I'd say it's both. The physics make the margins tighter which makes dealing
with the energy trickier and the power issue can't be held in isolation as
it can have a compounding effect to the energy ie velocity>dynamic loads &
thermal loads. What's more, much of that power is required to haul the
*energy*. The challenge is highlighted via the closest apples to apples
comparison you can draw - a typical long haul HTHL airliner vs the proposed
HTHL SSTO capable Skylon. Note the cooling and thermal complexity
requirements of the intakes and engines, of the body and the fuel
requirements and the development costs.
Everything in orbital capable rockets are run much harder than for
commercial airliners - the turbines in the pumps are exposed to much more
energy and deliver more power than those of typical airliner turbofan
engines. The dynamic loads experienced during takeoff are much higher, the
reactivity of the propellants is much higher, the chamber temperatures are
much higher and energy density within is much much greater placing
considerable importance on the cooling of such.
Troy
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of Rand Simberg
Sent: Friday, 21 October 2016 11:46 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: fault tolerance (was Re: thinking big once more)
I'd say that to the degree getting into orbit is a physics problem, it isn't
an energy problem; it's a power problem. How do you release that energy fast
enough to do the job, and for entry, how do you absorb it?
On 2016-10-20 17:25, Jim Davis wrote:
On 10/19/2016 10:59 PM, Henry Spencer wrote:
It would be good if you'd explain why you think the *physics* doesn't
permit this.
I'll take a stab at it.
The specific energy (kinetic and potential) of a unit mass in a
circular orbit is given by:
eo = g0 * R * (1 - R / (2 * r))
where
eo - specific energy of object in circular orbit
g0 - acceleration of gravity at surface of earth R - radius of surface
of earth r - radius of circular orbit
The minimum energy required for a unit mass to travel a distance is
given by:
ed = g0 * s / (L/D)
where
ed - minimum energy to overcome aerodynamic drag s - distance to be
traveled L/D - lift to drag ratio
For an object in a 160 km circular orbit (LEO) the specific energy is:
eo = 32.0 MJ
For an object traveling 14,000 km (the range of the longest commercial
service) at an L/D of 15 (conservative) the specific energy is:
ed = 9.2 MJ
The ratio is
eo/ed = 3.5
That's the basic physics. When one proceeds to engineering the ratio
remains about the same, between 3 and 4, depending on the exact
details. The actual energy required is much greater in both cases, of
course.
This ratio explains why it is necessary to use heroic measures
(staging, expendable hardware, thin margins, etc) to place anything in
LEO or beyond. Placing things in orbit is a much greater engineering
challenge than even the longest commercial flights.
This is why fault tolerance is more challenging in the case of a
launch vehicle when compared to aircraft.
It's a matter of physics.
Jim Davis
