[AR] Re: landing legs
- From: Ben Brockert <wikkit@xxxxxxxxx>
- To: "arocket@xxxxxxxxxxxxx" <arocket@xxxxxxxxxxxxx>
- Date: Tue, 25 Feb 2014 13:36:27 -0700
You don't need thrust to go below weight to land a VTVL rocket with
sufficiently good guidance. By public reports SpaceX doesn't plan to
hover, and demonstrated landing with thrust greater than weight with
the first Grasshopper. Hovering is a waste of propellant.
Ben
On Tue, Feb 25, 2014 at 1:10 PM, J. Cameron Cooper <jccooper@xxxxxxxxx> wrote:
> On Mon, 24 Feb 2014 12:20:24 +0100 Miklos Szeredi <miklos@xxxxxxxxxx>
> wrote:
>>
>> I was just thinking about spacex's reusability plans and wondering if
>> legs are actually necessary or not.
>>
>> It is returning to base, so the "legs" could be waiting right there to
>> capture and hold the rocket during landing.
>>
>> Advantages:
>>
>> - no need to carry the weight of the legs
>> - one less failure point in the rocket
>>
>> Disadvantages:
>>
>> - it can't "emergency land" anywhere else
>> - the landing must be precise and slow enough that capture can succeed
>>
>> Not sure how problematic these are in real life.
>>
>> Thoughts?
>
>
> I have no particular expertise, but based on some simple research and math,
> here's my understanding:
>
> A Merlin 1D is said to be able to throttle down to about 70%, where 100% is
> around 650 kN. So you have a 1-engine range of 455-650 kN.
>
> An empty F9 1.1 first stage is estimated at about 28000 kg, so it hovers
> with 274 kN. With say 22t of propellant left (just over 5%), you only need
> 490kN to hover.
>
> This accords well with the reports of a fair amount of propellant used as
> ballast in Grasshopper. Once too much propellant is gone, an F9-1 cannot
> hover--it'll take off again even at the bottom of its throttle range. It
> needs about 19t of ballast to be able to descend.
>
> (I'll note all these figures are guesses; the dry mass specifically is not a
> figure published by SpaceX. They seem plausible, though.)
>
> From this we can say:
>
> 1: there's plenty of weight to work with for the landing system. Since you
> need ballast, you might as well substitute dry mass for wet, if it makes
> your life easier. (Besides function, I would think dry mass to be better for
> predictability: no sloshing.) I think I've seen that the legs are 3t ea,
> though I can't remember where. That fits nicely in the ballast window.
>
> 2: the window for landing is going to be fairly small, and the throttle
> rather dynamic as the weight is quickly dropping. I think it will be a lot
> easier to drop onto a "big" pad with legs through a nice window of
> velocities (up to several m/s probably) than to achieve a precise enough
> hover to be hooked or a precise enough position to hit a cradle.
>
> SpaceX also appears to have rotation concerns, which the legs will also help
> with, both aerodynamically and by changing moment of inertia when extended.
>
> Legs seem to make a lot of sense given the circumstances. They aren't
> actually adding weight, and seem to make the landing a good deal easier.
>
> In the future, once there's plenty of experience landing with legs, it may
> look feasible to do cradle or crane landings, and get back that dry mass.
> But you'd need to modify the vehicle or motor to require less ballast to do
> so. That might mean larger throttle range, a smaller middle engine, a
> dedicated landing engine, or ... something else. (The Merlin is a pintle
> engine, I note, and the LMDE throttled down to 10%! They only need to get to
> 40% for a "dry hover".)
>
> Lack of legs will rather restrict the flexibility of the system, though, by
> requiring non-trivial ground infrastructure. It may make more sense
> operationally to just add some more propellant if you need more performance.
> A second stage, however, would be a different story.
>
>
> --
> J Cameron Cooper
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