Cancelling the sonic boom, a different way


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6 June 2006
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Remember RHEINBERRY ? it was to fly between 200 000 and 130 000 feet, horizontally, but on rocket power - not with wings, and not with turbofans, as above 100 000 feet there is not enough oxygen left for airbreathing and lift.

And the Bell X-1 could fly horizontally at Mach 0.95 on pure rocket power, provided it was throttled the right way. Bottom line: with enough (rocket) thrust, even bricks can fly.

Aaaaaall right, I wanted to ask... whatif a rocketplane coming down from orbit into reentry, managed to get into horizontal ROCKET-SUSTAINED flight way above 100 000 ft, and then gradually throttled down the rocket engine to go subsonic ?

The idea: cancelling the sonic boom by going "subsonic" outside the atmosphere, since: no atmosphere, no boom.

Would that ever work ? (I guess the sonic boom (well, physics) would have a word to say, akin to "Hell, NO, we refuse to cooperate".)

Just asking in passing, if the idea is absurd, then shoot it down.
as above 100 000 feet there is not enough oxygen left for airbreathing and lift.

Sez who? Lift is proportional to air density times velocity squared. The SR-71 clearly trucks along just fine at Mach 3 and 90,000 feet. According to the Standard Atmosphere, the density at 90,000 feet is 0.56. At 200,000 feet, it's .0053. A difference of approximately 100, so an SR-71 would need to fly at approximately Mach 30 (ten squared times Mach 3) to generate the same lift, a clearly impractical solution, as that's beyond orbital velocity. But at 150,000 feet, density is 0.037, a difference of about 15.15, requiring the SR-71 to fly 3.9 times faster, or about Mach 12. Flying at Mach 12 is conceivable, especially on rocket power or with a practical scramjet.

As for negating sonic booms via suborbital hops, that was one of the selling points of using Starship for that purpose. There'd be booms near launch and landing, but silence along the bulk of the route.
That's the point, I want to try and cancel the landing booms by going subsonic very, very high - horizontal flight, on rocket power. Is there a way to achieve that ? How high would it take to get no sonic boom on landing ? at least one not heard on the ground ? 150 000 ft ?
That would likely take too much fuel. Now, an OK-92 type Shuttle 2-an all passenger version-could re-enter over the ocean, and use jets to fly subsonic like any other passenger plane if designed well.
The fundamental problem with this idea is shedding kinetic energy. A de-orbiting spacecraft must slow by something like 15 times the speed of sound before it is travelling slow enough not to melt in the lower atmosphere. You need as much rocket fuel to do that as it took your second stage to accelerate into orbit in the first place. Plus the fuel needed to accelerate that deceleration reserve into orbit with you, and a first stage at least twice as big as before to get it all up there. It is utterly impractical and would be ludicrously expensive. Hence the development of aerodynamic braking and heat shielding. Sonic booms are not a problem anyway, as the craft is already subsonic by the time it gets low enough to worry about them.
SpaceX relies on a combination of the two techniques, maintaining a small fuel reserve for final braking and touchdown. But for a heavy lift their orbiter cannot carry enough reserve even for that.
I remember standing in the parking lot of our local public library (a fairly quiet place ;)) in Southern California underneath the return flight path of one of the last Shuttle missions to land in Edwards AFB, waiting to hear the double boom. If you knew what you were listening for, you could clearly discern it, but a misfiring car engine or even a slammed car door nearby would easily have masked it.
True…but you always have busybodies like “GSE Hound” attacking Musk over methane…
Same with the coming supersonic bizjets—-much weeping over noise pollution. You KNOW that’s coming.
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