[AR] Re: thinking big once more
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Thu, 29 Sep 2016 23:30:09 -0400 (EDT)
On Fri, 30 Sep 2016, Peter Fairbrother wrote:
However perhaps the biggest gain is in the sea-level thrust coefficient -
1.7769 at 20MPa and 1.8416 at 30MPa. That's an increase of 3.5% in takeoff
thrust.
Uh, remember that thrust is thrust coefficient, times throat area, *times*
*chamber* *pressure*. So that's a 55% increase in thrust, not 3.5%.
The traditional reason for wanting high pressure in a first-stage engine
is not higher Isp, but getting more thrust out of an engine whose size is
constrained by packaging issues, e.g. the size of the vehicle base. This
is why the SSME's pressure was so high -- three of them had to fit in the
lee of a dense, compact orbiter during reentry. Similarly, the RD-170 was
constrained to fit within the maximum allowable cross-section of a Russian
railroad cargo, so a fully-assembled Zenit first stage could be moved by
rail without restrictions.
Historically, designs for big rockets tend to gain width faster than they
gain height, simply because they need the extra base area. Elon's
megarocket would have to be a good deal shorter and fatter without that
alarmingly high chamber pressure.
Henry
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