Manned Dragon to land via rocket braking?

On 1/18/2011 5:14 PM, Jochem Huhmann wrote:

I'm still wondering what kind of engines and what amount of fuel they
want to squeeze into Dragon. The current RCS/OMS thrusters have 90
pounds of thrust each. Dragon has a mass of about 6000 kg and an LES
should be able to get the capsule away from an exploding launcher with
at least 10 g or so. This means some hefty engines which then need to
be able to throttle down deeply for a landing.

If they were using the same engines for a LES and landing, you would
think they would just add some more propellants and use them for the
retro burn also. There will probably be a minimum of four LES/landing
engines to help the descending capsule maneuver itself via varying their
thrust for final touchdown, and by firing one opposed pair as retros,
deceleration g's for the retrograde burn could be kept down to
acceptable levels.
(This is follow-up reply to the SpaceX announcement regarding they are
considering landing the manned Dragon variant via pure rocket power:
http://spaceflightnow.com/news/n1101/18spacex/ I'm cross-posting it to
sci.space.policy.)

Pat

Pat Flannery writes:

On 1/18/2011 5:14 PM, Jochem Huhmann wrote:

I'm still wondering what kind of engines and what amount of fuel they
want to squeeze into Dragon. The current RCS/OMS thrusters have 90
pounds of thrust each. Dragon has a mass of about 6000 kg and an LES
should be able to get the capsule away from an exploding launcher with
at least 10 g or so. This means some hefty engines which then need to
be able to throttle down deeply for a landing.

If they were using the same engines for a LES and landing, you would
think they would just add some more propellants and use them for the
retro burn also.

The same engines for a 10 g LES burn, for a 1 g landing and for a retro
burn which needs to be *very* precise? You'd need engines that can
throttle down to 1% or so for that.

There will probably be a minimum of four LES/landing engines to help
the descending capsule maneuver itself via varying their thrust for
final touchdown, and by firing one opposed pair as retros,
deceleration g's for the retrograde burn could be kept down to
acceptable levels.

Here's the layout of the current thrusters:

http://www.spacenews.com/images/092809Dragon02.jpg

The four downward pointing engines have a thrust of 90 pounds each. Try
to squeeze in engines with at least two orders of magnitude more
thrust and the fuel tanks for them there.

Again: Dragon has a mass of 6000 kg. To accelerate it with 10 g you'd
need engines with about 130000 lbf thrust (that's about as much as the
single Merlin engine in the first stage of Falcon 1 or the second stage
of Falcon 9). The current engines have 4 x 90 lbf.


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

On 1/19/2011 2:15 AM, Jochem Huhmann wrote:


The four downward pointing engines have a thrust of 90 pounds each. Try
to squeeze in engines with at least two orders of magnitude more
thrust and the fuel tanks for them there.

Again: Dragon has a mass of 6000 kg. To accelerate it with 10 g you'd
need engines with about 130000 lbf thrust (that's about as much as the
single Merlin engine in the first stage of Falcon 1 or the second stage
of Falcon 9). The current engines have 4 x 90 lbf.

Could you do it with massed solid rockets of small size, short duration
burn time, and very high thrust?
The Dragon antitank missile use large numbers of small rocket engines to
both maintain its flight velocity and direct its course:
http://www.designation-systems.net/dusrm/m-77.html
In this scenario, you would fire all of them at once for LES, and fire
only a few at a time in sequence for landing.
I wouldn't trust landing that way, but who knows?

Pat

In sci.space.history message , Wed, 19 Jan
2011 11:15:57, Jochem Huhmann posted:

Pat Flannery writes:

On 1/18/2011 5:14 PM, Jochem Huhmann wrote:

I'm still wondering what kind of engines and what amount of fuel they
want to squeeze into Dragon. The current RCS/OMS thrusters have 90
pounds of thrust each. Dragon has a mass of about 6000 kg and an LES
should be able to get the capsule away from an exploding launcher with
at least 10 g or so. This means some hefty engines which then need to
be able to throttle down deeply for a landing.

If they were using the same engines for a LES and landing, you would
think they would just add some more propellants and use them for the
retro burn also.

The same engines for a 10 g LES burn, for a 1 g landing and for a retro
burn which needs to be *very* precise? You'd need engines that can
throttle down to 1% or so for that.

Not really.

Have a ring of solids (perhaps in various sizes) under the heat-shield,
plus liquids for the final kilometre or so of the 1g landing.

For launch escape, fire all of the solids at once .

Once escape is no longer needed, perhaps fire some for ascent or orbit
insertion.

For re-entry, fire a few at a time, then another few, and so on,
finishing with a trimming burn from the liquids or the existing
verniers.

Carefully size everything so that most situations, including those
desired, get Dragon down with the solids all having been used and the
liquids with just some spare fuel left - i.e. everything lifted is
either crew, payload, reusable structure, or essentially all used up at
landing.

--
(c) John Stockton, nr London, UK. Turnpike v6.05 MIME.
Web http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms and links;
Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

On 1/20/2011 12:56 PM, Dr J R Stockton wrote:

Once escape is no longer needed, perhaps fire some for ascent or orbit
insertion.

That was the concept one of the original Shuttle designs was going to
use; a solid-fueled rocket mounted above the rear of each wing that
would allow the orbiter to pull itself free of the launch stack in case
of an ascent emergency, that would also be used to carry it on the final
stage into orbit after ET jettison if no abort had occurred.
If they had gone with liquid-fueled strap-on boosters that could be shut
down, or found a way to vent the SRB's to stop their combustion, like on
the Titan III MOL/Dyna-Soar design, it would have been a workable
concept; but sticking solid engines on the orbiter that could generate
enough thrust to overcome the thrust of the still-ascending ET/SRB combo
- after the loss of weight of the orbiter - to prevent the detached
orbiter from sliding back into the SRB exhaust, would have necessitated
very great strengthening of the orbiter wing structure, and a big
increase of orbiter weight.

Pat