Is a revolutionary advance in spaceflight imminent?

[darthgently]

8 hours ago, Exoscientist said:

, rather than just a few tens of times for rockets and weeks between flights.

What decade is the calendar on your wall?

[sevenperforce]

10 hours ago, Exoscientist said:

14 hours ago, sevenperforce said:

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. 

[snip]

 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.

The propulsion performance curve you've provided, while accurate as to the engine cycle itself, fails to account for the impact of drag vs acceleration on the vehicle in relation to gravity.

I do suggest you take a look at that link I provided above. (The original page is dead but I've given the path to Wayback.) The logarithmic curve takes the shape it does because of the nature of airbreathing engines. Imagine first that you have a ducted fan propeller-based engine driven by an electric motor. At a standstill, your engine is essentially acting like a fan, simply blowing air out the back at some particular velocity, which produces thrust via momentum transfer. Let's say for simplicity that the fan velocity is 200 m/s. Your maximum thrust is achieved at a standstill, because once you start moving, the amount of additional velocity you can impart to the air starts to drop. At 50 m/s, you can only impart 150 m/s so your max thrust has dropped to 75%. At 100 m/s, you can only impart 100 m/s so your max thrust has dropped to 50%. At 200 m/s, your engine can be operating at max thrust and yet you won't be accelerating at all, even if you disregard friction entirely.

(Obviously in real life propellers have differentially curved blades to allow them to continue operating at higher speeds with reduced efficiency, but this is a first-order approximation anyway.)

This point ALSO applies to jet engines. You're sucking in an airstream with some inlet velocity, mixing it with fuel, igniting it, and then allowing it to expand out the back. The thrust is the mass flow times the DIFFERENCE between the inlet velocity and the exhaust velocity. Although the amount of energy you're adding to the airstream remains essentially constant, the net thrust drops as you move faster because that difference shrinks as the inlet velocity gets closer and closer to the exhaust velocity.

But that's only half of the problem.

Airbreathers have extremely poor thrust-to-weight ratios compared to rocket engines, so you'll need to use aerodynamic lift to continue climbing. Lift-to-drag drops as Mach number increases: a Boeing 747 has a L/D ratio of around 15:1 at its cruising speed of Mach 0.85 while the Concorde and SR-71 had L/D ratios of just over 7 at Mach 2+. The X-15's L/D ratio was 4:1. The very best designs for a hypersonic aircraft might be able to achieve 5:1 at high Mach numbers.

But if your T/W ratio is 0.3 gees (which is still really impressive for a hypersonic engine), then two-thirds of your thrust is being converted into lift-induced drag just to resist gravity. If two thirds of your thrust is being wasted, then that means your effective specific impulse is just a third of what you expected. If your expected ramjet specific impulse is 1300 seconds at Mach 5.2, you'll be rather disappointed when you get an effective specific impulse of just 433 seconds. At that point you're better off just using a pure hydrolox engine for your spaceplane.

10 hours ago, Exoscientist said:

 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.

Where are you getting the idea that a hypersonic ramjet engine would be able to boast thousands of reuses with one-hour turnarounds?

[.50calBMG]

6 hours ago, sevenperforce said:

Where are you getting the idea that a hypersonic ramjet engine would be able to boast thousands of reuses with one-hour turnarounds?

Probably the same place he gets the ideas about raptors' reliability, out of date videos from the same time as when we thought shuttle would be cheap, reliable, and reusable.

[Exoscientist]

On 6/26/2024 at 1:36 PM, sevenperforce said:

…

Where are you getting the idea that a hypersonic ramjet engine would be able to boast thousands of reuses with one-hour turnarounds?

 

 It’s just based on the fact the turboramjets are derived from existing supersonic jet engines, which have lifetimes of thousands of hours. The turnaround time estimate too is based on the fact supersonic jet fighters have turnaround times between flights of only a few hours.

 About the Robert Zubrin analysis, remember he is looking at the SSTO case where an air breathing engine is making the entire flight to orbit. However, here we are discussing whether a hypersonic air breather being used as the first stage with the final stage to orbit being a hydrolox rocket, can cut the cost of spaceflight by being reused thousands of times.

  Bob Clark

Edited June 27 by Exoscientist

[sevenperforce]

3 minutes ago, Exoscientist said:

About the Robert Zubrin analysis, remember he is looking at the SSTO case where an air breathing engine is making the entire flight to orbit.

Well, no, an airbreathing engine can't make the entire flight to orbit at all.

The Isp-vs-speed math does not particularly care whether you are an SSTO or a first stage.

3 minutes ago, Exoscientist said:

However, here we are discussing whether a hypersonic air breather being used as the first stage with the final stage to orbit is a hydrolox rocket, can cut the cost of spaceflight by being reused thousands of times.

Presumably we would need to demonstrate the reuse of a hypersonic airbreather one time before supposing that a hypersonic airbreather could be reused thousands of times.

5 minutes ago, Exoscientist said:

the turboramjets are derived from existing supersonic jet engines, which have lifetimes of thousands of hours

The only turboramjets ever flown achieved an average of 234 hours of high-supersonic flight time per airframe. 

11 minutes ago, Exoscientist said:

here we are discussing whether a hypersonic air breather being used as the first stage with the final stage to orbit is a hydrolox rocket, can cut the cost of spaceflight

Let's compare to the Falcon 9 and use the typical 16 tonne Starlink type delivery. Based on your numbers the upper stage needs to deliver 6.3 km/s. This is too beefy of a job for a closed expander cycle, so let's go with a bigger engine running on staged combustion, like the currently-in-dev Chinese YF-90 boasting 2.2 MN of vacuum thrust and 453 seconds of specific impulse. The rocket equation helpfully tells us that we need a propellant fraction of 76% to make that work. The engine comes in at 4.8 tonnes so we're looking at an upper stage with a mass of at least 87 tonnes, triple the size of the Delta IV 5-meter Cryogenic Second Stage. We're looking at a length of probably 30 meters or more.

The XB-70 Valkyrie, the largest flight-tested supersonic bomber of all time, had a huge bomb bay...at just under 30 feet. So your hypersonic carrier aircraft would need to be something like triple the size of the Valkyrie.

[Exoscientist]

25 minutes ago, sevenperforce said:

Well, no, an airbreathing engine can't make the entire flight to orbit at all.

The Isp-vs-speed math does not particularly care whether you are an SSTO or a first stage.

Presumably we would need to demonstrate the reuse of a hypersonic airbreather one time before supposing that a hypersonic airbreather could be reused thousands of times.

The only turboramjets ever flown achieved an average of 234 hours of high-supersonic flight time per airframe. 

Let's compare to the Falcon 9 and use the typical 16 tonne Starlink type delivery. Based on your numbers the upper stage needs to deliver 6.3 km/s. This is too beefy of a job for a closed expander cycle, so let's go with a bigger engine running on staged combustion, like the currently-in-dev Chinese YF-90 boasting 2.2 MN of vacuum thrust and 453 seconds of specific impulse. The rocket equation helpfully tells us that we need a propellant fraction of 76% to make that work. The engine comes in at 4.8 tonnes so we're looking at an upper stage with a mass of at least 87 tonnes, triple the size of the Delta IV 5-meter Cryogenic Second Stage. We're looking at a length of probably 30 meters or more.

The XB-70 Valkyrie, the largest flight-tested supersonic bomber of all time, had a huge bomb bay...at just under 30 feet. So your hypersonic carrier aircraft would need to be something like triple the size of the Valkyrie.

 

 What I meant was Zubrin was saying an air breather couldn’t do the entire flight to orbit as an SSTO  because of the loss of efficiency at the high orbital speeds in the range of Mach 20 to Mach 25 being carried out by the airbreather. At lower Mach values say around Mach 5 to 6 an air breather can offer significant advantages as just the first stage because the ISP is so high in that range.

 What I’m envisioning is that most of the market won’t need the high payload capacity of the Starship. The launch cost would be reduced so much with airbreathers as the first stage that having payload capacity in the range of say 10 tons would cover most of the market. In other words for most of the market you won’t need a huge airbreather as the first stage to loft an upper stage large enough to get 100 tons to orbit.

 Once Hermeus is actually flying we’ll have a better idea of how much can be it’s reusability and turnaround time. 

  Bob Clark

Edited June 27 by Exoscientist

[sevenperforce]

15 hours ago, Exoscientist said:

 What I meant was Zubrin was saying an air breather couldn’t do the entire flight to orbit as an SSTO  because of the loss of efficiency at the high orbital speeds in the range of Mach 20 to Mach 25 being carried out by the airbreather.

Well, "loss of efficiency" is a bit of a misnomer. Some estimates for the maximum speed of a hydrogen-based scramjet go all the way up to Mach 24, but the theoretical maximum speed is probably closer to Mach 12-16,  with practical considerations kicking in even lower than that. So it's not "loss of efficiency" so much as it's a total shut-off in thrust well below orbital velocity.

15 hours ago, Exoscientist said:

At lower Mach values say around Mach 5 to 6 an air breather can offer significant advantages as just the first stage because the ISP is so high in that range.

Airbreathing engines are good for cruise, but not for acceleration. Mach 5.5 is less than a quarter of orbital velocity, so you've burned a LOT of props and still haven't gotten very far.

As noted, the effective specific impulse between Mach 5 and Mach 6, for an airbreather, is often less than hydrolox because of the low L/D ratio.

15 hours ago, Exoscientist said:

 What I’m envisioning is that most of the market won’t need the high payload capacity of the Starship.

I thought we were comparing to Falcon 9, not Starship.

15 hours ago, Exoscientist said:

The launch cost would be reduced so much with airbreathers as the first stage that having payload capacity in the range of say 10 tons would cover most of the market. 

10 tonnes to LEO is still pretty solidly toward the upper end of medium-heavy lift vehicles.

A dual-engine Common Centaur upper stage with a 10-tonne payload develops 4.3 km/s of Δv, well short of the 6.3 km/s you said is needed to bridge the gap between a Mach 5 hypersonic ramjet and orbital velocity. Bump up to a Centaur V and you get 6.7 km/s, which is enough...but you're now looking at dimensions of 5.4 meters wide and 33.3 meters long, plus your payload. That's at least twice the length and diameter of the bomb bay on the B-52 Stratofortress.

15 hours ago, Exoscientist said:

In other words for most of the market you won’t need a huge airbreather as the first stage. . . .

A hypersonic ramjet aircraft eight times the size of the B-52 is a pretty huge airbreather..

Let's go back to the XB-70 Valkyrie, and let's suppose that upgrading the engines and changing the fuel type can get us up to Mach 5.5 with no OML changes whatsoever. After all, it was designed to cruise for half an hour at Mach 3 but it doesn't have to cruise at all in this instantiation. Let's also suppose that the range-reducing changes can more than double both the width and length of the bomb bay, from 3x9 meters to 4x20 meters, all still without changing the OML. Finally, let's suppose that the release of a highly sensitive hydrolox rocket stage in the atmosphere at hypersonic airspeeds is a simple, easily solved problem.

A pure hydrolox solution just isn't going to work here. What we can do, however, is use a two-stage solution. Put a Centaur III on the front and a solid kick stage on the back. The Castor 30B might work. Coming in at 3.5 meters long, 2.3 meters wide, and 14 tonnes, it has 270 seconds of specific impulse, which should allow it to nudge our Centaur-III-based terminal stage from Mach 5.5's 1.9 km/s up to around 2.8 km/s. That should be enough for Centaur III to take over and get up to 7.8 km/s (neglecting gravity drag and air resistance) if we limit the payload mass to about 7 tonnes or so.

In today's dollars, each XB-70 cost over $8 billion to produce (not including operational costs), and achieved (on average) just five high-supersonic flights before being retired. That's a pretty hefty pricetag for theoretically getting less than 8 tonnes to LEO.

15 hours ago, Exoscientist said:

 Once Hermeus is actually flying we’ll have a better idea of how much can be it’s reusability and turnaround time. 

Hermeus looks like a fantastic company with some cool ideas. I have sketches of a combined-cycle turboramjet dating back years; being able to build a working one around an existing jet engine is really impressive. But these speeds still just aren't useful for orbital applications. The Halcyon's planned quad-engine, 125-passenger hypersonic business jet puts it in a payload class smaller than the P-8 Poseidon bomber, which can only carry about 10 tonnes of bombs...not really enough to get anything at all to orbit, even starting at Mach 5.5.

[Exoscientist]

17 hours ago, sevenperforce said:

…

10 tonnes to LEO is still pretty solidly toward the upper end of medium-heavy lift vehicles.

A dual-engine Common Centaur upper stage with a 10-tonne payload develops 4.3 km/s of Δv, well short of the 6.3 km/s you said is needed to bridge the gap between a Mach 5 hypersonic ramjet and orbital velocity. Bump up to a Centaur V and you get 6.7 km/s, which is enough...but you're now looking at dimensions of 5.4 meters wide and 33.3 meters long, plus your payload. That's at least twice the length and diameter of the bomb bay on the B-52 Stratofortress.…

 The Centaur V is only 13 meters long, 41 feet:

 While it is a bit wider than the Space Shuttle’s payload bay at 18 feet compared to the shuttle’s 15 feet, the shuttle’s bay was longer at 60 feet.  So a skinnyfied version of the Centaur V with the same propellant mass could fit in a vehicle with a space shuttle sized payload bay.

  Bob Clark

[domenprevc]

I believe we're on the brink of a revolutionary leap in spaceflight. With ongoing advancements in reusable rocket technology and plans for lunar and Martian missions, the future looks promising. Companies like SpaceX are pushing boundaries with Starship and Starlink initiatives, aiming for frequent space travel and satellite internet access. However, challenges like sustainable propulsion and orbital debris remain. If these hurdles can be overcome, we may witness unprecedented developments in space exploration within the next decade. Exciting times ahead for space enthusiasts and innovators alike!