What "weed need" is a space mission!

[chalsall]

Originally Posted by Dubslow

There's been multiple three engine landing burns for purposes of efficient use of extremely limited fuel.

Thanks for the correction. Sincerely.

Originally Posted by Dubslow

Conversely, I can't think of a reason to do a single engine reentry burn (excepting the earlier instances where only one engine had reignition equipment, which hasn't been the case for several launches).

One possibility I can think of is to slow the reentry speed over a longer period (applying the breaks slowly) such that the heating experience by the vehicle because of atmospheric interaction is lessened to a defined tolerance envelope while it is falling.

I could, of course, be completely wrong.

[Dubslow]

Originally Posted by chalsall

One possibility I can think of is to slow the reentry speed over a longer period (applying the breaks slowly) such that the heating experience by the vehicle because of atmospheric interaction is lessened to a defined tolerance envelope while it is falling.

Heating and stress are related to speed by a square? cube? something like that.

Using less thrust means higher speeds for longer times. The higher the thrust, the less total speed, the less total stress. Yeah it would be shorter, but that's also a good thing. Less stress over less time > more stress over more time.

[xilman]

Originally Posted by Dubslow

Conversely, I can't think of a reason to do a single engine reentry burn

I can, but it's pretty contrived.

Higher tthrust from three engines rather than one means higher g-forces on the structure. If some component is, or has become, weakened sufficiently ...

[retina]

Originally Posted by Dubslow

There's been multiple three engine landing burns for purposes of efficient use of extremely limited fuel.

Why stop at three? Why not four? Or six? Why not extend that logic to all nine engines?

I guess there are some very good reasons, but I'm not so sure you have touched on them here.

[Dubslow]

Originally Posted by retina

Why stop at three? Why not four? Or six? Why not extend that logic to all nine engines?

I guess there are some very good reasons, but I'm not so sure you have touched on them here.

Here's the primary answer:

Originally Posted by xilman

I can, but it's pretty contrived.

Higher tthrust from three engines rather than one means higher g-forces on the structure.

The empty mass of the first stage is somewhere around 25 tons (549 while fueled), while all 9 Merlin 1D engines have a thrust around 775 tons. If they use 5% of the remaining fuel for landing operations (it's a ballpark guess), then 3 engines of the 9 will be producing around 5gs of acceleration (775 / 3 / 50), which is generally the structural limit. As the fuel decreases further, they throttle the three engines, then shutdown two and go full thrust on the center engine, and eventually that one throttles down for landing.

Same limitation for the entry burn. More thrust is more efficient fuel use, up to structural loading limits.

Secondary issues include requiring re-igniters for more than three engines, and symmetric thrust requirements will exclude certain engine counts from being usable (if the structural acceleration limit hadn't already been reached). [Given that the center engine is required in any engine use pattern, and the ring around the center has 8-fold symmetry, any even count is automatically eliminated (since it would require an odd number of outer engines). 1, 3, 5 and 9 are the only viable counts (7 might be doable, but it would be far more sensitive to perturbations than 3 I think).]

[chalsall]

Originally Posted by Dubslow

Same limitation for the entry burn. More thrust is more efficient fuel use, up to structural loading limits.

Respectfully, I think this analysis is ignoring the variable of the atmospheric drag as a function of altitude (and the associated heating, which is a cubed function to speed).

Surely they're using drag to help the vehicle slow down (at least, here on earth with our thick atmosphere).

Originally Posted by Dubslow

Secondary issues include requiring re-igniters for more than three engines, and symmetric thrust requirements will exclude certain engine counts from being usable (if the structural acceleration limit hadn't already been reached).

I didn't realize that the Merlin engines needed to be "armed" with re-igniters. It doesn't surprise me though, thinking about it.

At the end of the day, I'm just a vicarious observer watching in awe what SpaceX is doing, and trying to figure out how they're doing it without any inside information. Thanks to those more knowledgeable sharing.

[only_human]

I think all the Russian engines used in the US use hypergolic fuel. Those wouldn't need igniters but a Rapid Unplanned Disassembly would be more worrisome.
Russian Rocket Explosion Releases Toxic Fuel Cloud

[Dubslow]

Originally Posted by chalsall

Respectfully, I think this analysis is ignoring the variable of the atmospheric drag as a function of altitude (and the associated heating, which is a cubed function to speed).

Surely they're using drag to help the vehicle slow down (at least, here on earth with our thick atmosphere).

I think you misunderstand the point of the re-entry burn. Obviously the "most" efficient way to return the booster is to do one and only one landing burn and let drag do what it can before that. But that's about as viable as fitting a ladder into a barn by running at relativistic speeds.

For the simplistic purposes of this post, we can regard the Earth's atmosphere as being essentially zero above ~35-40 km (not strictly true for orbiting and station keeping spacecraft over periods of days months and years, but not a terrible approximation for hypersonic things over the course of a few minutes) and dense below such height. See for example this graph.

After stage separation for GTO missions, the booster is travelling ~2300 m/s +- 100 m/s. (This is a good time to contemplate that highway speed is ~30 m/s, the speed of sound [at sea level] is ~340 m/s, and this booster velocity is still a ~quarter of LEO regime orbital velocity.) In other words, it is extremely hypersonic.

Were the booster to impact the atmosphere (in the binary approximation described above) at this speed, it would more or less immediately RUD. The aerodynamic and heating forces involved would tear it to bite size shreds in seconds (not unlike, for example, the Columbia orbiter). (In order to survive such forces requires designing a spacecraft's shape entirely around this point of survival -- hence capsules with their blunt bottoms and special materials and the space shuttle with its blunt bottom and special materials.)

The obvious way around this is to slow the booster down from "oh my god running into this wall of air will kill me" to "oh I'll just take a leisurely stroll into this here air". To wit, see the attached graphs (sourced from here, you will need to play with the scales on the altitude and velocity graphs to get what's below. The data is simulated to best fit post facto data publicly available, primarily from the streams they do). In particular, the speed is roughly halved (more than 1000 m/s delta v!), and the burn lasts from ~70 to ~50 km (varies from mission to mission, this video mentions 40 km as the shutoff). In fact, you can see from the velocity graph that for a short while after the re-entry burn shuts off, gravity still overpowers drag until roughly ~30 km altitude, after which yes drag does indeed do the majority of the energy dispersion.

At the end of the day, the re-entry burn is never more than the bare minimum it needs to be for the rocket to survive impacting the atmosphere, for precisely the reason of getting best fuel efficiency by using drag. Among other things, the bottom of the rocket is in fact mildly heat-shielded to help reduce the burn time required.

Though this was rather long winded, I hope it was very informative. As a bonus, the site I linked also offers animations of the whole launch (again post facto simulated), see e.g. JCSAT-14 launch here (largely similar launch, payload, and trajectory).

I didn't realize that the Merlin engines needed to be "armed" with re-igniters. It doesn't surprise me though, thinking about it.

Indeed, in the replay of any SpaceX launch, around T-4 to T-3 or so you can see the green glow of the TEA-TEB hypergolic ignition materials. Given what the public knows about the Merlins requiring individual re-ignition systems onboard, I would hazard to guess that the launch ignition TEA-TEB is provided from the pad itself rather than onboard.

Originally Posted by only_human

I think all the Russian engines used in the US use hypergolic fuel. Those wouldn't need igniters but a Rapid Unplanned Disassembly would be more worrisome.

The recent geopolitical issue about Russian sourced engines is exclusively about the RD-180 in use to power ULA Atlas V rockets, and the RD-180 is RP-1/LOX (like SpaceX's Merlin). The Delta rockets use American engines, and as far as I know, Russia has never exported its hypergolic engines (excluding Kazakhstan of course).

[xilman]

[only_human]

While reading about hypergolic applications earlier today I read about ullage motors. One of them blew up in space on June first.
RUSSIAN ULLAGE ROCKET ENGINE EXPLODES IN SPACE

Ullage motors, like the one that exploded, are used for flight stabilization, altitude control, and propellant settling activities. During a typical Proton flight, a pair of these motors remain attached to the Blok DM-2 and separates when the upper stage’s main engine reaches full thrust.

Each Ullage motor that is installed on a Blok DM-2 upper stage is filled with nitrogen tetroxide (N2O4) and unsymmetrical dimethylhydrazine (UDMH) propellants. This combination causes spontaneous explosions when propellant tanks corrode and these two fuels mix with each other. Under these conditions, such an explosive event could occur years after the rocket was launched, when the corrosion process is well underway.

Explosions of Ullage motors are relatively common. According to Spaceflight101.com, the first such event took place in 1984. This latest break-up is the 45th known explosion of this motor so far.

I keep wondering about a vented brown gas Dubslow noticed on a rocket launch. Maybe this (hypergolic propellant)?

The corrosiveness of nitrogen tetroxide can be reduced by adding several percent nitric oxide (NO), forming mixed oxides of nitrogen (MON).

Originally Posted by Dubslow

The recent geopolitical issue about Russian sourced engines is exclusively about the RD-180 in use to power ULA Atlas V rockets, and the RD-180 is RP-1/LOX (like SpaceX's Merlin). The Delta rockets use American engines, and as far as I know, Russia has never exported its hypergolic engines (excluding Kazakhstan of course).

Ok. Thanks for the correction.

[Dubslow]

Originally Posted by only_human

While reading about hypergolic applications earlier today I read about ullage motors. One of them blew up in space on June first.
RUSSIAN ULLAGE ROCKET ENGINE EXPLODES IN SPACE

That's cool, I've never heard of things like that. I suppose the greatest risk is adding more debris to what is effectively already a (lethal) junkyard.