Skylon triumphant!

RGClark · 2025-07-18 10:13:08

Skylon is set to be reborn:

INVICTUS - Leap into the future of space
https://vimeo.com/1101204441/719f1e7261

A major problem with Skylon was it supposed a hydrogen turbojet and such still does not exist. Another major problem was its $12 billion price tag. Both of those problems can now be resolved. First, adapt an existing jet-fuel turbojet. The American hypersonic transport concern Hermeus is taking that approach.

Better, use cryogenic methane since turbomachinery in general can be switched between hydrocarbon fuels, including natural gas, i.e., methane. The cryogenic methane would also have greater cooling capacity and be lighter than jet-fuel.

About the $12 billion price tag, when REL estimated program costs it used standard NASA cost metrics. But SpaceX and multiple other space start-ups have shown development costs can be cut by a factor of 10 by *private financing*. Then, beyond that, use existing and operational space systems. A well known fact in the space industry is the individual cost of a space system, rocket stage, spacecraft, etc., can be 1/10th to 1/30th of its development cost.

Together, the development cost might be cut down to only $120 million to $40 million(!) See:

From supersonic jet to hypersonic transport. Page 2: F-104 Starfighter.
https://www.linkedin.com/feed/update/ur … 2192621569

I've been informed by a member of the Invictus team that while they will be using Skylon's hydrogen precooler, they will use a COTS(commercial off the shelf) jet engine.

I had suggested using instead of hydrogen a hydrocarbon fuel such as cryogenic methane for the jet engine. The reason is hydrogen-fueled turbojets don't yet exist. In fact the leading company researching them, Rolls Royce, poured cold water on their own research program suggesting one might not be fielded for anther 20 years.

Nothing beats having that engine in front of you where you can experiment with various characteristics of the engine to improve performance. Note this is the route Hermeus is taking with their precooler and jet engine, and why they are already proceeding to flight testing. What the REL team has only been able to do though is simulate the airflow from their precooler to a turbojet intake since the actual hydrogen-fueled engine was not available.

The Invictus team though will be able to do actual engine tests with their design. Still, I'd like to see the trades of their approach of hydrogen precooler with jet-fueled turbojet vs. an all cryogenic methane approach where the precooler and jet engine are both adapted to use cryogenic methane.

To save development costs and time I wanted also to adapt an existing airframe. Using the fact also that fully private financing can cut the development cost by 1/10th, the two factors together of private funding and using existing and operational systems, can cut the development cost by a factor of 1/100 or more, to the $120 million to $40 million range. In the prior post I suggested the F-104 Starfighter. But several different supersonic airframes could be used. For a UK based research program one possibility might be the Saunders-Roe SR.53:

https://en.wikipedia.org/wiki/Saunders-Roe_SR.53

By swapping out its single jet engine for two of the current highest performance jet engines, it too could reach Mach 5. Note my estimate of ca. $120 million development cost was for a Skylon-sized vehicle of ca. 50 ton dry mass. But these two small jet fighters the F-104 Starfighter and Saunders-Roe SR.53 are at approx. 1/10th that dry mass. Then the development cost might be only in the range of $12 million(!) Admittedly a development cost of a hypersonic transport being that low would be quite remarkable. But note that fully operational F-104's are available for private ownership at less than $100,000. And two high performance jet engines can be obtained for ca. $4 to $5 million each:

From ChatGPT:

____________________________________________________________________________________________

Query: What is price of some recent military jet engines?

Response:

Here’s a breakdown of the unit prices (approximate) of some recent military jet engines, especially those used in current-generation fighter jets. Prices can vary by contract, production batch, and included accessories (like spares or maintenance), but these estimates are in the right range:

? U.S. and Allied Military Jet Engines
Engine Aircraft Estimated Unit Price (USD) Notes

F135-PW-100 /-600 F-35A/B/C (all variants) $12–15 million Most expensive; includes stealth/nozzle tech
F119-PW-100 F-22 Raptor $10–12 million Twin-engine; advanced thrust vectoring
GE F414-GE-400 F/A-18E/F Super Hornet, Gripen E ~$4 million Mature, reliable design
F110-GE-129 / -132 F-16 Block 50/52+, F-15 variants $4–5 million Powerful versions used in upgraded fighters
Eurojet EJ200 Eurofighter Typhoon ~$6–8 million Multinational development, modular
Klimov RD-33MK MiG-29K/M ~$3–4 million Russian naval fighter engine
Saturn AL-41F1S Su-35 ~$5–6 million High thrust; precursor to Su-57 engine
Saturn Izdeliye 30 Su-57 (future standard engine) ~$7–10 million (projected) Still under development; thrust vectoring
____________________________________________________________________________________________

Finally the F-104 Starfighter and the Saunders-Roe SR.53 both had variants where actual rockets were attached at the back to increase speed and/or altitude. Then when the purely airbreathing hypersonic flight is achieved, progressing to actual orbital flight could be reached by attaching a separate rocket to the back. Skylons original approach was to incorporate a rocket mode into the airbreathing engine. However, the thrust to weight ratio of rocket engines is so high, only a relatively small amount in payload would be lost by adding a separate rocket engine for the purpose.

Bob Clark

Last edited by RGClark (2025-07-18 10:14:34)

tahanson43206 · 2025-07-18 10:21:27

This post is reserved for an index to posts that may be contributed by NewMars members.

Index:

(th)

tahanson43206 · 2025-07-18 18:02:36

This is a follow up to the opening announcement ...

https://www.yahoo.com/tech/science/arti … 00502.html

Space
Europe working to launch 'Invictus' hypersonic space plane by 2031 (video)
Mike Wall
Thu, July 17, 2025 at 3:00 PM EDT
3 min read
Artist's impression of a space plane that could result from Europe's Invictus program.
Credit: Frazer-Nash Consultancy
Europe doesn't want to be left out of the space plane party.
The European Space Agency (ESA) is funding the development of a hypersonic space plane pathfinder, which will start flying by 2031 if all goes according to plan.
The work is being done via a research program called Invictus, which is led by the consulting firm Frazer-Nash. Invictus will leverage technology developed by the English company Reaction Engines Ltd., which aimed to build a huge space plane called Skylon but went bankrupt last year.
illustration of a grayish-silver space plane flying above the clouds
Artist's impression of a space plane that could result from Europe's Invictus program. | Credit: Frazer-Nash Consultancy
The key piece of Invictus tech is a "pre-cooler," which Reaction Engines built and tested for its Synergetic Air-Breathing Rocket Engine (SABRE). SABRE combined aspects of jet and rocket propulsion; it was designed to pull oxygen out of the air during flight at lower levels of Earth's atmosphere, reducing the need to carry propellant and therefore increasing efficiency.
"Aircraft that fly at hypersonic speeds — more than 5 times the speed of sound — face extremely high temperatures due to shock heating and the friction from the air. Typical aircraft engines cannot operate in these conditions, as the air is too hot to handle," Frazer-Nash representatives said in a statement.
This pre-cooler solves this problem, cooling "the air before it reaches the engine, allowing conventional aircraft engines to travel at hypersonic speeds," Frazer-Nash added.
Invictus isn't a huge project; its funding is 7 million pounds (about $9.4 million US at current exchange rates), according to the company. But it could have a big impact on European spaceflight, according to ESA.
"Hypersonic flight is not just the next frontier of aerospace — it is the gateway to a new paradigm of mobility, defense, and space access," Tommaso Ghidini, head of the Mechanical Department at the agency, said in the same statement.
"With Invictus, Europe is seizing the opportunity to lead in technologies that will redefine how we move across the planet and reach beyond it," he added. "By mastering reusable, air-breathing propulsion, we are laying the foundation for aircraft that take off like planes and reach orbit like rockets — revolutionizing both terrestrial and orbital transportation."
— Facts about Reaction Engines' Skylon space plane
— Space Force aims to launch 1st 'Foo Fighter' satellites in 2027 to track hypersonic threats
Advertisement
The plan calls for the Invictus team — a consortium led by Frazer-Nash that includes Spirit AeroSystems and Cranfield University, among other partners — to deliver "the concept and elements of preliminary design of the full flight system" 12 months from now.
The "full flight system" will be a reusable vehicle that takes off from, and lands on, a runway like an airplane. It will be up and running by early 2031, if all goes to plan, and could have a variety of uses and applications.
"We look forward to seeing how the work develops and the opportunity it presents for boosting economic growth and national security," Tony Forsythe, head of space technology at the U.K. Space Agency, said in the same statement.
Invictus isn't the only European space plane in development. Last month, for example, the French government and the French company Dassault Aviation announced plans for a demonstrator called VORTEX.
Space planes are experiencing something of a resurgence after the retirement of the most famous such vehicle — NASA's space shuttle — in 2011. The U.S. military operates a robotic orbital space plane called the X-37B, for example, and China has a similar vehicle, called Shenlong. Virgin Galactic flies a suborbital space plane for tourism and research purposes.
A number of other companies are developing space planes as well, including Sierra Nevada Corp., Dawn Aerospace and Radian Aerospace.

(th)

RGClark · 2025-07-27 14:18:26

Skylon Triumphant! Towards the SSTO.

The new program just announced called Invictus to continue the Skylon project will use a standard jet-fuel engine instead of hydrogen-fueled though it will still be precooled using hydrogen.

This may be an unrecognized blessing. My opinion is the major stumbling block from Skylon progressing is it assumed a hydrogen-fueled jet engine and such did not and still does not exist.

Nothing beats having that engine and precooler right in front of you where you can vary engine and precooler parameters to optimize performance. Skylon instead had to simulate this since they did not have a hydrogen engine at hand.

By taking the approach of using an existing jet-fuel engine, the American hypersonic concern Hermeus is rapidly proceeding to actual test flights. I predict the same will be possible with Invictus.

However, a question I had involved an aspect of the graphic attached below showing the T/W ratio and specific impulse of the Skylon compared to other propulsion methods.

The notable aspect of the Skylon Sabre engines is they used a precooler ahead of the turbojet engines. But what’s so remarkable is it increases the thrust by a factor of 3. The specific impulse is actually reduced(though still high as an airbreather) but that tripling of the thrust for an airbreathing engine is surprising.

The importance of this is that it has been frequently argued about producing an SSTO of why not just put some jet engines on it for the first part of the flight? The problem is jet engines are so heavy. Commonly their T/W ratio is only ca. 5 to 7.

The key question arises from the fact that graphic shows with a precooler the T/W ratio of a hydrogen airbreathing engine can be increased by a factor of 3. Will that increase also apply for a jet-fueled turbojet for the thrust to be increased 3 times?

The highest T/W ratio jet engine we have is at ~11.5 in the F135 engine. Then with precooling could the T/W ratio be increased to the range of 35 to 1??? If so, then that would be a radical advancement in the feasibility of an SSTO, since the weight of the jet-engines will be approaching that of rockets while still having the high Isp of airbreathers.

1-s2.0-S1000936120302144-gr1_lrg.jpg

Bob Clark

GW Johnson · 2025-07-27 16:18:41

Those Isp charts are marketing hype. I first saw them about 50 years ago. No one design will ever follow any of those curves, even at low altitude, which low altitude is what the T/W ratio chart really is.

I have said it before, and I will say it again: ANY airbreather of any type whatsoever, has a service ceiling! This is because ALL airbreathers (of any type whatsoever !!!) have a combustion chamber pressure that is pretty much fixed as a ratio to the local atmospheric pressure. That combustion chamber pressure ratio to ambient is pretty well proportional to the engine thrust, whatever type of airbreather it is. That's just basic thermodynamics, a subject few ever take. I majored in it, among other things.

At very high altitudes, thrust is low because ambient atmospheric pressure is low. That is INHERENT with any type of airbreather whatsoever. This has been known since the end of WW1 (yes, I said WW 1 !!!) and there have been no violations of that trend since then. Only the ratios vary with engine type. But once you are high enough, there is no significant thrust, because your ratio multiplied by essentially nothing for an ambient pressure is essentially nothing as thrust.

The highest airbreathing engine operation ever achieved was in a French ramjet test many decades ago, at 125,000 feet. I am entirely unsure whether thrust was greater than drag up there, but I am entirely sure that thrust was less than weight. This was a zoom climb test that apogeed out at no speed, and fell back. Even scramjets are VERY UNLIKELY to develop significant thrust at such an altitude.

Service ceiling is defined by a 200 fpm climb rate under the FAR's, but effectively it is the altitude at which your design is barely generating lift equal to weight at a relatively-efficient angle of attack near that for high vehicle L/D, and your thrust, being all you can deliver, is essentially just barely equal to your drag. You cannot accelerate to higher speed, and you cannot climb at any humanly-sensed rate. PERIOD. End of issue.

Rockets are not subject to that limitation, since their chamber pressure is utterly independent of the ambient atmospheric pressure, at any altitude whatsoever. You cannot presume that a ramjet or a scramjet is going to give you enough thrust to both climb and accelerate at altitudes in the 100,000-125,000 foot range. If you don't have thrust to accelerate your vehicle mass, Isp is worthless BS. Simple as that!

Do not be fooled by names: an "air turborocket" (ATR) is an airbreathing engine, not a rocket, and not a very efficient airbreathing engine at that. I know, I literally have worked on air turbo rockets, as well as ramjets and gas turbines, and of course piston engines.

There is no "magic" here with any kind of airbreather, ramjet, scramjet, or anything else remotely conceivable! It has a speed limit beyond which efficiency and Isp reduce. But the altitude limitation for ANY of them is down nearer 100,000 feet (30 km), than anything you might ever need to reach orbit.

Second point I have covered before: the Skylon airframe will NEVER survive reentry, not with wingtip-mounted engines! No entry spacecraft has EVER featured parallel-mounted nacelles, and no entry spacecraft ever will! That answer has been known since the 1968 X-15A-2 flight that reached Mach 6.67 at about 100,000 feet altitude. It is called "shock impingement heating", and it is fatal, even at low hypersonic speeds.

Those who fail to learn from history are doomed to repeat history's failures. PLEASE wake up and smell the coffee! Talk to some of us oldsters who were there, when the mistakes and the successes were made! Not everything was ever written down in the final reports, not by a long shot!

GW

Last edited by GW Johnson (2025-07-27 16:23:14)

RGClark · 2025-07-28 08:20:59

GW Johnson wrote:

Those Isp charts are marketing hype. I first saw them about 50 years ago. No one design will ever follow any of those curves, even at low altitude, which low altitude is what the T/W ratio chart really is.
I have said it before, and I will say it again: ANY airbreather of any type whatsoever, has a service ceiling! This is because ALL airbreathers (of any type whatsoever !!!) have a combustion chamber pressure that is pretty much fixed as a ratio to the local atmospheric pressure. That combustion chamber pressure ratio to ambient is pretty well proportional to the engine thrust, whatever type of airbreather it is. That's just basic thermodynamics, a subject few ever take. I majored in it, among other things.
…
GW

A majorly important advantage of the new Invictus reboot of Skylon is it will use an existing jet-fuel turbojet rather than a hydrogen-fueled one that Skylon had been supposing. A hydrogen-fueled turbojet engine still does not exist and in fact the leading jet engine manufacturer studying them Rolls Royce has stated one won’t be fielded for another 20 years.

On the other hand, having the engine and precooler actually in place and undergoing testing means you can vary the different parameters of both and confirm, or disconfirm, its estimated performance values.

By the way an interesting question occurred to me looking at those performance curves: can precooling break the Mach 6 ramjet barrier?

Prior analysis of ramjet propulsion had led to the conclusion they can only be effective up to the range of ca. Mach 5.5 to Mach 6.

After that, you need scramjets to get positive net thrust with airbreathers. The problem is nobody has been able to get scramjets to provide positive net thrust for longer than just a few seconds.

But if you look at the graphs of the T/W ratio and Isp of the precooled Sabre engine compared to that of ramjets approaching Mach 6, you notice the Isp for the ramjets is rapidly trending downwards to that of just rockets, while the Sabre’s Isp is gradually leveling off. Does this mean the precooled Sabre could still get effective thrust past Mach 6?

The attached image here show specs for the original Sabre that used a high level of cryogenic precooling, while the image in my previous post was for a later version of Sabre with lower precooling. The first version with the high precooling has higher thrust but lowered Isp. But what’s notable is the Sabre’s Isp graph seems to be leveling off approaching Mach 6, while the ramjet’s Isp graph is rapidly dropping off.

So there arises the question: Is there a combination of precooling level at a prescribed outside air pressure level that would allow the Sabre to maintain positive net thrust even past Mach 6?

Bob Clark

GW Johnson · 2025-07-28 09:44:56

The rationale for even trying scramjet is that keeping the air supersonic lowers its thermodynamic static temperature, delaying ionization. That is the fuzzy barrier that ramjet faces at about Mach 6: combustion energy release starts partitioning more and more into ionization instead of static temperature rise. You do not get ionization energy back out as speed in a nozzle. I'm unsure as to why; I just know that it does not work, experimentally. Probably a severe residence time mismatch, but that's just an educated guess.

What that really says is that scramjet also faces the same kind of ionization-induced speed barrier, just at a far higher speed. Another educated guess says that's why the "speed limit" quoted for hydrocarbon-fueled scramjets is somewhere near Mach 10, when hydrogen-fueled scramjets can fly faster (nearer Mach 20). The species from hydrocarbon combustion are different and some of them ionize easier.

When you add some sort of pre-cooler to the system to cool the air, you delay the onset of ionization problem to higher speeds. But you do it with a loss of some of the energy in that captured air, unless you can contrive to put it directly into the fuel to be burned. There are limits to that, and also process inefficiencies. Those act to limit what you can do and how fast you can really fly. I suspect those are what limits the Sabre engine to about Mach 5 in any practical sense, and maybe Mach 6 in a best-case theoretical sense.

Yeah, you might add a pre-cooler to a ramjet, but if you did, you would negate its main advantage of very high thrust/weight. That pre-cooler stuff is heavy. It only works precisely because it has to use hydrogen fuel. That's also heavy.

--- UPDATE 7-29-2025: The tanks are heavy because they have to be so large, liquid hydrogen being of such low density. But the pre-cooler DEPENDS FUNDAMENTALLY on the fuel being liquid hydrogen! There is no fast pre-cool without the intense cold of liquid hydrogen. That's not me saying that, it's the Sabre engine people!

NASA's X-43 scramjet tests were 3 second burns of gaseous hydrogen, 2 successes for 3 attempts, one at almost Mach 7, and the other at almost Mach 10. These were rocket-boosted to test speed, and done above 100,000 feet where the thrust and drag were quite small compared to weight. Why? To lower airframe aeroheating. These did NOT accelerate as airbreathers.

USAF's X-51 scramjet tests were 3 minute burns of JP-7 thermally-stable kerosene, 2 successes for 4 attempts, both at Mach 5. These were rocket-boosted to test speed, and done above 100,000 feet where the thrust and drag were quite small compared to weight. Why? To lower airframe aeroheating. These did NOT accelerate as airbreathers.

UPDATE 7-29-2025: JP-7 was a unique early version of thermally-stable kerosene, used in the SR-71 variants to resist wet-wing exposures to very hot skins in the 400-500 F range. The X-51 tests used up the last of those stocks, there is no JP-7 being made anymore. Even so, thermally stable kerosenes are being made, but today they are additive packages in JP-8.

In contrast, the one hypersonic flight of ASALM was an accidental throttle runaway on the very first flight test. This was a liquid ramjet burning RJ-5 (a.k.a. Shelldyne-H), a synthetic resembling very strongly kerosene, but slightly denser than water. It was intended to cruise at Mach 4, at 80,000 feet, and then average Mach 5 in a terminal dive onto its target, not having time to heat up and melt itself. The runaway happened in ramjet after boost to takeover, launched at only 20,000 feet at Eglin AFB over the Gulf, and accelerated away dramatically in ramjet to just about Mach 6. It was still accelerating when it ran out of fuel, reporting by telemetry that its skins were beginning to melt.

UPDATE 7-29-2025: We lost the bird for 3-4 days, then found it stuck like a giant steel dart, in a farmer's field, about 10 miles outside of Eglin AFB. ASALM-PTV was 20 inches in diameter, and about 15 feet long. The weaponized form was longer, with a longer fuel tank section.

This was a wingless chin inlet design with a wave rider flattish bottom (a technology that goes back to the F-100 Super Sabre in 1953, by the way). It was aerodynamically very clean, so its top speed was likely somewhere between Mach 6 and 7, we'll never really know. It was made of martensitic stainless steel construction, limited to about Mach 4 steady state by aeroheating. Made instead of something like Rene 41, it might have survived steady-state at about Mach 5.5-to-maybe-6. The inlet chin cowl and buried subsonic air duct passage would have been most at risk from the aeroheating.

GW

Last edited by GW Johnson (2025-07-29 10:44:00)

New Mars Forums

Announcement

#1 2025-07-18 10:13:08

Skylon triumphant!

#2 2025-07-18 10:21:27

Re: Skylon triumphant!

#3 2025-07-18 18:02:36

Re: Skylon triumphant!

#4 2025-07-27 14:18:26

Re: Skylon triumphant!

#5 2025-07-27 16:18:41

Re: Skylon triumphant!

#6 2025-07-28 08:20:59

Re: Skylon triumphant!

#7 2025-07-28 09:44:56

Re: Skylon triumphant!

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