Henry,
Highly informative! I guess I’m curious if there was ever a solid
engineering reason to shy away from straight wings with ballistic reentry.
Crossrange seems to be the main reason why it was abandoned, but the
concept with its compromises seems acceptable.
On Sat, Mar 18, 2023 at 6:37 PM Henry Spencer <hspencer@xxxxxxxxxxxxx>
wrote:
On Sat, 18 Mar 2023, Matthew JL wrote:
Of course - I’m thinking of, for example, why it was never touched upon
in the 1990s-2000s for XPRIZE proposals. Or why some startup hasn’t put
forward a straight-wing spaceplane.
Well, note that Spaceship One and its descendants *do* use a Faget-style
belly-first reentry, so there are some echoes of it there.
Back in the 70s, there were some concerns about hypersonic stability --
such configurations apparently tended to spin -- and about reentry thermal
protection. And a lot of people simply distrusted the idea of having no
lift, and thus essentially no control of the flight path, until the
pitch-down at circa 40kft and low subsonic speed.
Faget's concept was actually a fairly radical idea, not something familiar
and straightforward. The very late transition from flat-on drag-brake
mode to airplane mode bought a lot of simplicity, but it clearly came from
a capsule guy, comfortable with an essentially ballistic descent until the
last minute, rather than an airplane guy. And a lot of the folks
evaluating it were airplane people, or at least people who thought that
for an operational space transport to operate like an airplane, it had to
fly like one -- which meant a lifting reentry, or at least pitching down
much earlier.
And to be honest, I'd say that there was some room to argue that the
near-uncontrolled essentially-ballistic descent was uncomfortably
dependent on getting everything right at the start, with very little fault
tolerance -- a relic of the manned-missile days, inappropriate for routine
operations of what was meant to be a reliable reusable vehicle. (As it
happened, of course, the final Shuttle ended up retaining a lot of that
make-everything-perfect-and-you-don't-need-fault-tolerance approach
anyhow, but originally it wasn't supposed to be that way.) Now, to what
extent that was true and to what extent that was just appearances, would
take a careful evaluation. But appearances certainly were against it.
The whole issue became moot once the USAF, with a launch site on the West
Coast and thus a requirement for major crossrange, was accepted as a
customer. Note, the crossrange wasn't just for once-around missions; you
also needed that crossrange for an Abort Once Around returning to the
launch site, or for a downrange abort site well away from the nominal
ascent path. Both assume greater importance at Vandenberg, where the
downrange abort options are so few -- the Eastern Pacific is very empty.
And do bear in mind that the crossrange issue *hasn't* gone away for
modern orbital-spaceplane proposers, because the guys in the blue uniforms
(okay, some of them now wear black uniforms) are still an important
customer. For that matter, there's now considerable commercial activity
in polar and near-polar orbits, and even if you don't *want* to fly from
Vandenberg, there's a problem finding a polar-orbit launch site with good
downrange abort options.
Rocketplane kind of counts towards that, possibly, but as far as I know
that was intended to be built out of a bizjet.
Only in the early days, when it was going to be a heavily-modified Learjet
for the sake of fast development. When the concept came off the back
burner after the company bowed out of the COTS program, they decided to do
a full fuselage redesign for more volume and simpler structure. (Not only
is the Learjet fuselage relatively small, but although it looks like it
has a circular cross-section, very little of it actually does.) That's
what they said at the time, anyway.
Henry
