Jeff Findley writes:

Curiously, it had a thrust of 155,000 lbs, which is just about what
you're saying Dragon would need, so this ought to be interesting. Total
impulse of the launch escape motor was 515,000 lb-sec, which works out
to about 3.3 seconds of thrust at 155,000 lbs.

The launch escape motor had a weight of 4850 lbs.

The Apollo CM had less mass than dragon with down payload, so to attain
about the same escape acceleration as Apollo, Dragon needs about the
same thrust, yes.

Now compare it to LOX/kerosene numbers:

According to Wikipedia, when configured for Falcon 1, Merlin IC's thrust
was 78,000 lbs at sea level, and its fuel consumption is about 300 lb of
propellant per second. Scaling that up to 155,000 lbs of thrust gives a
fuel consumption number of about 600 lbs per second which is about 2000
lbs for 3.3 seconds of thrust.

So, the total weight of LOX/kerosene engines plus propellant would be
3500 lbs, which is 1350 lbs lighter than the solid. Obviously we've got
to add in the weight of tanks, plumbing, and etc., so there may not be
much in the way of weight savings.

3500 lbs is about half of the Dragon down payload and a quarter of the
up payload. And you'll need turbopumps (which might be a bit slow to
spin up in an escape system and anyway heavy) or pressurized tanks
(which also are heavy). And plumbing and then there's the landing gear.
And getting away with about 30 seconds worth of fuel for a landing seems
to be a bit daring, too. Especially if you don't have any other way to
control your descent (like control surfaces), you'll need the fuel for
more than just breaking.

And for a manned landing there will be a very nasty black zone where
you're too low to switch to the backup parachute and still high enough
to have a failing engine or control system lead to loss of vehicle and
crew. But if you have thrust enough for accelerating with 10 g, you may
have got enough engines and redundancy to compensate for many failure
modes, OK.

This back of the envelope calculation shows that liquid fueled engines
for launch escape isn't ridiculous at first glance. I know that "the
devil is in the details", so this may be quite a challenge to work out.
Pluse, if these liquid fueled engines double as landing engines, that's
a "win" over a solid system which is simply discarded if it's not
needed.

I think solids are better for escape systems if you throw them away
unused nearly all the time. Just because they're probably cheaper.
Maybe more reliable, too. For a reusable capsule liquid fueled is
certainly a good idea.

Dragon is a great spacecraft and would totally deserve powered precision
landings. I'm just saying that this is nothing anyone would dare to try.
If it works out: Great!

Squeezing something with 150000 lbf thrust into the bottom of a Dragon
capsule along with the fuel tanks for it just seems impossible to me.

We'll see what SpaceX comes up with. Results are what will matter.

I'm certainly looking forward to that and in any case trying and failing
is totally better than not even trying.


Jochem

--
"A designer knows he has arrived at perfection not when there is no
longer anything to add, but when there is no longer anything to take away."
- Antoine de Saint-Exupery