[wenyuanyu]

Not sure if I understood correctly, but does this mean that instead of going up vertically, with this engine, the "rocket" should fly near horizontally and stay in the atmosphere at the right altitude until it reached the highest possible speed given the air resistance, and then lift up by the 2nd stage rocket engines?

[lmm]

Rockets already mostly do this - they start pitching over at a fairly low altitude (10-30km), or sometimes immediately on launch, and thrust near horizontally. But yeah, theoretically an air-breathing rocket would fly lower for longer, or for some designs even dive down for part of the trajectory.

[dredmorbius]

That's actually pretty backwards.

Rockets take off vertically, then pitch over not for aerodynamics but because reaching orbital velocity requires going sideways VERY fast. They don't pitch over at very low altitudes (with rare exceptions) because the air resistance from high-speed movement is simply too great.

Among the exceptions was the Nike Hercules missile interceptor. As its target was ballistic missiles on a hypersonic ballistic trajectory, the Nike Sprint had to go very fast, in the lower atmosphere, going from 0 to Mach 10 in 15 seconds, sustaining 100 Gs and reaching a skin temperature of over 6,000°F, glowing white, within seconds of launch:

<https://yewtu.be/watch?v=kpHE9O8ckno&t=168>

Sounding rockets, used in early rocketry and atmospheric / astronomic research would in fact launch near vertically. Their goal wasn't to go orbital, but merely to get above (most) of the Earth's atmosphere.

Early US sounding rockets were the WAC Corporal (max altitude ~235,000 ft / 72 km) and Aerobee (260 mi / 418 km), each with about 60 kg payload capacity. Neither was an orbit-capable launcher.

<https://en.wikipedia.org/wiki/WAC_Corporal>

<https://en.wikipedia.org/wiki/Aerobee>

[cm2187]

I was wondering how a missile interceptor was even possible in the 50s given the insane precision required to hit a bullet with a bullet at those speeds, but it actually uses a nuclear warhead.

[pfdietz]

Spartan used a 500 kT thermonuclear warhead, I believe. It was a special warhead designed to minimize immediate beta emission in the debris cloud (which would create ionization that would interfere with radar). To that end, the tamper in the H bomb's secondary was not uranium, but solid gold.

[lmm]

> Rockets take off vertically, then pitch over not for aerodynamics but because reaching orbital velocity requires going sideways VERY fast. They don't pitch over at very low altitudes (with rare exceptions) because the air resistance from high-speed movement is simply too great.

I know. I don't know why you think that contradicts what I said.

[dredmorbius]

Because the comment you'd responded to asked "does this mean that instead of going up vertically, with this engine, the 'rocket' should fly near horizontally and stay in the atmosphere at the right altitude?"

Your response suggested that rockets do this (though your altitude comment negates some of that). They in fact don't, and get above most of the atmosphere before their horizontal-to-the-ground vector becomes significant. A key clue is that fairing separation (shedding excess weight, but constrained by the aerodynamic advantages and protections of the fairing itself) tends to occur before major pitch-over.

Note that pitch-over is not the same as the azimuth "roll program" which most launches execute immediately after clearing the launch tower itself, which is for purposes of aligning navigation, in part for the later pitch-over maneuver. Roll is not pitch-over. Everyday Astronaut has a good explainer (~22m long):

<https://yewtu.be/watch?v=kB-GKvdydho>

The problem with air-breathing engines is that they work best where the atmosphere, and aerodynamic effects, are still relatively thick, as compared to the elevations at which pitch-over occurs. Commercial flights and even very-high-altitude surveillance craft (U-2, SR-71) still operate where aerodynamics and high-speed skin heating (a factor for they hypersonic SR-71, but not the subsonic U-2). Max altitude for the SR-71 was about 25 km (82,000 ft).

Ramjets can attain altitudes of ~30+ km (record: 27.7 km, 123,500ft by a MiG-25 per StackExchange: <https://space.stackexchange.com/questions/35858/how-much-of-...>). Scramjets might be able to reach 100k ft (<https://www.nasa.gov/missions/research/x43_schedule.html>). That's getting to be close to what's useful for space launch, but whilst the altitude is useful, the velocity remains low relative to orbital velocities.

[lmm]

> fairing separation (shedding excess weight, but constrained by the aerodynamic advantages and protections of the fairing itself) tends to occur before major pitch-over.

Not for the launches I've watched, e.g. SpaceX pitches through 45 degrees at ~61km of altitude, whereas fairing separation doesn't happen until 82km altitude (by which time it's of course pitched down significantly further). Is that unusual?

> That's getting to be close to what's useful for space launch, but whilst the altitude is useful, the velocity remains low relative to orbital velocities.

True, but also potentially positive; if (big if) you can figure out the other issues, then the faster you go the higher you can continue to take in enough air to be useful.

[dredmorbius]

FWIW, I tried to find an altitude-velocity diagram of a typical rocket launch without luck. Lots of diagrams, none with specific altitude & velocity components.

61 km altitude is FL200, a/k/a 200,000 feet altitude. That's above the operating altitude of any air-breathing so far as I'm aware.

As I'd noted earlier, the SR-71 (in regular operation) was limited to FL85, and the all-time altitude record was FL123, still 77,000 feet below your SpaceX Falcon pitch-over. The SR-71 saw significant thermal heating given its speed. The only aircraft that have gone higher are the rocket-powered X-15, with an all-time record of 347,400 ft (105,900m) in 1963, and Spaceship One, at 367,490 ft. (112,010 m), in 2004. Both the latter were themselves air-launched, though largely to gain initial altitude given the power and speed achieved under rocket power.

I'm unable to read the Twitter thread itself, so if there's any specific technical capability mentioned, I'm missing it. I'd be very surprised if the designs would exceed FL100, let alone FL200.