Is a revolutionary advance in spaceflight imminent?

[Exoscientist]

  No, I don’t mean SpaceX. Several companies rather are investigating hypersonic air-breathing vehicles. These are vehicles that draw the oxidizer from the air like jet engines. Reaching hypersonic speeds would be to Mach 5+. The easiest approach to these speeds is to use ramjets. Scramjets can reach even higher speeds but are a much more difficult technological task.

I found the most promising approach that taken by Hermeus. They were able to keep costs down by using already existing turbojet engines and modifying them to operate in a ramjet mode:

 

But a key fact about the hypersonic air-breathing vehicles is how much they can subtract off from the delta-v needed to reach orbit. 

Mach 5.5 is about 5.5*340 m/s = 1,870 m/s. But ramjets can also operate at quite high altitudes, well above standard turbojets, to about 30+ km, 100,000+ feet. The equivalent delta-v for that altitude is about 800 m/s. Then ramjets can supply 1,870 + 800 = 2,670 m/s delta-v to orbit. It turns out the trajectories that use aerodynamic lift can subtract some amount from the needed delta-v to orbit, so call it 9,000 m/s needed for orbit using lifting trajectory.

Then the delta-v that needs to be supplied by the upper stage would be 9,000 - 2,670 = 6,330 m/s. For expendable rockets the first stage commonly supplies about 4,000 m/s total delta-v speed+altitude with about 5,000+ m/s being supplied by the upper stage. But for rockets for which the first stage will be reusable like the Falcon 9 and the Superheavy booster, they supply a smaller total delta-v speed+altitude  to the flight of about 3,000 m/s. This is so they can boost back to the launch site more easily. 

Then this means the air-breathing hypersonic vehicle can supply about the same total delta-v, speed+altitude, as the reusable booster. But the majorly important advantage is the air-breathing vehicle can be reused thousands of times, compared to only a few ten's of times for the rocket booster. Since the first stage by virtue of its large size commonly takes up 75% of the cost of a launcher, this means the cost of  launch will be greatly reduced when that first stage can be reused thousands of times.

The implications of this will be immense. It means for example despite SpaceX spending billions developing the SH/SS it may already be obsolete just by 2025 when hypersonic vehicles become operational and are used as the first stage of an orbital vehicle. 

 The implications don’t seem to be recognized by these companies developing the hypersonic aircraft. They only discuss using them for hypersonic transports or hypersonic missiles. The applications to low cost, high reusability first stages does not seemed to be realized.  If Hermeus succeeds, then it will become important to recognize how that success can be repeated by other companies in the aerospace industry. The adapting of existing turbojet engines to hypersonic turbo-ramjet hybrids as described by Hermeus is not particularly advanced technology nor of high cost.  Other companies can repeat it as well. I advise though not just adapting the existing turbojet engines but also adapting existing supersonic aircraft. Hermeus built their own hypersonic test vehicle. Costs could be cut further by adapting an existing supersonic airframe as well as its supersonic capable engine.

 The result will be the democratization of hypersonic transport aircraft, and thereafter the democratization of spaceflight.
   
 I think the hypersonic advance will be successful, and it will thereby be used to launch payloads to orbit as a first stage, greatly reducing costs.

However, I think it possible it will also make possible another key advance: combined air-breathing/rocket SSTO's. The designers of Skylon have argued this will be possible with their Sabre engine able to reach Mach 5.5. The problem with Skylon is it's projected $12 billion development cost. But by adapting already existing turbojets to ramjets, and utilizing existing airframes plus an existing hydrolox stage for the rocket portion of the flight this cost can be radically reduced, perhaps to only 1/100th of that, a few hundreds of millions of dollars.

See discussion here:

Low cost approach to winged, air-breathing and rocket SSTO's, Page 1.
https://exoscientist.blogspot.com/2024/06/low-cost-approach-to-winged-air.html

  

Robert Clark

Edited June 21 by Exoscientist
Edited for readability

[NFUN]

that's quite an invisible wall of text

[magnemoe]

The problem is that you still need an second stage you have to release at hyper sonic velocity. An falcon 9 second stage with payload is over 100 ton. 
For smaller rockets its easier, for larger one like starship its 1300 ton. Over 5 times the AN-225 cargo weight so forget it. 

Now if you reach mach 15 second stage size go down a lot, but this is stuff who is pretty theoretical as in no prototypes has flown. Just making fast planes are expensive, making an high high capacity hyper-sonic transport / bomber is much more expensive. 
They will eat the small launcher market, then they want to build an 20 ton payload hyper-sonic heavy bomber / first stage and it would be an hard sell. 

Its much harder to scale fast planes than rockets. 
Now if you launched 1000 ton to orbit daily its an marked for stuff like this. 

[Nuke]

i renumber burt rutan's tech talk about how fast we went from shoddy gliders to landing on moon, and that the past few decades have done nothing nearly as impressive as that burst in development. with spacex and others pushing the envelope again, this slump seems like its to come to an end. these kind of development cycles seem to happen in bursts. we just get to the point where we dont feel like barking up the same tree even though there might be juicer fruit on the higher branches. nuclear is in a similar situation and because of a couple bombs and 3 accidents at powerplants were afraid a coconut will land on our head and we leave that tree alone.

based entirely off the half a paragraph i managed to read before my eyes wigged out.

also i do kind of like the idea of winged boosters. its kind of the role skylon should be designed for rather than ssto. it doesnt even need to go to orbit or even build up enough speed where reentry becomes a hot mess. it just needs to loft stage two high enough so it has time to complete a circularization burn.  of course the end result to that is an overcomplicated falcon that can take off from any airport. what we need to do is figure out how to make that kick stage recoverable. but it has to deal with the same issues that starship does. so again it feels like a hard way to reinvent the wheel.

still the landing part of reusable second stages is going to be a huge problem both here and on other bodies (heat shield is still an issue but it looks like we have made some headway at least, eg move the flap roots leeward as planned, even make them retractable, eg on linear bearings). think we really need to start looking at lateral engine mounting, except now the turbopumps have to be able to move propellant half way up the ship under thrust gravity. as a second stage this can be reduced depending on whatever loft time your first stage (winged or otherwise) gave you. also fine for lifting off of moon/mars and keeps the regolith out of the engines. but then your structural loads from thrust need to be handled and more mass required (possibly also from a slightly bigger turbopump) again less of a problem at low gravity (real or otherwise) but you must also support the weight of the engine. and losing one of the two pods would be bad. multiple engines per pod would be better but then you need to isolate them better so you dont get a cascade failure in the pod. complex geometry like this also makes re-entry problematic (see starship's liquid metal flaps). heat shields like to be simple.

Edited June 19 by Nuke

[magnemoe]

58 minutes ago, Nuke said:

i renumber burt rutan's tech talk about how fast we went from shoddy gliders to landing on moon, and that the past few decades have done nothing nearly as impressive as that burst in development. with spacex and others pushing the envelope again, this slump seems like its to come to an end. these kind of development cycles seem to happen in bursts. we just get to the point where we dont feel like barking up the same tree even though there might be juicer fruit on the higher branches. nuclear is in a similar situation and because of a couple bombs and 3 accidents at powerplants were afraid a coconut will land on our head and we leave that tree alone.

based entirely off the half a paragraph i managed to read before my eyes wigged out.

All of the early space race was an sprint with lots of money behind it. Probably inspired by the WW 2 peoples thinking just build more than the enemy and we win, it works if your economy is much larger than the enemy. 
But technology did not progress much, shuttles was cool but an bridge to far, design was over engineered like an F1 car as the margins was so slim but F1 cars who breaks down rarely kill the driver. 

Again I don't disagree, but outside of an electron killer this is way down the line.

[Hannu2]

I do not believe that those hypersonic planes would be important in foreseeable future. They may get some very special application for example in military use but not massive commercial success. New rockets (Starship, New Glenn, Vulcan) will be next step and make new things possible but I would not say it revolutionary to send million cheap satellites for entertainment use. 

I do not believe any large scale manned operations in my lifetime (about 30-40 years). There are not real will and attitude needed for pioneering exploration anywhere in the world today. There will not be manned Mars trips or large operations on Moon before states and funders learn to handle failures, losses and deaths. There may be some short propaganda Moon operations from USA and China but not more.

[Exoscientist]

On 6/19/2024 at 1:52 PM, NFUN said:

that's quite an invisible wall of text

Re-sent using unformatted text.

 Bob Clark

Edited June 21 by Exoscientist

[Exoscientist]

On 6/19/2024 at 3:09 PM, Nuke said:

i renumber burt rutan's tech talk about how fast we went from shoddy gliders to landing on moon, and that the past few decades have done nothing nearly as impressive as that burst in development. with spacex and others pushing the envelope again, this slump seems like its to come to an end. these kind of development cycles seem to happen in bursts. we just get to the point where we dont feel like barking up the same tree even though there might be juicer fruit on the higher branches. nuclear is in a similar situation and because of a couple bombs and 3 accidents at powerplants were afraid a coconut will land on our head and we leave that tree alone.

based entirely off the half a paragraph i managed to read before my eyes wigged out.

also i do kind of like the idea of winged boosters. its kind of the role skylon should be designed for rather than ssto. it doesnt even need to go to orbit or even build up enough speed where reentry becomes a hot mess. it just needs to loft stage two high enough so it has time to complete a circularization burn.  of course the end result to that is an overcomplicated falcon that can take off from any airport. what we need to do is figure out how to make that kick stage recoverable. but it has to deal with the same issues that starship does. so again it feels like a hard way to reinvent the wheel.

still the landing part of reusable second stages is going to be a huge problem both here and on other bodies (heat shield is still an issue but it looks like we have made some headway at least, eg move the flap roots leeward as planned, even make them retractable, eg on linear bearings). think we really need to start looking at lateral engine mounting, except now the turbopumps have to be able to move propellant half way up the ship under thrust gravity. as a second stage this can be reduced depending on whatever loft time your first stage (winged or otherwise) gave you. also fine for lifting off of moon/mars and keeps the regolith out of the engines. but then your structural loads from thrust need to be handled and more mass required (possibly also from a slightly bigger turbopump) again less of a problem at low gravity (real or otherwise) but you must also support the weight of the engine. and losing one of the two pods would be bad. multiple engines per pod would be better but then you need to isolate them better so you dont get a cascade failure in the pod. complex geometry like this also makes re-entry problematic (see starship's liquid metal flaps). heat shields like to be simple.

 Thanks for the informative post. I’m a fan of Peter Diamandis series of books on “exponential technological advance”. He makes the point that some tech advance can seem very slow, in fact stagnant. This was the case with nuclear fusion research for example for decades, and also with hypersonics. But then it reaches an inflection point and the technology advances exponentially. I believe that is where we are at with fusion and hypersonics. 

The Future Is Faster Than You Think: How Converging Technologies Are Transforming Business, Industries, and Our Lives (Exponential Technology Series)
https://www.amazon.com/Future-Faster-Than-You-Think-ebook/dp/B07TD6GPVR/

 Bob Clark

Edited June 21 by Exoscientist

[JoeSchmuckatelli]

1 hour ago, Exoscientist said:

the technology advances exponentially

I posted a thread a while back looking for the 'killer app' of the current period (much like the railroad, electricity, internal combustion engine and microchip were the killer apps of decades past).  The technology had to be disruptive but also allow / enable advances in other areas - as well as being able to mature and permeate the economy. 

The consensus was that AI is the likely candidate. 

With people harnessing AI, we may indeed see advances in the technologies you mentioned - or we might not.  Hypersonic is a solution to the desire to get somewhere faster - but it's not the only path to that end.  With fusion - I agree that it is exciting to live in a time when major advances are likely in the next 10-20 years 

[Nuke]

i think fusion is coming but the process of getting there is expensive and begrudgingly slow. most of the delay is just putting the ginormous puzzle together. even just building up the highway infrastructure to make it possible to ship the coils was pretty impressive, not to mention a huge delay and extra cost. the always 20 years away seems like the amount of time it takes to build a massive tokamak. from first funds to first plasma. then we have to do it again for demo, unless they can perhaps do a retrofit of the existing reactor. 

focusing mostly on iter because i dont think the startups are going to pull this off. most of the updates are in the form of "fusion can save the world but first we need money". iter has at least solved the funding problem.

Edited June 21 by Nuke

[Exoscientist]

17 hours ago, Nuke said:

i think fusion is coming but the process of getting there is expensive and begrudgingly slow. most of the delay is just putting the ginormous puzzle together. even just building up the highway infrastructure to make it possible to ship the coils was pretty impressive, not to mention a huge delay and extra cost. the always 20 years away seems like the amount of time it takes to build a massive tokamak. from first funds to first plasma. then we have to do it again for demo, unless they can perhaps do a retrofit of the existing reactor. 

focusing mostly on iter because i dont think the startups are going to pull this off. most of the updates are in the form of "fusion can save the world but first we need money". iter has at least solved the funding problem.

 The irony of the rapid development of hypersonics is that it will be replaced not very long after with fusion propulsion.

 That’s the thing with being in the exponentially rapid advancement part of the curve. Why spend a great deal of money on something when by the time you have it operational it will be made obsolete by another  advancement?

  Bob Clark

[Nuke]

fusion propulsion might actually come before fusion power, as break even is not needed.  especially if were just doing mars/moon for the most part and can use solar with next gen cells (multi junction or even quantum dot). further out and you need some kind of power plant, this can be fission.

waiting for the next new thing you can easily miss opportunities to do stuff now. like if i applied that to my computer id never upgrade and id still be playing '90s games. there is no reason not to pursue intermediate technologies.

Edited June 22 by Nuke

[magnemoe]

6 hours ago, Exoscientist said:

 The irony of the rapid development of hypersonics is that it will be replaced not very long after with fusion propulsion.

 That’s the thing with being in the exponentially rapid advancement part of the curve. Why spend a great deal of money on something when by the time you have it operational it will be made obsolete by another  advancement?

  Bob Clark

Fusion engines is likely to be large as in assemble in space even with Starship and most realistic designs are open designs who would not work in atmosphere.
Also low trust until we have good fusion. 

[Nuke]

low thrust is fine if your travel time is like 18 months anyway.

[Entropian]

Rockets are flying.  These hypersonic aircraft are not.  I think it is incredibly naive to presume that a novel system will go from not even having a single flight test to "obsolete" a flight-proven design... all by the end of next year.  It could happen, but certainly not over such a minuscule timescale.

[Exoscientist]

1 hour ago, Entropian said:

Rockets are flying.  These hypersonic aircraft are not.  I think it is incredibly naive to presume that a novel system will go from not even having a single flight test to "obsolete" a flight-proven design... all by the end of next year.  It could happen, but certainly not over such a minuscule timescale.

Agreed, saying it will take over spaceflight in 2025 is overly optimistic. But if Hermeus succeeds at their hypersonic  test flight in 2025 it presages hypersonic transports within a few years, with accompanying low cost flights to space.

  Bob Clark

[Nuke]

i think the general misconception is that focusing on one thing detracts from the other thing. you never know what kind of spinoffs you can get out of any line of research and some of those might be useful in other lines of research.  you put x amount of resources into one thing and you get a new technology out of it. you put 2x into one thing and you get that tech a little faster, likely with diminishing returns. however if you put 2x resources into two somewhat related things, you might get both things faster but also a third thing. its multiplicative rather than additive.

[magnemoe]

18 hours ago, Nuke said:

low thrust is fine if your travel time is like 18 months anyway.

Who is true, but getting into orbit who is  the discussion here very different from deep space and will always be I say as long as technology is realistic. 
Say you have good high trust fusion, then an SSTO plane makes perfect sense, you would want to switch to an deep space ship with variable spin gravity for an month long trip to Jupiter. 

[sevenperforce]

On 6/19/2024 at 1:50 PM, Exoscientist said:

Mach 5.5 is about 5.5*340 m/s = 1,870 m/s. But ramjets can also operate at quite high altitudes, well above standard turbojets, to about 30+ km, 100,000+ feet. The equivalent delta-v for that altitude is about 800 m/s. Then ramjets can supply 1,870 + 800 = 2,670 m/s delta-v to orbit. It turns out the trajectories that use aerodynamic lift can subtract some amount from the needed delta-v to orbit, so call it 9,000 m/s needed for orbit using lifting trajectory.

Then the delta-v that needs to be supplied by the upper stage would be 9,000 - 2,670 = 6,330 m/s. For expendable rockets the first stage commonly supplies about 4,000 m/s total delta-v speed+altitude with about 5,000+ m/s being supplied by the upper stage. But for rockets for which the first stage will be reusable like the Falcon 9 and the Superheavy booster, they supply a smaller total delta-v speed+altitude  to the flight of about 3,000 m/s. This is so they can boost back to the launch site more easily. 

Then this means the air-breathing hypersonic vehicle can supply about the same total delta-v, speed+altitude, as the reusable booster. But the majorly important advantage is the air-breathing vehicle can be reused thousands of times, compared to only a few ten's of times for the rocket booster. Since the first stage by virtue of its large size commonly takes up 75% of the cost of a launcher, this means the cost of  launch will be greatly reduced when that first stage can be reused thousands of times.

The only hypersonic aircraft that has ever been reused, the X-15, used rocket-based propulsion and required a drop from a carrier aircraft. No attempt has ever been made to reuse scramjets. The fastest actually-reusable aircraft capable of taking off under its own power, the SR-71, had a maximum speed of Mach 3.5 (1.2 km/s) and flew an average of 540 times each across the 32-craft fleet, with at least a week of maintenance required between each flight.

It is much easier to build a reusable rocket stage than it is to build a reusable hypersonic airbreathing vehicle. Hypersonic airbreathing makes full-flow combustion look simple.

Also notable that the payload fraction for a hypersonic aircraft is extremely low. The SR-71  had a max takeoff weight of 78 tonnes and carried just 1.6 tonnes of actual mission payload (let's round up to 2 tonnes to account for the pilots and cockpit weight), a payload fraction of 2.6%. Compare this to Falcon 9 -- the first stage has a mass of 443 tonnes compared to the second stage mass plus payload of 131 tonnes, a payload fraction of 22.8%. To loft a second stage and payload comparable to the Falcon 9, your notional hypersonic launch vehicle would need to have a rolling takeoff mass of over 5,000 tonnes, nearly nine times the size of the largest plane to ever leave the ground, the Antonov An-225 .

Edited June 25 by sevenperforce
forum glitch

[sevenperforce]

On 6/22/2024 at 11:36 PM, Exoscientist said:

if Hermeus succeeds at their hypersonic  test flight in 2025 it presages hypersonic transports within a few years

Who's to say that hypersonic jet transport is any more feasible -- as a business case -- than hypersonic rocket transport?

[farmerben]

1 hour ago, sevenperforce said:

Who's to say that hypersonic jet transport is any more feasible -- as a business case -- than hypersonic rocket transport?

Obviously it does better by not carrying oxidizer.  Surely that's cheaper.

[sevenperforce]

3 minutes ago, farmerben said:

Obviously it does better by not carrying oxidizer.  Surely that's cheaper.

Not when your vehicle has to be ten times larger because the engines have a TWR that is a dozen (or more) times smaller.

[StrandedonEarth]

It’s the challenge of getting air (only 20% oxygen, remember) into the engine at hypersonic speeds, still flowing smoothly and without melting things. At least a rocket can get above the soup, to where things won’t melt from Mach heating. 

[sevenperforce]

2 hours ago, sevenperforce said:

2 hours ago, farmerben said:

Obviously it does better by not carrying oxidizer.  Surely that's cheaper.

Not when your vehicle has to be ten times larger because the engines have a TWR that is a dozen (or more) times smaller.

By way of elaboration:

The engines have a TWR that will start fairly low, increase modestly with velocity as ram effect compression initiates, then drop to extremely low -- along with specific impulse -- at hypersonic velocities. This is all due to the airbreather's burden. At hypersonic velocities, an airbreathing spaceplane is fighting like crazy to even maintain speed, because it has to somehow gulp up air, accelerate that air to its own speed, then burn and push that air out the back end faster than it entered, while a rocket continues to merrily accelerate without a care in the world. The effective specific impulse of an accelerating spaceplane drops so low at hypersonic velocities that it will generally require several TIMES more fuel than a pure rocket, despite avoiding the need to carry oxidizer.

[Exoscientist]

10 hours ago, sevenperforce said:

By way of elaboration:

The engines have a TWR that will start fairly low, increase modestly with velocity as ram effect compression initiates, then drop to extremely low -- along with specific impulse -- at hypersonic velocities. This is all due to the airbreather's burden. At hypersonic velocities, an airbreathing spaceplane is fighting like crazy to even maintain speed, because it has to somehow gulp up air, accelerate that air to its own speed, then burn and push that air out the back end faster than it entered, while a rocket continues to merrily accelerate without a care in the world. The effective specific impulse of an accelerating spaceplane drops so low at hypersonic velocities that it will generally require several TIMES more fuel than a pure rocket, despite avoiding the need to carry oxidizer.

 That will happen at the higher scramjet speeds, and even then only for hydrocarbons. Scramjet theoretically could operate beneficially up to say Mach 15. That is to say, if they could be made to operate reliably. 

 Ramjets have been seen to operate at Mach 3.5 to 4. Theoretically they should be able to reach Mach 5.5. That is why the test by Hermeus next year is so important. If they can reach Mach 5.5 even if not  used as a first stage for an orbital rocket then it can be used as a hypersonic transport.  Instead of 6 hour flights cross-Atlantic or cross-continental USA they could be done in 1 hour.

 For the business case compared to rockets, such flights would be less costly being able to be reused for thousands of flights and within, say, a one hour reuse time like jet engines, rather than just a few tens of times for rockets and weeks between flights.

 

 Bob Clark

Edited June 26 by Exoscientist