Examine Figure 2 in the linked article. This is a very useful plot and the first time I've seen something like this (in a previous life I worked on scramjets for a living).

Notice that hydrogen fueled scramjets perform much better than JP fueled versions. Hydrogen fuel requires a very large storage volume due to its very low density. This means a higher vehicle surface area (skin friction) drag due to the bigger fuel tank.

The blue trace on the graph shows the performance of a "combined cycle" (turbojets and scramjet). The black trace shows the combined cycle of scramjets and rockets. Scramjets flake out somewhere below Mach = 6 and somewhere above Mach = 10 (the red traces shown in the plot for scramjets are speculative). The combined cycles are necessary to get the vehicle from Mach = 0 to orbital velocity. The author quietly assumes that a scramjet powered vehicle would not be Single Stage to Orbit (SSTO) because he does not show the combined rocket --and-- turbojet combined cycle with scramjets.

The vertical axis is Specific Impulse although the author calls it "Propulsion Efficiency" (why?). Specific Impulse is only one parameter-of-merit. Thrust is another parameter-of-meter. You want high thrust to get something off the ground and overcome atmospheric drag. However, once you have sufficient altitude, you then want maximum specific impuse. For example, rocket based expendable launch vehicles (ELV) typicallly uses a low Isp / high thrust first stage and then lower thrust / high Isp upper stages. Another graph which is not shown and possibly never has been created is a bar graph showing the ratio of payload mass (to Low Earth Orbit) divided by propulsion system mass for each propulsion system type. There is probably not enough experimental and flight data available to produce a believable graph (shame on NASA, ESA and the USAF). I suspect a scramjet based system would compare very poorly against a well designed ELV using ordinary rocket motors.

This sort of stuff is high level rocket science and not enough R&D funding is being directed towards it.

Gary