you mean any vertical take off rocket powered SSTO.

Where a) Thrust must exceed GTOW or no TO to begin with b)Rocket grade Isp's mean mass ratios are high and payload mass fractions low.

What people can't seem to grasp is that air breathing gives  you an Isp about 6x that of the best (useable) rocket propellant. That "buys" a lot of structure. So you can use wings, that give you benign failure modes. You don't need staging (eliminating separation failure modes) and you get a payload mass fraction equal to the same sized ELV. SSTO's historical inability to match TSTO payload fraction has been the killer to the concept.

That's what Skylon gives you. SSTO convenience with ELV payload fraction, on your launch schedule, not anyone else's.