Re: How big would an SSTO be?

On Jun 30, 7:08 pm, Sylvia Else <syl...@xxxxxxxxxxxxxxxxxxx> wrote:
David Cornell wrote:
Sylvia Else wrote:

You didn't say so, but I'm assuming you mean a reusable craft.
Disposable SSTO's seem a waste of effort.

The most developed design I've seen for a reusable SSTO is

http://www.reactionengines.co.uk/skylon_vehicle.html

It has a payload of 12 tonnes, and a maximum takeoff weight around 280
tonnes, similar to that of a 777-300. It uses a new engine design with
some technological challenges, but they seem to have made some progress
with it. They're obviously financially constrained, so if you have a
spare $billion, I'm sure they be interested in talking.

Skylon is an automated system, and as such is not designed to have a
crew, but could carry people as payload. This document

http://www.reactionengines.co.uk/downloads/JBIS_v56_118-126.pdf

discusses that application using a module carrying 40 people, though
that's obviously in a transport application (to a space hotel, perhaps).
If you have space tourism in mind, with passengers not leaving the craft
and floating around the cabin, then presumably they'd need more space
per passenger.

It's hard to say how this scales for a smaller payload, but at a guess,
I'd say you could get a craft to carry four people that was the size of
a small airliner in the 50 seat range.

Sylvia.

Yes, I was assuming a reusable craft. I guess I took that for granted.

Thank you for the information on Skylon. I had heard the name but
didn't know much about it. I wish I had either money or talent to
contribute to the cause, but all I have is an interest in the subject
and a little bit of imagination. In any case,
Skylon's proposed performance sounds too good to be true, but I would be
delighted to be proved wrong.

Well, the performance comes from using an engine that is air breathing
for part of the ascent. It all comes down to whether they can build that
engine, and that appears to be where they're spending what money they have.

Sylvia.

[Sylvia, I'm going to repost the following comment.
On my computer, at least, the previous posting
seems to have gotten filed under a non-relevant part
of the thread]:

Perhaps this is a good place to respond futher to your
concerns about my doubts about Skylon, Sylvia.
If Skylon is anything like other airbreathers, then
it doesn't really come down to whether or not they
can build the engine. Most of us that have done
analyses of both rocket-powered and various
hybrid and other airbreathing engines have generally
given the airbreather the benefit of the doubt with
respect to both feasibliity and reasonable performance.
Even with such concessions, the airbreather tends
to fall far short when one is honest about the rather
horrendous drag and structural penalties
associated with trying to accelerate the
system to high supersonic or hypersonic speeds
within the atmosphere. For acceleration
type missions, it generally pays to get out of
the atmosphere as painlessly as possible.

Ironically, using the atmosphere for lift (not
propulsion) for the early portion of the trajectory
can be beneficial for some design strategies
such as our Space Van--but other
rocket-powered design strategies are
promising as well. A big clue here is that
the air needed for an airbreathing engine
is generally dependent upon the first power
of the velocity, whereas lift is depedent upon
the square of the velocity. Wings and inlets
do not want to be in the same layer of the atmosphere
over significant mach-number excursions.

Airbreathing and hybrid approaches tend to
be Loreleis with respect to first impression
and superficial analysis. However, with more
detailed, realistic (and expensive) analyses,
the aribreathing concepts tend to go downhill
from the first optimistic estimates, while the
rocket-powered accelerators tend to hold
their own or even improve with detailed
analyses and appropriate feedback redesigns.
I have never found airbreathing accelerators
to be viable beyond the first naive stage.
I am far from alone with this observation;
however, I first came to this conclusion
about 1962, when I was project engineer
for space transportation systems at the
LA Division of what was then North American
Aviation. The promise of airbreathing drew
me to North American in the first place,
since they were building the B-70. I avoided
being part of the North American bid to look
at the first Aerospaceplane concepts. IIRC,
this bid assumed a delta vee ideal requirement
to get to orbit of about 29,000 ft/sec. This was
incredibly naive, since a typical, real number is
more like 40,000 or 45,000 ft/sec--compared to
about 9500 m/s (31,200 ft/sec) for a typical
rocket powered space transport. Nowadays,
even a relatively crude spread*** trajectory
analyis would show high drag losses.

Incidentallly, the carrier stage of our Space Van
2011 is rather inefficient from the delta-vee-ideal
point of view. This is due to our strategy of using
a very large (1000 m^2) wing and constraining climb
and initial acceleration to relative low dynamic
pressures that are generally inappropriate for
rocket propulsion. However, the resulting
penalties are of little consequence since
the extra propellants are placed near lifting
(and ground-cart-support) loads and the
extra tankage is staged with the carrier.
The net result of this patent pending
approach is that both carrier and orbiter
structure can be much lighter than would be
typical of a higher-dynamic-pressure trajectory.
It's basically is--and always been--a mass-ratio
game--at least until some presently unknown
technolgy comes along.

Len

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