On 16/07/2011 12:23 AM, Jeff Findley wrote:
In , lid
says...
On 15/07/2011 12:30 AM, Jeff Findley wrote:
In , lid
says...
sigh It's not a rocket in airbreathing mode.
It's some ******* engine that does nothing well. It's not as efficient
as a turbofan (likely even a turbojet) aircraft engine when in air-
breathing mode and it's not as efficient as a liquid fueled rocket
engine when operating in pure rocket mode (LOX from internal tanks).
I fail to see how an engine which operates worse than the state of the
art in either mode is better than having two separate stages with two
separate types of engines on each.
All of this silliness is in pursuit of SSTO. Fully reusable TSTO would
be far easier to implement than this because it would require no new
technologies (i.e. fundamentally new engine) to be developed.
It's all about cost per kg payload in orbit. An SSTO, particularly one
that takes off and lands horizontally presents considerable operational
advantages.
Please enumerate the advantages of HTHL over VTVL as they apply to SSTO.
At this point in time, I'm simply not convinced that the advantages are
worth the cost, especially for intact abort scenarios.
With VTVL you have to carry fuel for landing. Of course, you don't have
to carry wings, so there's an element of swings and roundabouts.
With vertical takeoff you have a period during which the rocket exhaust
is impacting a launch pad, which thus has to be protected both from the
temperature thereof and the shockwaves therein. In a horizontal takeoff
the direction of the exhaust is different, and the time of exposure for
any given section of runway is much reduced.
A horizontally landing vehicle necessarily has wheels, so after it
stops, it can be towed away from the landing area without further
complication. A vertically landing vehicle either has to carry wheels as
dead-weight, or additional ground equipment is required to allow the
landed vehicle to be moved from the landing site.
Skylon also uses its own wheels for takeoff, so it can be towed to the
takeoff point. Compare with the equipment and time required to get a
vertically launched vehicle like the shuttle to the launch site.
Both systems would need the ability to abort intact at any point, but I
don't see any particular benefit to a VTVL system in that regard.
While the SABRE engine is more complex than a standard rocket,
complexity is removed in other areas - there's no stage separation, for
example.
Stage separation is an existing technology that's been in place on the
very first orbital launch vehicle. It's at least a fairly well known
quantity, especially if you do your stage separation above the bulk of
the atmosphere.
Stage separation is an existing technology as regards disposable
rockets, and for some reusable components of the shuttle. Impact by
separation debry is a not-insignificant hazard for the shuttle. Avoiding
explosive bolts would perhaps reduce the debry hazard, but would
increase the chance of an incomplete separation, which would likely
cause loss of vehicle.
Stage separation above most of the atmosphere, but with the first stage
then returning to the launch site seems a questionable proposition. If
the first stage lands somewhere else, then provision has to be made for
returning it to the launch site, which increases the operating cost.
It also eliminates a whole class of risks associated with
separation, such as collisions between the two parts of the vehicle,
partial separation, etc.
A VTVL SSTO would have the same advantages without the high cost of
SABRE development.
Perhaps, but who has a VTVL SSTO?
In any case, it's far from clear that a fully reusable TSTO is so easy
to achieve.
Possibly, but so far, no one has tried to build and fly such a vehicle.
That's not entirely true. The shuttle was orignally meant to be a fully
reusable TSTO. The tank and SRBs were substituted for the first stage
due to cost. Now, that was NASA, with intereference being run by the
USAF for good measure, so I'll concede that perhaps it could be done for
substantially less in the hands of private industry.
It's at least on firmer technological ground than SABRE and Skylon, so
I'd say the chance of success would be much higher for the reusable
TSTO.
Getting Skylon to work in practice may prove more difficult than RE
think. It may prove impossible. But I don't understand the sheer
antagonism towards it evidenced by some in this group. Unless it's a
manifestation of a fear that RE will achieve a disruptive technology.
No, it's mostly the acknowledgment that SABRE is a bleeding edge
technology in much the same way as a hypersonic air breathing engine
(e.g. NASP and more recent technology demonstrators). We've been down
this road more than once, and it's burned us each and every time.
Hypersonic air breathing engines are horribly expensive to test,
typically requiring a rocket launch. By contrast, and awful lot of
SABRE's development can be done on the ground.
Don't get me wrong, I'd really *like* to see Reaction Engines succeed
and produce a working, economically viable, launch vehicle. I'm just
extremely skeptical due to the inclusion of an engine design which has
never been fully tested *and* has not been successfully integrated into
an actual vehicle.
Aerodynamics is a p.i.t.a. and it will rear its ugly head during the
design phase where the engine is integrated into a *real* vehicle. So
far, I find the "artist's concept" type drawings of Skylon to be lacking
in sufficient aerodynamic detail. In order to provide enough air for
the engines at flight conditions, the engine will need to be *much* more
integrated into the vehicle aerodynamics than current drawings and
renderings suggest.
I can't see any reason for thinking that integration is required for
there to be enough air. The intakes have a particular size. The airflow
speed is known.
The design deliberately does NOT integrate the engines with the
airframe, because that hugely simplifies development.
This should come as no surprise since Reaction Engines is focusing on
building a working engine first. The problem is still that of the
chicken and egg. Without an engine fully integrated into an actual
vehicle, it's awfully hard to test either the engine or the vehicle.
Due to the nature of the engine (and the aerodynamics needed for the
inlet), the two designs are almost inseparable.
The inlets are behind the nose shock. I have to wonder a bit about the
shock from the canard, but otherwise I can't see that there'll will be
any interaction between the airframe and the intakes when the vehicle is
supersonic. The subsonic regime will persumably require some attention,
but it's one that's well understood.
Sylvia.