There is a conundrum associated with launching to low Earth orbit from an airplane. The illustration tries to sum up the various parts of it. That is not to say that it cannot be done, because it already has. But, it may, or may not, be an attractive way to do the mission.
The first part of this conundrum is the low speed of the
launch aircraft (which for the Pegasus system is a wide-body subsonic
airliner). That forces the dropped
rocket vehicle to be at least two-stage,
despite the advantage of the low stratospheric launch altitude. As it says in the illustration, speed at drop is the biggest influence on the
rocket vehicle design, and altitude the
least, although both are
beneficial. Mach 0.85 at 45,000 feet is
but 822 feet/sec (0.25 km/s). The drag
loss of the rocket vehicle is (at least theoretically) less, because it starts in thinner air up high.
The second part of this conundrum is not so obvious: the level path angle of the carrier
airplane at the drop point. A
low-loss non-lifting ballistic trajectory begun at stratospheric altitude would
need a path angle at ignition on the order of 45 degrees, maybe even a little more. So,
either the carrier airplane, or
the rocket vehicle, has to pull up
rather sharply, to reach that path angle
from level flight. One or the other must
do this!
The usual airplane flying high in the stratosphere is at or
near its “service ceiling”, where there
is barely enough wing lift being produced at an efficient angle of attack, to hold up the weight, and essentially all the thrust the
airbreathing engines can make is just overcoming drag at the flight speed! The airplane can neither accelerate
path-wise, nor can it climb! That is the definition of “service ceiling”, and for most planes, it falls in the 45,000-55,000 foot altitude
range, at high subsonic speeds. There have been exceptions: the U-2 variants and the SR-71 variants could
fly higher, being very specialized
designs.
Left unaddressed in the airplane, the service ceiling problem puts the sharp pull-up
task squarely upon the rocket vehicle to be dropped. There are only two choices: put wings on the rocket vehicle, or fly it at very large angles of attack,
so that the cross-path vector component
of its thrust is effectively a large lift force.
Pegasus used large wings,
on the first stage of a two-stage rocket vehicle. Those add both weight and drag, especially drag-due-to-lift at the large
lift coefficient needed to pull up sharply.
That pretty-well eats up the advantage gained by launching the rocket at
elevated altitude in the thin air. The
wings are bigger than you would want,
precisely because of that thin air!
And that problem is why there have just not been that many Pegasus
launches.
Leaving the wings off of the rocket vehicle forces you to
pitch it up to very large angles of attack,
in the 45-75-degree range, to get
enough of a cross-path vector component of the rocket thrust, to serve as the necessary lift force for a
sharp pull-up maneuver. That reduces the
path-wise vector component of thrust,
while at the same time greatly increasing vehicle drag. So,
you accelerate slowly( if at all) in rocket thrust during the pull-up
maneuver, using up a great deal of
rocket propellant that adds nothing to your speed. That also eats up any advantage of launching
in the thin air, way up high!
The only other feasible alternative is to add another large
source of thrust to the launch airplane,
so that it can execute the pull-up maneuver into a zoom climb, without stalling and falling out of the
sky, out-of-control. Generally speaking, you would add a source of thrust immune to
the service ceiling effect, and that is
rocket thrust! Your launch
airplane would have to be modified for mixed (parallel-burn) rocket and gas
turbine propulsion, somewhat
similar to the NF-104 and some of the early high-speed X-planes.
So far, no
air-launch carrier plane has had this design approach, but it certainly would be possible! And it would take care of the high path angle
requirement that is second only to speed at launch in importance, while keeping the wings on the airplane
where they belong, and not on the
rocket vehicle!
That leaves speed at launch,
the most important variable affecting the rocket vehicle design. There are (or have been) very few supersonic
aircraft designs that are also large enough to serve as a drop aircraft for a
rocket vehicle of any significant size.
Those would include the B-58 Hustler (long-retired, and none are left), the SR-71 (also retired, but very expensive to operate indeed), and the B-1B bomber (currently in service as
a military strategic bomber).
The modifications to include rocket propulsion to the SR-71
likely would not fit within its very-critical shape. The M-21 variant that launched the D-21 drone
was limited in payload size, to the size
of that drone (not very large). A rocket
might be added in the tail cone of a B-1B,
but its payload would be limited to that which would fit in its bomb
bay. That B-1B option would reach a low
supersonic launch speed at the high path angle needed, with a rather-dangerous zoom climb and
recovery after drop.
That brings up the danger of supersonic store
separation. There is a very good
reason that most military aircraft, even
those capable of supersonic flight, are
limited to high-subsonic weapon release speeds.
That is because the inherent wobbles of a released store will include
pitch-up, thus developing lift. At high enough speeds, that lift generated by the wobbling store
will exceed its weight, and it can
easily fly up and collide with the drop aircraft, before the store’s drag can pull it behind.
It cost a destroyed airplane and the life of one of the two
crew, to learn this lesson with the M-21
trying to launch a D-21 drone (without a booster) at just about Mach 3. That is why the drone was re-fitted with a
big booster, and launched subsonically
from B-52’s instead. It’s not that
supersonic store separation cannot be done (because that booster separated at
Mach 3 from the D-21). But successful
supersonic store separation is very difficult to achieve, and the risks of doing it are inherently very
high.
So how fast a drop speed can be obtained? That depends upon the gas turbine engines
powering the launch aircraft. Those are
seriously limited by the high air temperatures associated with capturing
supersonic air. Most are limited to
about Mach 2.5. There are a very few
that went faster: those powering the
XB-70 at Mach 3, those powering the
SR-71 variants at Mach 3.2, and the 500
hour short-life, replace-don’t-overhaul engines
in the Mig-25 at Mach 3.5. So, to have a wide range of possible engines
available for new designs, it looks like
Mach 2.5 at drop is “about it” with gas turbine. Maybe Mach 3.
So, the answer would
seem to be a mixed-propulsion airplane with gas turbine propulsion, augmented by parallel-burn rocket
propulsion, added to enable the
zoom-climb by a sharp pull-up maneuver.
This would be at high altitude near 45,000 feet, for the drop of the rocket vehicle. To do this successfully, the very difficult supersonic store
separation problem must be very carefully addressed! Both aircraft and crews are at serious risk.
Mach 2.5 at that altitude would be 2419 feet/second (0.737
km/s), less than 10% of low circular
orbit speed, so one is still looking
at a two-stage rocket vehicle to reach orbit. Deliverable payload would be limited in size
by the size of the drop aircraft, since
that in turn limits the size of the rocket vehicle it can drop.
In a word, this has
already been done with subsonic carrier aircraft, although it has proven no more attractive
than vertical rocket launch, at
best. The supersonic release has yet to
be tried, and will prove both difficult
and dangerous, although the improvement
in attractiveness may be worth that effort and risk. No one yet knows.
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