Why not four pairs of SuperDracos in the F9 S1 interstage?

[whvholst]

Given that the difficulty of landing the F9.1 first stage is that a single Merlin still has a T/W ratio larger than one, even when throttled down to its minimum throttling, how stupid/silly would it be to mount four pairs of SuperDracos on the interstage? A single SuperDraco has a vacuum thrust of 7.4 kN while a 40% throttled Merlin D has a vacuum thrust of about four times that. Which means that four pairs of SuperDracos are comparable to a 80% throttled Merlin D and can together throttled much lower.

Given the amount of tinkering that still appears to be going on with the interstage (grid fins and related hydraulics), how far-fetched is this idea?

[Mongo62]

I would guess that the mass penalty would be prohibitive.

You save the mass of the landing kerolox propellant, but have to add the mass of the hypergolic SuperDraco propellant, which due to lower ISP and cosine losses, might be around 50% greater than the mass of the replaced kerolox.

Plus you have to add the mass of the SuperDracos themselves, the mounting hardware, the tanks for the hypergolic propellant and the connecting valves and piping.

[JasonAW3]

Too bad a Kerolox version of the SuperDraco don't exist.  While there would be a mass penalty, it may be easier to land a legged pendulm rather than a broomstick on a fingertip.

[neoforce]

Given that the difficulty of landing the F9.1 first stage is that a single Merlin still has a T/W ratio larger than one, even when throttled down to its minimum throttling,

It is difficult.  But it is so simple, and the smallest weight penalty.

This is a test program, but with high probability of successful recovery.  Don't jump to conclusion that it can't be done based on the first test.

[Coastal Ron]

Given that the difficulty of landing the F9.1 first stage is that a single Merlin still has a T/W ratio larger than one, even when throttled down to its minimum throttling, how stupid/silly would it be to mount four pairs of SuperDracos on the interstage?

The Falcon 9 1st stage doesn't need to hover, it only needs to time the arrest of it's descent to coincide with the stage meeting the ground.  The challenging part is getting that timing down to when you stop the engine the stage is already on the ground, and not starting to go back up again.

SpaceX has been practicing this with the Grasshopper test vehicle, and has been doing pretty good.  What they are trying to do now is figure out how to do it on a moving platform that could be raising or lowering in elevation fairly rapidly.  But I think it's something that can be solved with software instead of adding more mass to the stage.

Given the amount of tinkering that still appears to be going on with the interstage (grid fins and related hydraulics), how far-fetched is this idea?

The grid fins address the twisting of the stage, and likely add some ability to move the top end of the stage around without having to use the engines on the lower end.  Adding Draco engines likely would not do that, so the stage would still need grid fins regardless what engines you used to arrest the descent velocity of the stage.

You have to trust SpaceX with what they are doing.  They know what they are capable of, and they know what the important factors are.  No doubt they have spent a lot of time debating what the trade-offs are, and what we see today is the result of a lot of experimentation.  And there is more experimentation to come.

[llanitedave]

I would guess that the mass penalty would be prohibitive.

You save the mass of the landing kerolox propellant, but have to add the mass of the hypergolic SuperDraco propellant, which due to lower ISP and cosine losses, might be around 50% greater than the mass of the replaced kerolox.

Plus you have to add the mass of the SuperDracos themselves, the mounting hardware, the tanks for the hypergolic propellant and the connecting valves and piping.

Not to mention the changes to the center of gravity that would entail, with possible added stability and control issues.

[NovaSilisko]

Too bad a Kerolox version of the SuperDraco don't exist.  While there would be a mass penalty, it may be easier to land a legged pendulm rather than a broomstick on a fingertip.

https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy

[Robotbeat]

Given that the difficulty of landing the F9.1 first stage is that a single Merlin still has a T/W ratio larger than one, even when throttled down to its minimum throttling, how stupid/silly would it be to mount four pairs of SuperDracos on the interstage? A single SuperDraco has a vacuum thrust of 7.4 kN while a 40% throttled Merlin D has a vacuum thrust of about four times that. Which means that four pairs of SuperDracos are comparable to a 80% throttled Merlin D and can together throttled much lower.

Given the amount of tinkering that still appears to be going on with the interstage (grid fins and related hydraulics), how far-fetched is this idea?

They aren't having (undue) difficulty; they will achieve success. Fins are way less complicated operationally than a bunch of superdracos would be (unless you need them anyway, like for Dragon).

It's myopic fixation on problems that have already been solved or will be very shortly. Just like when people were all worried about valves all of a sudden after a scrub (on Delta IV then on Falcon 9, though a completely different issue), the problem is already fixed and will soon be forgotten.

EDIT: (On the other hand, your idea has some merit on some future upper stage recovery, where you have difficulty getting the single engine to have anywhere near low enough thrust to land propulsively.)

[Burninate]

I would like to see the SuperDracos available in this capacity specifically to aid in solving all ~12 degrees of freedom that need to be correct for a soft touchdown, amidst a moving reference frame. 3D position, 3D orientation, 3D velocity, 3D rotational velocity all need to be near-zero relative to the landing zone at impact, and long after the grid fins cease to be effective.  Fighting any significant wind from terminal velocity to ship velocity with the main engines essentially requires that some of these variables diverge from zero.  More wind, more problems.  SuperDracos provide extra thrust in the right directions at very high frequency.  This is *far* more useful than merely slowing down the hoverslam descent to lower G-ratings, because it expands the operational envelope of first stage reuse, something that exponentially increases the lifespan of a first stage in SpaceX's fleet given a fraction of missions targetting specific orbital windows.

Two problems with this, though:
1) While CoM does not have a huge effect during the flight of a pure rocket, contrary to intuition...  it does have a large effect on stability for the duration that the landing legs touch the deck.

2) Nitrogen tetroxide and monomethyl hydrazine.  If the landing requires these in quantity, the landing zone is a HAZMAT zone, and approval for such landings is even harder.  Green hypergolic propellants would be very strongly preferred.

[Jim]

because it expands the operational envelope of first stage reuse, something that exponentially increases the lifespan of a first stage in SpaceX's fleet given a fraction of missions targetting specific orbital windows.

It does nothing of the sort. 
1.  First, you don't know what is limiting the operational envelope, much less knowing the life span and what increases it.
2.  Don't know what orbital windows are much less what they have to do with launch vehicle reuse.  If you mean launch windows, they too have no effect on stage reuse.
3.  Allocating more first stage propellant for the return is a better trade than adding a completely separate system.
4.  A Draco system complicates reuse by adding more complexity.

And it is not 12 degree as you stated:  roll orientation is not constrained and you forgot acceleration

[Burninate]

because it expands the operational envelope of first stage reuse, something that exponentially increases the lifespan of a first stage in SpaceX's fleet given a fraction of missions targetting specific orbital windows.

It does nothing of the sort. 
1.  First, you don't know what is limiting the operational envelope, much less knowing the life span and what increases it.
2.  Don't know what orbital windows are much less what they have to do with launch vehicle reuse.  If you mean launch windows, they too have no effect on stage reuse.
3.  Allocating more first stage propellant for the return is a better trade than adding a completely separate system.
4.  A Draco system complicates reuse by adding more complexity.

And it is not 12 degree as you stated:  roll orientation is not constrained and you forgot acceleration

The operational envelope of first-stage reuse is at least sometimes limited by winds at the landing pad.

Launch windows which require eg synodic or Lunar period phasing or a rare rendezvous, launch windows which are not just tightly bound to a few seconds, but *sparse*, with long periods between them, mean you don't have much flexibility in when the launch occurs (or what the weather is at the landing pad).  If F9R's first stage becomes frequently reused, this minority of missions which have a go on launch but do not meet conditions for landing, become the limiting factor for first stage lifetime.

Allocating more first-stage propellant does not help with the aerodynamic / thrust controls problem of landing at the pad in a strong wind;  It's not a matter of fuel being scarce, but there not being enough degrees of freedom, and also not enough quantity in the horizontal direction, of thrust.

A Draco system does indeed complicate reuse by adding complexity.

Roll orientation is indeed unconstrained.  Acceleration is not constrained - 'Hoverslam' has substantial acceleration right up to touchdown, and a dynamic solution that requires high acceleration or angular acceleration to very briefly zero out all the things that need to be zeroed out, is just fine.

[Jim]

1.  The operational envelope of first-stage reuse is at least sometimes limited by winds at the landing pad.

2.  Launch windows which require eg synodic or Lunar period phasing or a rare rendezvous, launch windows which are not just tightly bound to a few seconds, but *sparse*, with long periods between them, mean you don't have much flexibility in when the launch occurs (or what the weather is at the landing pad).  If F9R's first stage becomes frequently reused, this minority of missions which have a go on launch but do not meet conditions for landing, become the limiting factor for first stage lifetime.

3.  Allocating more first-stage propellant does not help with the aerodynamic / thrust controls problem of landing at the pad in a strong wind;  It's not a matter of fuel being scarce, but there not being enough degrees of freedom, and also not enough quantity in the horizontal direction, of thrust.

1.  Same winds would also prevent launch

2.  not true see #1

3. wrong.  There are plenty of degrees of freedom.  Extra propellant allows for more time of main engine burn to counter the wind, including in the  horizontal direction

[drzerg]

Thay could make just central engine capable of doing 20%. No need to make all of 9. Or replace one Merlin D with one or two Kestrel. Or add them from each side of the bottom section. Dry weight   52 kilograms, (31 kN). May be additional small turbo pump will be needed.

[Jim]

Thay could make just central engine capable of doing 20%. No need do make all of 9. Or replace one Merlin D with one or two Kestrel. Or add them from each side of the bottom section. Dry weight   52 kilograms, (31 kN). May be additional small turbo pump will be needed.

Replacing an engine is a non starter, especially with Kestrels.  A turbo pump can't just be added.  It has to been integrated into the engine.  A Kestrel is pressure fed.  Adding a turbo pump means it is a completely new engine. 
Adding them to the side is also a non starter, there is no room and would require a major redesign of the thrust section.

[JamesH]

Any mileage in developing a modification of the Merlin that is lower thrust to stick in the centre of the octaweb, giving more ability during the landing process? There seems to be some excess performance of the 9 engines currently, could they take a small hit on one to give them better manoeuvrability?

[llanitedave]

Geez, people, you're driving me crazy here!  Is everyone Rube Goldberg in disguise?

They have a system, it's going to work.  Let them tweak it.

[oiorionsbelt]

Geez, people, you're driving me crazy here!  Is everyone Rube Goldberg in disguise?

They have a system, it's going to work.  Let them tweak it.

Absolutely, they have come up with an elegant solution and everyone seems to want to turn it into a kludge.

[llanitedave]

"Look, if we built this    

[JamesH]

Geez, people, you're driving me crazy here!  Is everyone Rube Goldberg in disguise?

They have a system, it's going to work.  Let them tweak it.

They have a system, it's *probably* going to work. Fixed that for you. Unless you have a crystal ball and already know.

But the question is... is there an advantage of being able to hover the stage (F9 and BFR) to give more time to land accurately? More relevant to BFR since the Rapture is going to be a powerful bit of kit with I suspect a higher T2W ratio than the Merlin.

* I personally think the 'hoverslam' will work. I'm just wondering about hovering advantages.

[cambrianera]

High gee landing has higher efficiency (less gravity losses).
What is needed is:
More experience.
Easier landing place.
(IMHO) A sturdier landing gear.