Re: How big would an SSTO be?
- From: Alex Terrell <alexterrell@xxxxxxxxx>
- Date: Sun, 01 Jul 2007 08:07:15 -0700
Len wrote:
On Jun 30, 6:03 am, Alex Terrell <alexterr...@xxxxxxxxx> wrote:Having been out argued on this in my younger days, the only mid term
On 30 Jun, 04:38, Sylvia Else <syl...@xxxxxxxxxxxxxxxxxxx> wrote:
Len wrote:
On Jun 29, 6:55 pm, Sylvia Else <syl...@xxxxxxxxxxxxxxxxxxx> wrote:
David Cornell wrote:
If someone were to build an SSTO using realistic assumptions about massYou didn't say so, but I'm assuming you mean a reusable craft.
ratios and available power systems, how big a vehicle would be needed to
send (say) three people and a modest amount of cargo into LEO? I have
seen Apollo capsules in museums, so I am using them as my baseline.
Would such a thing be the size of a regular jetliner? Or the new Airbus
super jumbo jet? Or are we talking about a Zeppelin on steroids?
Also, how would these things scale? If we wanted to increase the crew
from three to four, would the vehicle size go up by a third? Or more?
Thanks
David Cornell
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.
I have probably looked at as many launch vehicle concepts
--rocket powered and airbreathers--as anybody in the world.
The devil is in the details. I would not consider Skylon
anywhere close to realistic. As for purely rocket-powered
approaches, I have never been able to convince myself
that any SSTO having a gross mass of
less than about 800 tonnes was very realistic. And for
HTOL, some type of ground cart to support the vehicle
at gross mass is probably necessary--thus making it
really an assisted SSTO, rather than a pure SSTO.
Staging--even subsonically at altitude or at low supersonic
speeds greatly relieves the challenge. IMO, staging
can sometimes be beneficial from the operations point
of view--as wsll as the performance point of view. SSTOs
are undoubtedly appealing from the psychological point
of view. However, they may not be a good way to run
an airline.
At some combination of size and yet-to-be-discovered
technology, SSTOs will make technical, economic and
marketing sense; but I don't see this happening soon.
Len
Would you care to expand on your concerns. At the moment, you've
basically said that you're an expert and that we should believe your
claim that Skylon is not realistic.
Sylvia.
I believe Len is an expert and would take his word for it.
Nevertheless, an expansion on the concerns would be of interest.
However, I don't see the benefit in SSTO when concepts like Quickreach
2 (http://www.astronautix.com/lvs/quieach2.htm) could reach orbit for
relatively low cost.
Len - how does Quickreach 2 compare to the latest space van proposals?
I have been out of town. I'll respond in more detail
tomorrow.
As a quick answer, Quickreach should be able
to launch a nearly twice as large payload a
couple of years earlier than the Space Van 2011.
We expect to carry 2000-kg--or eight passengers
--to an ISS-type orbit (not our main mission) for
a price (including ROI) of $2,000,000 per flight
in 2007 dollars. This compares to a Quickreach
cost (price?) of $20,000,000 in 2005 dollars.
The Space Van should be able to achieve much
more frequent flights, since there are no expendable
parts or reusable parts that require extensive
refurbishment between flights.
The Space Van should have good abort options
throughtout its flight regime--starting with
engine-out abort capablility just after liftoff
with derated engines. The engines are
derated for much improved time between
overhaul.
As for Sylvia's request, I am not sure
exactly what cycle Skylon plans to use,
but I suspect that it is some type of
combined-cycle engine. The poor-man's
approach to analyzing combined cycle
performance (except for potential benefits
from saving installation space through
integration) is to imagine separate rocket
and airbreathing engines. The resulting
thrust and specific impulse usually equals
the goals for the combined cycle engine.
This anaylis trick allows a quick
assessment of how much airbtreathing
and how much rocket the designer would
like to have. If one then goes through some
tradeoffs of different ratios and allows for
real-trajectory estimates of drag losses and
real-structure mass estimates allowing for
realistic inlets and the impact of flying the
whole vehicle at relatively high dynamic
pressures and velocities, I have always
found that the best ratio is 100 percent
rocket. Many others--including highly
knowledgeable people like Henry Spencer
--have noted that the airbreathing appeal
is rather superficial and vanishes under
realistic analyses.
hope I can see for air breathing would be to attach military derived
ramjets or scram jets onto the outside of the rocket, and have these
drop off at Mach whatever. These could drop off with the wings, which
starts to move towards a 2 stage model.
The investment would only make sense if there are suitable missiles
that can be adapted. Some countries are experimenting with supersonic
ramjet missiles, IIRC for cruise at about Mach 3.
.
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