[AR] Re: thinking big once more
- From: Peter Fairbrother <zenadsl6186@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Fri, 30 Sep 2016 03:11:20 +0100
On 29/09/16 01:33, Alexander Ponomarenko wrote:
Still wondering why they selected champion
chamber pressure.
Probably for lots of reasons, starting off with that they thought they
could do it.
But I think the driver may have been takeoff thrust. 42 takeoff engines,
after all...
The Raptor engine has three roles: Earth sea-level booster, vacuum
spacecraft, Mars lander/takeoff.
These roles are served by two variants; one has a sea-level nozzle with
an expansion ratio of 40, the other a vacuum nozzle with an expansion
ratio of 200.
A typical Mars mission might involve seven boosters, six tankers, and
one craft.
That's 258 sea-level engine firings (252 for the boosters and 6 for Mars
landing/takeoff) and 48 vacuum engine firings (36 for the boosters, 12
for Mars go/return).
(I am unclear as to whether the tanker has sea-level engines - I assume not)
With reuse, the number of engines of each type produced might be
approximately equal in time; but initially I assume many more sea-level
engines will be required.
This may be wrong, as someone (USAF?) gave SpaceX $30 million towards
developing the vacuum Raptor, and presumably they may become a customer
for vacuum engines.
I digressed. Playing around with RPA :), comparing a 20MPa engine with a
30MPa engine:
For the vacuum engine, such a high chamber pressure doesn't seem to make
much sense. Decreasing the chamber pressure from 30 MPa to 20 MPa would
only decrease Isp by less than a quarter of one percent.
Decreasing the chamber pressure from 30 MPa to 20 MPa would save a lot
of mass, expense, complication, operating constraints,
risk/unreliability, and development cost.
For the sea-level first stage/booster engine, the expansion ratio of 40
is quite high.
Vacuum Isp for the sea-level engine is 369s at 30 MPa, and only falls to
367s at 15MPa - so no great gain from a heroic chamber pressure there.
At an expansion ratio of 40, sea-level Isp is 331s at 20MPa, and 344s at
30MPa. That is considerably more significant, at 4% increase - but the
engine doesn't operate at sea level for long, lessening the effect on
the mission of the higher chamber pressure.
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.
Plus, unlike the overall effect of the increased sea-level ISP, the
overall effect on the mission of this thrust increase is magnified, as
the booster accelerates faster, reaching vacuum conditions quicker (to
the extent permitted by MaxQ), and spending less time at thirsty
sea-level. Also, gravitational slowing is lessened.
But maybe SpaceX just don't think of 30MPa as being champion, heroic and
troublesome... and if it turns out to be, then, with a 20%-100% design
throttling range, simply *operating* them at 20 MPa should be easy enough.
-- Peter Fairbrother
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