Blue Origin Thread (merged)

[KerbonautInTraining]

I wonder what those little silver things are at the top of the shock absorbers?

[Rakaydos]

I'm guessing locking pins/release mechanisim for the legs in flight.

[wumpus]

On 5/7/2016 at 3:59 AM, codepoet said:

Musk confirms that they did indeed cut two of the engines at then end of the landing burn and land on one. Also that the engines can now throttle down to 40%.

So if 40% of one engine is imparting >9.8m/s2 then three at full belt would be capable of giving at least 73.5m/s2. That could really take the edge off your velocity in a hurry.

Cool.  I wonder if this will be standard procedure (even if the go back to ~1km/s re-entry).  I'm pretty sure they want to get the center engine halfway through the range when the outer two cut out, even if they are never throttled.

I'm kind of surprised that the Falcon can't "throttle" the engines down to hovering thrust.  Not so much by reducing the fuel to the engines (which is really only needed for long, lower trust burns such as the center of a Falcon Heavy when it still has its boosters), but by letting the fuel/fire escape elsewhere (SRBs do this to fake "engine out").  I'd also expect the response of this type of thing to be faster than trying to monkey with the turbopumps.  Any idea if this is how Blue Origin does it (spacex presumably can throttle down to save fuel, no idea how many issues would suddenly explode if you had two means of throttling).

[sojourner]

Unlike KPS, in the real world rocket engines can't throttle down to %1. The fact that Elon has stated the Merlin can now throttle  to %40 is really impressive.

[PB666]

50 minutes ago, sojourner said:

Unlike KPS, in the real world rocket engines can't throttle down to %1. The fact that Elon has stated the Merlin can now throttle  to %40 is really impressive.

It may only be the main engine that is set to do this. I was watching the Merlin vacuum engine at low speed, it appears to go through a prolonged power up, it does not reach its full power up for many seconds after initiation.

Go to +2:44 (21:17 in the CRS-8 Technical video) and set you you-tube settings to 0.25 rate. Its starts at a velocity of about 4/10ths of second later and velocity starts slowing down its drop, previously about -20 kmh/second. The initial quarter of a second dropped the decline to about -10 kmh/second, the next quarter second the drop is around 0 kmh/sec and it holds that for about 2/3rds of a second, then is ramps up power again to gain 3 kmh/secong (Remembering we start with -20, the angle of attack has risen after decoupling so is fighting more gravity) About 2 seconds after the original engine engage it increases to 6 kmh/second and by 5 seconds later is about 9 kmh/second, after this the power up slows. If we remember the boosting topped out at about 3.6g of ground relative acceleration. 9kmh/s is about 0.25g and given the ground deceleration and loss of potential (we are not aware of) the engine is only producing TWR of 0.8. The video makes the acceleration to look greater than it is, its actually kmh/3.6 which gives meters per second.

 

Edited May 8, 2016 by PB666

[PB666]

1 minute ago, WildLynx said:

In real world, engines tend to extinguish, or worse, vibrate themselves apart, when set to unfavorable power setting. Throttling down to 40% is really impressive for booster engine. Lander engines can throttle down even deeper, but they are not optimal as booster engines.

But there is also alot more bounce when the Merlins are at  (full power) versus cut to 90%.

[sojourner]

25 minutes ago, PB666 said:

It may only be the main engine that is set to do this. I was watching the Merlin vacuum engine at low speed, it appears to go through a prolonged power up, it does not reach its full power up for many seconds after initiation.

 

Don't presume that just because it takes a few seconds for the engine bell to start to glow that the engine is not at full power.

[PB666]

2 minutes ago, sojourner said:

Don't presume that just because it takes a few seconds for the engine bell to start to glow that the engine is not at full power.

I don't think that is the cap, I think they are holding power until the nose cone is deployed, then they go to full. There is actually some logic to this, because their space craft does not have an optimal, prograde angle angle. You can tell because the stage 1 rises first relative to the camera as the space craft has pitched up and view is pointing down, as the spacecraft gains thrust the stage 1 comes back into view and quickly disappears on the anti-horizon side of the image. So it kind of makes sense to push up deploy the nose cone while the remnant atmosphere is thinning then turn and go full power. You can tell when the Merlin is close to full power, the outside edge of the bell starts to flex About T+3:00, but if you want to see real flexing of the bell go to about 3:45 and watch the velocity increasing and the amount of flexing. By 4:20  minutes its gaining about 21kmh/sec with a TWR almost double (1.61) that of 2::52. The second stage has a burn time of 397 seconds and thus in has burned maximally 24% of the fuel, so yeah through 3 minutes and to 4 minutes the second stage engine is still powering up, not likely due to bell housing temperature effects either. Another thing is that you can see vibrational gimbling from 3:00 goes on and may be causing some of the optical illusion that the bell housing is flexing. 

 

 

[sojourner]

The question is how deep can it throttle?

[wumpus]

6 hours ago, WildLynx said:

In real world, engines tend to extinguish, or worse, vibrate themselves apart, when set to unfavorable power setting. Throttling down to 40% is really impressive for booster engine. Lander engines can throttle down even deeper, but they are not optimal as booster engines.

My point wasn't that it would be easy to convince the turbopumps to shove less fuel & oxidizer downstream (I'm assuming that the 40% happens close enough to this way).  My point is that there are other places said fuel and oxidizer can go and not push against the bell nozzles quite as hard.  Getting all the efficiency to maximum (or maximized average) is hard.  Finding some means to reduce said efficiency should be easier.  No idea if reducing power by a precise level is even possible this way.

As I said, this is how SRBs greatly reduce thrust when it is "engine cut-off time" (except that there it is all or nothing).  I wouldn't be too surprised if it is already done and "40%" is the limit for real control over the thing (a few times they need to throttle for fuel efficiency reasons, but that can be done with slow changes.  They also need fast changes which don't need such high efficiency).

[sojourner]

Where would you have that shunted fuel and oxygen go?  That wouldn't result in uncontrolled combustion in a place you don't want it?  Besides, the issue isn't with being able to control how much runs through the turbo pumps to get your throttle range, it's how much minimal fuel/oxygen is burning in the bell before you get a flame out/coking.

[softweir]

5 hours ago, wumpus said:

My point wasn't that it would be easy to convince the turbopumps to shove less fuel & oxidizer downstream (I'm assuming that the 40% happens close enough to this way).  My point is that there are other places said fuel and oxidizer can go and not push against the bell nozzles quite as hard.  Getting all the efficiency to maximum (or maximized average) is hard.  Finding some means to reduce said efficiency should be easier.  No idea if reducing power by a precise level is even possible this way.

As I said, this is how SRBs greatly reduce thrust when it is "engine cut-off time" (except that there it is all or nothing).  I wouldn't be too surprised if it is already done and "40%" is the limit for real control over the thing (a few times they need to throttle for fuel efficiency reasons, but that can be done with slow changes.  They also need fast changes which don't need such high efficiency).

That sort of throttle-down would be near useless for a SpaceX landing post-GTO launch: they are operating on the edge of not having enough fuel, and throttling down by throwing away fuel is an excellent way of making quite, quite sure there isn't enough fuel. The same applies to throttling down during launch.

Yes, in other circumstances venting fuel from the pumps might be an option, but I can't help but feel it isn't an option for SpaceX. It makes much more sense to find some way to controllably spin-down the turbopumps while maintaining a flame in the combustion chamber. Much harder, yes, but if you can do it then that is much more fuel efficient and sensible.

Finally, the vast majority of orbital-launch-capable SRBs don't reduce power by venting! They reduce power by careful shaping of plugs so that the area of grain available to burn reduces as the plug burns away. That's the way the Space-Shuttles and all other serious launch vehicles operate. SRB plug design is an art and a science - as is working out when you want the SRB to start to taper-off its thrust.

SRB venting has been experimented upon, not as a routine way to reduce power but as an emergency measure to kill thrust in an emergency. I don't know of any rockets that have been commercially launched using that technology, as it is a very dramatic and dangerous way to kill thrust.

[magnemoe]

5 hours ago, wumpus said:

My point wasn't that it would be easy to convince the turbopumps to shove less fuel & oxidizer downstream (I'm assuming that the 40% happens close enough to this way).  My point is that there are other places said fuel and oxidizer can go and not push against the bell nozzles quite as hard.  Getting all the efficiency to maximum (or maximized average) is hard.  Finding some means to reduce said efficiency should be easier.  No idea if reducing power by a precise level is even possible this way.

As I said, this is how SRBs greatly reduce thrust when it is "engine cut-off time" (except that there it is all or nothing).  I wouldn't be too surprised if it is already done and "40%" is the limit for real control over the thing (a few times they need to throttle for fuel efficiency reasons, but that can be done with slow changes.  They also need fast changes which don't need such high efficiency).

Some solid fuel rockets has blow out sections to kill trust, nice for an ICBM especially on submarines where you might want to strike an target pretty close. 
Only reason on an space rocket is self destruct so the rocket don't fly in over populated areas if out of controll, here you can be far more crude as you don't need an accurate shutdown. 

problem with venting fuel and oxidizer is that it will ignite. Yes it might be possible to inject it into the flame outside the engine chamber where it would not add much trust while getting burned safely.

[kerbiloid]

1 hour ago, magnemoe said:

problem with venting fuel and oxidizer is that it will ignite

They can vent them in turn. For example, any plane can vent its fuel (into an atmosphere full of oxygen)

Edited May 9, 2016 by kerbiloid

[Streetwind]

10 hours ago, sojourner said:

The question is how deep can it throttle?

Originally, the 1D sea level version was rated at 620 kN sea level thrust, and able to throttle to about 435 kN (~70%).

 

If you now assume that a minimum throttle setting is due to combustion instability and flow separation from the nozzle when not running with enough power, then that means that that 435 kN minimum thrust figure should not change significantly when you uprate the engine to more maximum thrust. Curiously enough, SpaceX now claims a minimum thrust of 530 kN for the v1.1 "Full Thrust" Merlins, which still is about 70% of the maximum specified 756 kN sea level thrust (source). Not sure why they can't simply drop it down to the original 435 kN, if there were no physical changes to the engine, as Elon Musk claimed...?

Meanwhile, SpaceX claims in the same document that the Merlin 1D Vacuum is able to go as low as 360 kN vacuum thrust, which is only 38.5% of its maximum rated 934 kN vacuum thrust. Now, the MVac is somewhat modified from the sea level version, but I would personally guess that the much lower minimum throttle setting here is more a result of the vacuum itself minimizing the risks of flow separation and such things, giving you more leeway.

How much can the sea level Merlin throttle in a vacuum, or the MVac at sea level? Those figures are not given - and honestly, those cases are not particularly relevant. The F9 first stage even has the option of shutting down engine pairs to limit its acceleration near the end of its burn (an option sometimes exercised with the v1.0, using unthrottleable 1C engines), so the whole stage's throttle range is extreme in any case, and not reliant on the engine's capabilities.

 

Ideally we'll get an updated User's Guide along with the further uprating later this year, so we can have throttling info for that use case as well

Edited May 9, 2016 by Streetwind

[sevenperforce]

3 hours ago, Streetwind said:

Originally, the 1D sea level version was rated at 620 kN sea level thrust, and able to throttle to about 435 kN (~70%).

 

If you now assume that a minimum throttle setting is due to combustion instability and flow separation from the nozzle when not running with enough power, then that means that that 435 kN minimum thrust figure should not change significantly when you uprate the engine to more maximum thrust. Curiously enough, SpaceX now claims a minimum thrust of 530 kN for the v1.1 "Full Thrust" Merlins, which still is about 70% of the maximum specified 756 kN sea level thrust (source). Not sure why they can't simply drop it down to the original 435 kN, if there were no physical changes to the engine, as Elon Musk claimed...?

Meanwhile, SpaceX claims in the same document that the Merlin 1D Vacuum is able to go as low as 360 kN vacuum thrust, which is only 38.5% of its maximum rated 934 kN vacuum thrust. Now, the MVac is somewhat modified from the sea level version, but I would personally guess that the much lower minimum throttle setting here is more a result of the vacuum itself minimizing the risks of flow separation and such things, giving you more leeway.

How much can the sea level Merlin throttle in a vacuum, or the MVac at sea level? Those figures are not given - and honestly, those cases are not particularly relevant. The F9 first stage even has the option of shutting down engine pairs to limit its acceleration near the end of its burn (an option sometimes exercised with the v1.0, using unthrottleable 1C engines), so the whole stage's throttle range is extreme in any case, and not reliant on the engine's capabilities.

 

Ideally we'll get an updated User's Guide along with the further uprating later this year, so we can have throttling info for that use case as well

Minimum thrust is now just over 300 kN per Elon's discussion of the three-engine suicide burn on the JCSAT-14 mission.

Being able to go from 2,268 kN to 302 kN (and everything in between) in a fraction of a second, controllably, is a pretty terrific achievement. That's the equivalent of throttling by 87%.

I wonder what's harder on the bird: coming through re-entry at nearly a mile per second, or burning retrograde at 2.3 million Newtons to brake for landing. 

[Streetwind]

1 hour ago, sevenperforce said:

Minimum thrust is now just over 300 kN per Elon's discussion of the three-engine suicide burn on the JCSAT-14 mission.

Really? Then why does their own user guide claim almost twice that?

Darnit SpaceX, make up your mind, and give us proper numbers!

[sojourner]

8 hours ago, kerbiloid said:

They can vent them in turn. For example, any plane can vent its fuel (into an atmosphere full of oxygen)

Um, aircraft don't have open combustion taking place where you'd be dumping that fuel and pure oxygen from. Unless of course we're talking about afterburners which sort of defeats the purpose intended in this conversation.

[PB666]

2 hours ago, Streetwind said:

Really? Then why does their own user guide claim almost twice that?

Darnit SpaceX, make up your mind, and give us proper numbers!

Well at the time 70% was not full thrust, if you then increase the FT number on the same rocket engine by 20% and lower you lower thrust limit tolerance its 40%.

Yeah, there is a bit of Darnit there, SpaceX does not seem to be fixated on accuracy.

[kerbiloid]

1 hour ago, sojourner said:

Um, aircraft don't have open combustion taking place where you'd be dumping that fuel and pure oxygen from.

And when combustion stops...

[sevenperforce]

58 minutes ago, kerbiloid said:

And when combustion stops...

Pretty lights!

[KerbonautInTraining]

The thing about this throttling conversation is, while it may seem like a large inconvenience, it really isn't.

During the final descent all it has to do is hit the ground at less than 6m/s (stated limits of the shock absorbers). 6m/s is child's play to a rocket's guidance computer.

Plus, the way they're doing it now is how they'd do it even if they could hover.

The reason each landing looks better than the last (more accurate) is because they learn from every flight. For instance, let's say the engine had to run at 98% the entire landing burn to keep the touchdown within limits. On the next flight the computer would be commanded to light the engine slightly earlier. 

[PB666]

4 minutes ago, KerbonautInTraining said:

The thing about this throttling conversation is, while it may seem like a large inconvenience, it really isn't.

During the final descent all it has to do is hit the ground at less than 6m/s (stated limits of the shock absorbers). 6m/s is child's play to a rocket's guidance computer.

Plus, the way they're doing it now is how they'd do it even if they could hover.

The reason each landing looks better than the last (more accurate) is because they learn from every flight. For instance, let's say the engine had to run at 98% the entire landing burn to keep the touchdown within limits. On the next flight the computer would be commanded to light the engine slightly earlier. 

98% is meaningless, instantaneous output for the merlin is not published and even if it was its not currently credible. The problem is flex on the bell, but you can over power the engine breifly and the bell should hold until temperature of the metal rises. 

What that means is that they can push the engine to 98% of its currently rated power, but in a pinch, probaly 115%, who knows, theres not a person in this forum who knows how powerful the one minute of operation maximum is, even Elon may not know, today. 

[sojourner]

2 hours ago, kerbiloid said:

And when combustion stops...

The engine is out. You only have so many restarts with the Merlin, so shutting it off to dump fuel is not going to work as a way to throttle thrust.Unless your goal is to you know, shut down the engine, then it's sort of pointless to be dumping fuel to reduce thrust in the first place.

[Kryten]

New video from the latest test;