originally posted by: penroc3
this is a nasa project using 24 ram jets as a first stage and than other off the shelf boosters to get various things to space in a quick way.
it claims to get sats and military craft to LEO for a very low cost.
this sounds like water downed TSTO system
Yup. This looks like yet another amateurish attempt by the airplane engineers at Armstrong to try to be relevant to the space launch problem.
For those who may not have read the article, it's actually a 3 stage system. The first 2 stages consist of a low-bypass turbofan powered stage,
followed by a pure ramjet stage, followed by a pure rocket powered third stage. The 2 air breathing stages only get the third stage to about 30 km
altitude and 1.2 km/sec velocity. The delta-V to get to low Earth orbit is about 8 km/sec, taking into account gravity losses, air drag, and whatnot.
That means the third stage still has to supply about 6.8 km/sec of delta-V. That means the third stage still has to be almost a single stage to
orbit rocket vehicle. That's why they had to assume a LOX-H2 upper stage--with "characteristics of the Centaur III".
The Centaur III is probably the most expensive upper stage in the US inventory.
They claim that a selling point is the fact that the first two stages use only Kerosene, thus avoiding the need for "hazardous cryogenic
propellants"and "complex reaction control systems". However, the third stage consists of exactly those kinds of technologies. So, they're avoiding
certain technologies in the first two stages because they're "bad", but using them in the third stage because they're "good". They're confused.
Of course, there have been numerous two-stage Earth-to-orbit system concepts designed that use an air breathing first stage, or perhaps an air
breathing/rocket propulsion hybrid approach, and they would seem to provide a much more optimal launch system. For example, there is the Boeing
approach patented in 1986:
Instead of separate turbojet and ramjet stages, this approach uses turbo ramjet engines in a single stage to accomplish about what this NASA approach
takes two stages to accomplish.
And of course, two stage to orbit designs tend to converge on minimum mass solutions when the split in delta-V between the upper stage and the first
stage is closer to 50-50 (about 4 km/sec, each). This is why it could make sense to add rocket propulsion to the first stage to get the separation
velocity up higher. That would probably allow the use of LOX-Kerosene in the upper stage, instead of the more expensive LOX-H2.
And finally, there's cost. SpaceX is already down to about $2500/lb and set to go lower. The NASA article presented no cost analysis of their
concept, but it requires developing 3 new stages, and landing one of them in the water (which probably renders it effectively expendable). I find it
hard to believe that this would come out cheaper than SpaceX is already demonstrating.
All in all, this concept is about what I would expect from a High School science fair.