A mach 6 composite ramjet/scramjet would replace not only that but part of the second stage as well; that is, it would allow the mass of the second stage to be significantly reduced.

Beyond that, mach 6 is by no means an upper limit for scramjets, it is just the proven capability as of this moment. Estimates of the maximum potential range from mach 10 to mach 17 depending on the assumptions, with a few best-case estimates going all the way to mach 25 (ie orbital velocity).

“Take the lower stage off a Saturn V, replace it with a scramjet. Then strap the whole thing to the biggest plane in the world (that doesn’t exist yet). Then strap a booster rocket to the scramjet in order to get to operating speed (once dropped from the plane).”

No. As Blackcat intimates, a ramjet can be started from subsonic speed. Any operational vehicle would be capable of operating in either ramjet or scramjet mode. The X-51A cannot because it is purely a scramjet test vehicle and boosting it to the required speed was more easily accomplished simply by adding a rocket.

The dropship in Aliens is pretty cool in that it had two sets of engines, conventional turbojets for takeoff and landing and a second pair of engine that have a movable spike that allows them to behave as ramjets, scramjets, or liquid-fueled rockets depending on the speed and air density. They called it a “ram-rocket” I believe. Since the craft was meant to get from the ground to low orbit it would start off in turbojet mode (using harrier-like VTOL thrusters for takeoff), accelerate to low subsonic speeds at which point the turbojet would shut off and the ramjet would take over. As it reached hypersonic speeds it would transition to scramjet mode. As the air got less dense the ramjet spike would shift forward and plug the air intake and the craft would behave as a rocket. When dropped from orbit the fall gave it enough speed for scramjet mode so the process would proceed in the opposite direction. It even had different sets of weapons specialized for different speed ranges, with small streamlined missile pods that would fold out at lower hypersonic speeds and bulkier missile and rocket booms that would fold out at low supersonic speeds.

Well, it was built before I was born, and that only went Mach 2.04!

Good response, but let’s take it further. These scramjets require forward movement in order to create a useful/stable combustion cycle. I mean, take a look at the OP video, they used a rocket (launched from a plane!) in order to take it up to the minimum operating speed.

So, what are we supposed to do? Take the lower stage off a Saturn V, replace it with a scramjet. Then strap the whole thing to the biggest plane in the world (that doesn’t exist yet). Then strap a booster rocket to the scramjet in order to get to operating speed (once dropped from the plane).

A single rocket, even with a massive first stage, starts to look pretty appealing. Fewer failure modes overall, less systems complexity.

Or, go the way that Orbital Sciences (Pegasus) and Scaled Composites (SpaceShip 1/2) did, and simply use a (fairly) conventional plane to drop a conventional rocket at altitude. That’s also what NASA did with the X-1 and X-15 ships.

Hmmm as a military weapon they could launch sheep and cows at the enemy and feed the innocent needy at the same time (only comes delivered as well done ground meat tho).

Dream use: Passenger flight anywhere in the world in a couple hours or low orbit booster.

Actual use: Recallable hypersonic weapons delivery platform.

Not against that…just sayin’.

Oooh! Toy! But seriously – where does a scramjet fit in with our current array of propulsion systems? I can imagine how it could be useful if a satellite wanted to propel a payload into the planet at high speed but I really can’t think of a use for them. It’s not useful in space, it’s not useful to get things into space. I don’t know of an instance in which a scramjet is a sensible alternative to existing systems.

In broad terms it can replace the lower staging of a booster, much as the L-1011 launch aircraft did for the Pegasus booster, but with a much higher delta/v. The scramjet stage would have a lower mass than the replaced rocket stage since it gets its oxidizer directly from the atmosphere. Pure rocket stages would, of course, still be required for acceleration beyond mach 6, but the lower (and slower stages) of a booster are naturally the largest and, all else being equal, the most expensive. Take a look at a Saturn V or any other large expendable booster to get an idea of the scale involved.
If launched from a conventional airplane, a booster incorporating the scramjet stage would be much smaller than an all-rocket booster of equivalent payload.

Are you saying any travel above 200mph isn’t safe? You ever fly on an airplane?

Oh man. Oh man. No more 14/18/20 hour international flights.

Er. Don’t get your hopes up. An airline based on this technology would probably be no more successful than the Concorde.

Who needs a bomb. The energy in that thing alone traveling at 7000kph would be enough to decimate your home!

@Grendel, i like that!

I’ll be first in line for the patented Phil Plait Scrampack, as soon as I get my hands on a patented Phil Plait Anti-Face-Melting Scramshield.

Wimp. You wear a helmet when riding a bike, don’t you? And you hold onto the railing when descending stairs too.

The human nervous system runs around 200mph.

Oh that’s a great combination and in three dimensions too.

I hereby await our < > overlords.

I can’t wait for supersonic air travel.

Forgotten about Concorde already?

Hello Scrampack – Goodbye clothes and face!

(Now there’s a disturbing image: the Bad Astronomer streaking across the sky naked with no face! That would make quite a UFO report!)