=space =rockets
How do wings compare to powered landings for rockets?
First, here's the naive case for flyback over powered landing.
mass
A 2:1 takeoff to empty mass ratio
is typical for aircraft. From the 50% empty mass during takeoff, subtract
15% for not having engines, and perhaps 10% for weight savings from
integration with tanks. The result is about 25% empty mass, which is 1/3 the
payload.
A Falcon 9 first stage has about a 16:1 fueled to empty mass
ratio. If wings and landing gear add 25% to the 1st stage empty mass, that
would reduce payload by about 8%.
A Falcon 9 landing on a ship
reduces payload by about 30% compared to when it's expended.
reliability
Empty rockets are much lighter
than full ones, and rocket engines don't throttle down very well. Falcon 9
rockets deal with this by using a cluster of engines, and only firing one
during landing. Even so, the minimum stable thrust is too high, so the
engine needs to be fired shortly before landing. This is the underlying
reason why SpaceX rockets have sometimes crashed on landing.
Aircraft
landings have a much lower failure rate.
Now, let's look at why SpaceX rejected that.
It's not fair to compare just landing. SpaceX does 2 or 3 burns to return stages:
- a boost-back
burn to reverse direction if returning to the launch site
- a reentry
burn to slow down before the atmosphere becomes dense
- a landing burn
China doesn't care if some rocket
parts
land
on villages sometimes, but traditionally, rockets are launched over the
ocean. SpaceX can eliminate the boost-back burn by landing stages on
ships. A runway to land horizontally would be too large for this.
With wings, it's cost-effective to glide up to maybe 150 km, but Falcon
Heavy boosters go farther than that without a boost-back burn, more like 400
km.
If you launch from a ship, you can arrange things such that a
ballistic trajectory lands in the ocean but stages can glide to a runway on
land. In certain locations, it's also possible to launch eastward over ocean
from land and glide north or south to a runway on land. But both of these
arrangements are limiting.
The
Space Shuttle solid rocket boosters separate at about the same speed as
the Falcon Heavy boosters. Why can't you just have liquid fueled boosters do
the same thing?
A solid rocket booster is similar to a pressure-fed
liquid rocket, but pressure is maintained by the hot gas. In a solid rocket,
unburned fuel protects the "tank" while recycling hot gas to the tanks of a
pressure-fed liquid rocket is more problematic.
The empty SRBs are
mostly steel tubes, thick enough to contain the 63 bar chamber pressure.
That's why they can survive reentry at that speed. Liquid fueled stages with
thin-walled tanks wouldn't survive so well, and wings would probably make
things worse.
So, this being the case, why do I like winged rockets?
The boosters of a Falcon Heavy launch separate at about 2 km/s and 70 km. If you look at its launch profile, separating at 1 km/s is about 115s burn time, vs 154s for 2 km/s.
If boosters separate at 1 km/s:
- they can
survive reentry with just special paint
- less distance is travelled
ballistically
With separation at 1 km/s, less
than 1/4 the distance is travelled ballistically because the rocket points
more upward earlier on, and boosters can realistically glide back to their
launch site.
Yes, the rocket engine utilization per launch is less,
but less burn time and fewer burns per launch presumably means more launches
between refurbishment.
So, when I say I like winged rockets better than powered landing for reuse, I'm talking about a very different overall system. You have to consider a lot of factors in the comparison, such as:
- development
costs
- how late you want to recover stages
- the value of recovery
reliability
- ideal acceleration profile considering gravity and drag
losses
- launch sites and runway availability
- the costs of staging
complexity
There are lots of options for
rocket configurations. Maybe you want a cluster of engines so an engine
failure is acceptable. Or maybe you want one big turbopump set with
multiple combustion chambers,
to minimize the chance of failure.
Maybe you want something durable
enough to survive reentry, like a solid rocket booster. But those are
explodey, so maybe a
hybrid
rocket? Maybe with an
electrically
driven turbopump? Or maybe it's better to make cheaper pumps instead of
trying to recover stages. Maybe you decide turbines are too expensive so
you'll use piston pumps.
With all these choices, what's the best option? The answer is, as long
as you choose a reasonable high-level design, what's usually more important
is design for manufacturing and organizational efficiency.