How can SpaceX test SuperDracos whilst hanging off a crane?

Question

NASASpaceFlight.com reports that the DragonFly test vehicle has arrived at McGregor, TX for testing.

They discuss that the FAA is limiting them to 80 feet above ground for initial testing. That they will hang it off a large crane and test the SuperDracos while hanging off a crane.

The problem is, the 8 SuperDracos have 16 Klbf thrust (vacuum) EACH!

The Dragon V2 vehicle is around 14,000 lbs mass which the article says includes steel legs attached for testing. I am not sure how low you can throttle a SuperDraco, but even with one SuperDraco firing, that thing is going to be going like a bat out of hell.

The SuperDracos are embedded in the side and do not thrust directly down, so firing just one is probably not an option if you do not wish to destroy the test vehicle.

How are they going to run this test in a way that makes sense? (Since they clearly have a plan...)

Answer 1

Like any good descent engine, SuperDraco throttles deeply; Wikipedia says down to 20%.

Various speculative sources put the thrust angle anywhere between 25º and 35º, which determines cosine losses. The sidewall angle is 15º, and the thrust angle is greater than that. Split the difference and assume it's 30º for 0.87 cosine.

I can't find a solid reference for the difference between sea level and vacuum thrust; Wikipedia says 240s ISP at SL, and some unsourced reference from 2013 suggested 300s vacuum, which seems plausible (if a bit low), so let's assume sea level thrust is 80% of vac.

73kN x 8 x 0.2 x 0.87 x 0.8 = 81kN total thrust, which is in the ballpark of 18,000 lbf. If throttling down that far isn't sufficient, they might have to shut down 4 or 6 of the engines.

With only 2 engines running, they'd lose one axis of differential-thrust control authority, although the Draco RCS thrusters can probably take care of that.

Wikipedia has the dry mass of Dragon V2 at 9300 lbs, and your linked article at 14000. Propellant adds another 4000 lbs or so, for a total of something like 18000 lbs at the start of the test.

In January of 2016, SpaceX released a video of a hover on a crane from November 2015 showing a successful hover. Information released by SpaceX about that test claims 147kN total thrust, implying about 36% throttle and a heavily ballasted capsule.

Answer 2

Think of it like this: how would you test the vehicle without a crane? They are testing out the powered landing capabilities so the idea is to work with slow, smooth trajectories that may be mostly vertical but will obviously incorporate some lateral movement too.

If anything were to go wrong the vehicle could diverge wildly off-course and probably necessitate self-destruction. Instead, the crane allows you to hold onto the vehicle with (ideally) minimal reaction between the cable and the vehicle.

Of course without any thrust the cable provides a reaction if the vehicle is hanging, but once the test is being run they will likely be adjusting the tension to be minimal (while being careful not to let it go completely slack).

The overall idea is like putting a leash on your dog so it doesn't run away, but still giving the dog slack to walk around comfortably (at least if your dog doesn't try pulling you around like mine!).

As for the overall thrust and throttling, this system is being designed to operate with thrust that reduces to less than 1 g acceleration (otherwise landing would be too dangerous since you would depend on shorter high thrust burns while still descending). That article you link to actually states the dry weight is 14000 lbs, so considering 4000 lbs of propellant you end up with a mass of about 8100 kg so an acceleration of about 1 g.

EDIT:

This document gives some more details about the DragonFly vehicle although it is primarily concerned with the subsequent 4 test phases after using the crane.

From this document we can pull out these numbers:

Dry mass = 6364 kg

Propellant mass = 1756 kg (assuming full 400 gal tank with 50/50 NTO and MMH which have SGs of 1.44 and 0.88 respectively, although it could be anywhere from 1332-2180 kg)

Maximum thrust = 546.7 kN

On page 97 they state "The four engines are modeled as a...", which could be just some confusion about the fact that the engines are configured in pairs, but might also suggest they are only firing four of the engines.

With the masses I've quoted, you would need a thrust of about 79.7 kN to hover, which corresponds to a throttle of 14.6% for all 8 engines or 29.1% for 4 engines.