a reply to: seasonal
There are many technical differences between the DC-X and SpaceX’s flyback booster that makes a direct comparison of the two of them somewhat
The DC-X was a technology demonstrator for what was supposed to be a Single Stage To Orbit (SSTO) launch vehicle. By definition, an SSTO vehicle has
to go all the way from the launch pad to Low Earth Orbit (LEO) and back. For that reason, it has to have a shape that is pretty good at rising up
through the atmosphere (starting from zero and ending up at about 8 km/sec) AND good at re-entering the atmosphere (going from 8 km/sec down to zero)
without burning up. You can see that the shape they chose is kind of a rounded conical shape with a cone half-angle of about 10 degrees and a
hemispherical nose. The shape is actually a scaled-up version of an advanced nuclear warhead entry vehicle design that was called AMARV.(See, for
). With an approximately 10-to-1 ratio
between length and diameter, this makes a reasonable good shape for a launch vehicle (similar to the Saturn booster, for instance).
The DC-X, like the AMARV utilized body flaps during re-entry to control the angle of attack and the roll angle, aerodynamically. This is the same
strategy that was used on Space Shuttle and would have allowed it to fly similar entry trajectories—meaning it could fly thousands of kilometers
cross range and maneuver to a precision landing point. The AMARV was highly tested and analyzed by McDonnell Douglas, so there was a lot of
confidence in its feasibility as an entry vehicle. That was an important factor in its selection.
Just like the Shuttle, an SSTO vehicle shaped like the DC-X would have decelerated from its entry velocity all the way down to its terminal velocity
entirely aerodynamically (in other words, without consuming any propellant). At the time it would have to pitch up and begin its terminal descent to
landing, it would have been going less than Mach 1. The rocket thrusters on DC-X were highly throttleable, meaning that the thrust-to-weight ratio
when the landing descent began was just slightly greater than 1-to-1. This means that soft landings were very easy, since you could just slowly turn
down the thrust level and allow the vehicle to settle slowly on the pad.
In the SpaceX case, it is only the first stage that flies back to the launch site. The first stage is basically just a cylinder—not streamlined at
all. It doesn’t have a good enough lift-to-drag ratio to fly very far aerodynamically. The strategy that SpaceX uses is to perform the major
course maneuvers with propulsion, not aerodynamics. That allows them to avoid adding lots of different gadgets like body flaps and simply carry more
fuel (which is cheap). In order to do their turn around maneuver, they have to fire their rocket motors after the second stage has separated and the
first stage is nearly at its highest speed. Also, they have to fire the rockets directly into the oncoming, supersonic airstream. That’s called
supersonic retropropulsion. Supersonic retropropulsion had never been demonstrated on a large scale until SpaceX did it in 2014, and NASA considered
it to be a difficult and scary problem.
The second problem is that the SpaceX booster did not have deeply throttleable engines. That means that they have to land with thrusters that have a
thrust-to-weight ratio much greater than 1. They have to time the landing burn very precisely so that they scrub off all the descent velocity
exactly and then turn off the motors at the exact moment they touch down. Otherwise, they start ascending again and then crash.
Overall, the SpaceX soft landing was much more technically difficult than the DC-X landing. SpaceX hired engineers that worked on DC-X, so they knew
what the DC-X program had learned and then they took it to the next level.