Who will compete with SpaceX? The last two and next two years.

[envy887]

To me the obstacle is not the first/second deltaV split (that can be overcome), it is the engine size on the first stage. Absent REALLY deep throttling, a 2 or 3 engine launcher can't decrease thrust far enough to land without some kind of auxiliary engine of lower thrust. Which adds mass that isn't used for the primary mission. And adding such an engine is a major vehicle redesign.

ULA and Ariane have too few engines on the first stage... IMHO.  This is a bit off topic so I'll leave it at that.

SpaceX has demonstrated landing with 3/9 engines, and they have quite high T/W ratio on F9.

So there should not be any problem with landing with 1/3 engines on a stage with a more common T/W ratio with similar throttling capacity.

Though Merlin's pintle injector allows deeper throttling than many other engines.

Is the injector type of BE-4 known?

Falcon 9 always lands on a single engine which throttles down to 40%, and even then it cannot hover.

I highly doubt anyone is going to land a 3 engine orbital boost stage on 1 main engine anytime soon.

Falcon 9 has landed on 3 engines multiple times, the last time being BulgariaSat-1.

The outer two engines shut down just before landing... the actual touchdown is always on a single engine.

[Ludus]

To me the obstacle is not the first/second deltaV split (that can be overcome), it is the engine size on the first stage. Absent REALLY deep throttling, a 2 or 3 engine launcher can't decrease thrust far enough to land without some kind of auxiliary engine of lower thrust. Which adds mass that isn't used for the primary mission. And adding such an engine is a major vehicle redesign.

ULA and Ariane have too few engines on the first stage... IMHO.  This is a bit off topic so I'll leave it at that.

It is kinda topical to near term competition with SpaceX. The more Ariane and ULA invest in their partial reusability systems the harder it will be to completely junk those designs and start over with something that's competitive with SpaceX and Blue Origin. Having 2 - 3 engine designs is what constrains them to "smart reusability" even as SpaceX makes it look more outdated every month.

[LouScheffer]

Falcon 9 always lands on a single engine which throttles down to 40%, and even then it cannot hover.

I highly doubt anyone is going to land a 3 engine orbital boost stage on 1 main engine anytime soon.

This is not clear to me.  What is needed technically is an engine that can throttle to 13% or so.  No current booster engine can do this, but it's not fundamentally impossible.  No one has tried it with a booster engine since there was no requirement.  It may well be easier than re-designing a booster with more and smaller engines.

The one time such deep throttling WAS a serious requirement, on the Descent Propulsion System for the moon landings, it was made to work.

[envy887]

Falcon 9 always lands on a single engine which throttles down to 40%, and even then it cannot hover.

I highly doubt anyone is going to land a 3 engine orbital boost stage on 1 main engine anytime soon.

This is not clear to me.  What is needed technically is an engine that can throttle to 13% or so.  No current booster engine can do this, but it's not fundamentally impossible.  No one has tried it with a booster engine since there was no requirement.  It may well be easier than re-designing a booster with more and smaller engines.

The one time such deep throttling WAS a serious requirement, on the Descent Propulsion System for the moon landings, it was made to work.

Except for Falcon none of the existing multi-engine booster designs could hover upright on a single engine with any amount of throttling, since none have a engine on the centerline of the vehicle. Landing those on a single engine would require a complete booster redesign.

Landing on the same number of engines as used for boost requires very deep throttling, at least 10% and probably lower. The major problem there is flow separation due to low chamber pressures, a problem the lunar descent engine did not have. Controlling the thrust vector on a separated exhaust stream is a major issue.

[spacenut]

To me, "old school" thought is fewer engines the better for less chance of failure.  But, it also meant throwing away the booster.  SpaceX and BO are new school thought.  Throttling down of the main engine as in New Sheppard, or multiple engines and using a single engine for landing as Falcon 9.  I also think partial reusable as the "old school" companies begin to try to compete on pricing will be a dead end in the long run.  Maybe if they parachute down the tanks separately after engines disconnect and try to retrieve them to re-assemble later might save some money. 

[envy887]

To me, "old school" thought is fewer engines the better for less chance of failure.  But, it also meant throwing away the booster.  SpaceX and BO are new school thought.  Throttling down of the main engine as in New Sheppard, or multiple engines and using a single engine for landing as Falcon 9.
...

New Shepard lands on it's single main engine, but it also throttles to 18% and has mass fractions highly unsuitable for an orbital booster.

[AncientU]

To me the obstacle is not the first/second deltaV split (that can be overcome), it is the engine size on the first stage. Absent REALLY deep throttling, a 2 or 3 engine launcher can't decrease thrust far enough to land without some kind of auxiliary engine of lower thrust. Which adds mass that isn't used for the primary mission. And adding such an engine is a major vehicle redesign.

ULA and Ariane have too few engines on the first stage... IMHO.  This is a bit off topic so I'll leave it at that.

It is kinda topical to near term competition with SpaceX. The more Ariane and ULA invest in their partial reusability systems the harder it will be to completely junk those designs and start over with something that's competitive with SpaceX and Blue Origin. Having 2 - 3 engine designs is what constrains them to "smart reusability" even as SpaceX makes it look more outdated every month.

That's why each company has spent about the cost of a PowerPoint on them.

[Nomadd]

To me, "old school" thought is fewer engines the better for less chance of failure.  But, it also meant throwing away the booster.  SpaceX and BO are new school thought.  Throttling down of the main engine as in New Sheppard, or multiple engines and using a single engine for landing as Falcon 9.  I also think partial reusable as the "old school" companies begin to try to compete on pricing will be a dead end in the long run.  Maybe if they parachute down the tanks separately after engines disconnect and try to retrieve them to re-assemble later might save some money. 

The F9 doesn't have 9 engines so it can be recovered. It has nine so an engine failure isn't a mission failure. Losing an engine means losing 100% of your missions with anybody else. 
 Recovery by landing came after the engine configuration decision.

[punder]

To me, "old school" thought is fewer engines the better for less chance of failure.  But, it also meant throwing away the booster.  SpaceX and BO are new school thought.  Throttling down of the main engine as in New Sheppard, or multiple engines and using a single engine for landing as Falcon 9.  I also think partial reusable as the "old school" companies begin to try to compete on pricing will be a dead end in the long run.  Maybe if they parachute down the tanks separately after engines disconnect and try to retrieve them to re-assemble later might save some money. 

The F9 doesn't have 9 engines so it can be recovered. It has nine so an engine failure isn't a mission failure. Losing 1% of your engines means losing 1% of your missions with anybody else. 
 Recovery by landing came after the engine configuration decision.

9 engines is definitely good for engine-out, but I think the real reason the F9 has that many, is because that's how many Merlin 1Cs (inherited from the F1) the configuration needed for the design performance. The original step up from the F1 had only 5, and IIRC they advertised engine-out for that one too.

[livingjw]

 9 engines so they can use the same engine on the second stage. Also, allows for engine out capability, but required them to design engines to fail in such a way as to not take out others.

John

[deruch]

To me the obstacle is not the first/second deltaV split (that can be overcome), it is the engine size on the first stage. Absent REALLY deep throttling, a 2 or 3 engine launcher can't decrease thrust far enough to land without some kind of auxiliary engine of lower thrust. Which adds mass that isn't used for the primary mission. And adding such an engine is a major vehicle redesign.

ULA and Ariane have too few engines on the first stage... IMHO.  This is a bit off topic so I'll leave it at that.

Note this issue is only for VTVL boosters, horizontal landing isn't dependent on engine size/throttling.  Maybe a competitor will try it differently from SpaceX?

[livingjw]

To me the obstacle is not the first/second deltaV split (that can be overcome), it is the engine size on the first stage. Absent REALLY deep throttling, a 2 or 3 engine launcher can't decrease thrust far enough to land without some kind of auxiliary engine of lower thrust. Which adds mass that isn't used for the primary mission. And adding such an engine is a major vehicle redesign.

ULA and Ariane have too few engines on the first stage... IMHO.  This is a bit off topic so I'll leave it at that.

Note this issue is only for VTVL boosters, horizontal landing isn't dependent on engine size/throttling.  Maybe a competitor will try it differently from SpaceX?

VTHL has been studied and it appears that its it performs about as well as VTVL, with two exceptions: 1) it is more complicated and 2) no barge landing, obviously.

John

[LouScheffer]

Except for Falcon none of the existing multi-engine booster designs could hover upright on a single engine with any amount of throttling, since none have a engine on the centerline of the vehicle. Landing those on a single engine would require a complete booster redesign.

Landing on the same number of engines as used for boost requires very deep throttling, at least 10% and probably lower. The major problem there is flow separation due to low chamber pressures, a problem the lunar descent engine did not have. Controlling the thrust vector on a separated exhaust stream is a major issue.

These are real problems, but have received little engineering effort.  There are many ways these issues might be addressed.  For example, the RD-107 has two vernier thrust chambers driven by the same turbopumps.  If you shut off the main chamber and used only these for landing, you could solve both the centerline and separated exhaust issues.

Or you could try to control exhaust separation.  The current completely symmetrical nozzles are awful for this.  You could make them slightly non-round, or perhaps add bumps to encourage certain patterns of separation, or try fluidic control by injecting liquid or gas from the side, or other tricks.

These tricks might, or might not, be easier than a complete redesign with more but smaller engines.  It seems at least plausible that a stage with a single large engine might be cheaper to manufacture than one with nine engines of almost equal complexity.  But no-one has considered this seriously, to my knowledge.

[spacenut]

Is the BE-3 the only engine with a wide range throttle?  If booster engines could be built this way and larger engines, then a 3 engine or 5 engine rocket could throttle down the middle engine for landing.  Seems as if SpaceX is mass producing engines with 3D printing less expensive than developing a larger engine with a wide throttle range.  If they can make 9 engines as cheap as one big engine with a wide throttle range, it seems to be a better idea.  Also it allows engine out capability during launch.  9 engines at $18 million vs one big engine at 18 million.  Seems the 9 would work better with engine out.  If big engine goes out, the mission is loss.  BO is going to use 7 engines on the New Glenn.  They can land on one and have engine out. 

[envy887]

Except for Falcon none of the existing multi-engine booster designs could hover upright on a single engine with any amount of throttling, since none have a engine on the centerline of the vehicle. Landing those on a single engine would require a complete booster redesign.

Landing on the same number of engines as used for boost requires very deep throttling, at least 10% and probably lower. The major problem there is flow separation due to low chamber pressures, a problem the lunar descent engine did not have. Controlling the thrust vector on a separated exhaust stream is a major issue.

These are real problems, but have received little engineering effort.  There are many ways these issues might be addressed.  For example, the RD-107 has two vernier thrust chambers driven by the same turbopumps.  If you shut off the main chamber and used only these for landing, you could solve both the centerline and separated exhaust issues.

Or you could try to control exhaust separation.  The current completely symmetrical nozzles are awful for this.  You could make them slightly non-round, or perhaps add bumps to encourage certain patterns of separation, or try fluidic control by injecting liquid or gas from the side, or other tricks.

These tricks might, or might not, be easier than a complete redesign with more but smaller engines.  It seems at least plausible that a stage with a single large engine might be cheaper to manufacture than one with nine engines of almost equal complexity.  But no-one has considered this seriously, to my knowledge.

For Vulcan, adding verniers to BE-4 might not be desirable as it reduces commonality with New Glenn while still requiring thrust structure for more chambers. It might be simpler to add a pair of RL-10 or Broadsword engines. RL-10 has demonstrated burning methane and deep throttling. RL-10 has also demonstrated air-start, multiple restarts and actually landing (DC-X). BE-4 will be capable of restarts and could scrub off the majority of velocity at landing in a very high-g maneuver before settling on the landing engines.

Alternatively, Vulcan already has SRB mounting points; I've always wondered if non-separating LRBs with restartable, throttleable liquid engines (e.g. BE-3, perhaps running methane) would work for landing. The LRBs could provide extra boost on the way up, and in theory make up the performance hit for reuse. But the aerodynamics of flying them backwards on return might be dicey though.

Ariane is looking at a clean-sheet methalox design, and I wouldn't be surprised if it turned into a multi-engine design capable of landing and replaced A6 - with A5 flying longer to make up the difference in development time.

The Russians are in kind of a pickle as transport issues make downrange landing very difficult, and the same issues force small diameters which aren't good for RTLS with even a medium lift vehicle. So while they could probably land a booster on a highly updated RD-108, I'm not sure where they would land it.

[AncientU]

So, when will we start seeing these direct-to-reusable efforts that skip the 'next generation' expendable launch vehicles?  2018 seems too soon for each of these establishment players...2020?  Or will they field interim 'next generation' expandables, buying time, but wasting billions?

[copper8]

For Vulcan, adding verniers to BE-4 might not be desirable as it reduces commonality with New Glenn while still requiring thrust structure for more chambers. It might be simpler to add a pair of RL-10 or Broadsword engines. RL-10 has demonstrated burning methane and deep throttling. RL-10 has also demonstrated air-start, multiple restarts and actually landing (DC-X). BE-4 will be capable of restarts and could scrub off the majority of velocity at landing in a very high-g maneuver before settling on the landing engines.

Carrying along the mass of a whole second set of engines (along with the complications to the plumbing) seems like an extraordinary penalty just for landing. (On top of the other mass penalties that the Falcon 9 is already paying)

[Lar]

Verniers presumably are lighter than main engines,, but yeah, it just doesn't make sense to this layman.

[envy887]

Verniers presumably are lighter than main engines,, but yeah, it just doesn't make sense to this layman.

For Vulcan, adding verniers to BE-4 might not be desirable as it reduces commonality with New Glenn while still requiring thrust structure for more chambers. It might be simpler to add a pair of RL-10 or Broadsword engines. RL-10 has demonstrated burning methane and deep throttling. RL-10 has also demonstrated air-start, multiple restarts and actually landing (DC-X). BE-4 will be capable of restarts and could scrub off the majority of velocity at landing in a very high-g maneuver before settling on the landing engines.

Carrying along the mass of a whole second set of engines (along with the complications to the plumbing) seems like an extraordinary penalty just for landing. (On top of the other mass penalties that the Falcon 9 is already paying)

It's not parasitic mass if it's used during ascent - more thrust during ascent generally results in a net gain in payload to orbit.

RL-10A-4-2 weighs 170 kg and produces 10,000 kgf of thrust burning hydrolox. On Vulcan they could plumb it into the main tanks and burn methalox for even more thrust, probably close to 15,000 kgf. On a 20,000 kg booster the extra weight of the engine(s), even with extra plumbing and thrust structure, is relatively tiny - the extra fuel for landing weighs a lot more.

Is the BE-3 the only engine with a wide range throttle?  If booster engines could be built this way and larger engines, then a 3 engine or 5 engine rocket could throttle down the middle engine for landing.

I wonder if BE-3 could burn methalox. It would make a good landing engine if located between the BE-4's on Vulcan, and extra thrust on ascent could reduce the need for SRBs.

A 5-engine hydrolox BE-3 booster could make an interesting reusable Delta II class launcher, but neither Blue nor ULA seem very interested in that market.

[speedevil]

Except for Falcon none of the existing multi-engine booster designs could hover upright on a single engine with any amount of throttling, since none have a engine on the centerline of the vehicle. Landing those on a single engine would require a complete booster redesign.

This assumes hovering is a requirement.
Falcon 9 doesn't do it, and has never approached this as a strategy. (grasshopper of course did)

(it decellerates at around 7m/s^2, all the way to the ground). (orbcomm launch)

In principle, all that is required is the control system can close the loop and get it to near zero roll-pitch-yaw, translational speed and vertical speed all at the same time.

You can do this with off-centre thrust, though probably requiring a larger landing pad. It means your  acceleration in the direction of the assymetry cannot be zero at the time of landing, but that's not something you're trying to minimise.
Making the landing target an explicit strip in the direction of approach may be required.

However, even if this was in principle possible, it does require significant - possibly implausible - modifications to existing flying rockets, the grid-fins (or upgraded thrusters) and legs are not trivial, nor may be adequately fast response on the main engine be plausible.

It would be _real_ fun to watch though, as it would be decellerating several times faster than F9, with very considerable lateral velocity until the end.