SpaceX FH : Falcon Heavy Demo : Feb 6, 2018 : Discussion Thread 2

[Prettz]

I understand that the center core did not have the titanium grid fins.  Why is that?

That has been addressed several times already. It did not need it, one of the reasons why titanium fins were needed for side boosters is for control authority as they have rounded nosecones instead of a cylinder which affects flow separation and thus controllability. The titanium fins are larger than the old aluminum fins so they provide more authority.

The center core on this flight apparently flew a conservative reentry profile, early MECO, long boostback burn. Very likely lower heating than a single stick GTO recovery.

Besides not needing them, it's also likely they don't have many to spare, and didn't want to risk losing 3 whole sets if the rocket blew up.

[Joffan]

I understand that the center core did not have the titanium grid fins.  Why is that?

one of the reasons why titanium fins were needed for side boosters is for control authority as they have rounded nosecones instead of a cylinder which affects flow separation and thus controllability. The titanium fins are larger than the old aluminum fins so they provide more authority.

Thanks ugordon.  I've also wondered why the side booster nose cones are not jettisoned like the fairing if that would add control.

No, it wouldn't add control, or not significantly. It's not so much the presence of the nose cones as the absence of the cylindrical interstage above the grid fins that makes the difference.

[GWH]

I think discussions on the center core expendable price are missing things and moved my reply since it isn't relevant to the demo mission: https://forum.nasaspaceflight.com/index.php?topic=43025.msg1788093#msg1788093

[Nomadd]

I understand that the center core did not have the titanium grid fins.  Why is that?  Did Musk not expect the center core to make it back?  Since the center core experienced higher re-entry velocities,  you would think it would have the titanium GF as a high priority.

The boosters needed the bigger fins because of their aerodynamics. The core didn't. Aluminum fins tend to get ruined in hot entries, but they still get the job done, and the titanium ones are very hard to make and cost a lot. It turns out they made a good call, since they'd hate to lose a set.

[envy887]

https://twitter.com/elonmusk/status/963107229523038211

Elon Musk
‏Compte certifié @elonmusk
16 minil y a 16 minutes
En réponse à @kerrbones @nextspaceflight

Not enough ignition fluid to light the outer two engines after several three engine relights. Fix is pretty obvious.

The centre core landing failure was caused by the fact that the need for ignition fluids (TEA/TEB) increases after several engine relights. They are adding more storage to fix the issue.

More critical about this.

One should always ask, if the fix was so simple, why wasn' t the issue determined before flight (i.e. simulation)?

People give SX too many "mulligans", too readily, at any time, and for any reason.

Yes it was good the demo launch succeeded.

Because the simplicity of the fix and the difficulty in predicting the failure have no relation to each other. Failures that are easy to predict can be insanely hard to fix, and those that are very difficult to predict can be very easy to fix.

[1]

Are you perhaps mixing up fixing versus improving ?

Its impressive that SpaceX managed to get every first of F9 and FH right the first time.

I won't argue if you'd prefer to use the word "improve" to describe the rest of the vehicle, but the center core needs fixing; even if the fix is simple. Elon's word, not just mine.

Personally, I'd use "fix" for any changes made during testing of a vehicle; and "improve" for changes made after a relatively mature design is servicing customers (barring any outright failure, of course).  But that's just me.

Completely agree that this flight was impressive as hell.

[Norm38]

https://twitter.com/elonmusk/status/963107229523038211

Elon Musk
‏Compte certifié @elonmusk
16 minil y a 16 minutes
En réponse à @kerrbones @nextspaceflight

Not enough ignition fluid to light the outer two engines after several three engine relights. Fix is pretty obvious.

It's not clear to me what was different about this center core's flight post MECO.  3 engine burns have been done before.  Why did they run out of ignition fluid on this flight, but not earlier ones?

[envy887]

The performance numbers in this database are not accurate. In process of being fixed. Even if they were, a fully expendable Falcon Heavy, which far exceeds the performance of a Delta IV Heavy, is $150M, compared to over $400M for Delta IV Heavy.

https://twitter.com/elonmusk/status/963076231921938432

So we now have an actual price for an expendable Falcon Heavy.

So that is $90M for 8 t to GTO or $11,250/kg or $150M for 26.7 t to GTO or $5,618/kg. That means if you are sending large payloads to the Moon or Mars, its much cheaper to do it expendable!

You are assuming the price-performance curve is linear between those points. This is obviously not true as Musk noted that they can get about 90% of expendable performance (about 24,000 kg to GTO) for a small price increase to $95M.

The price-performance curve is dependent on many factors including flight rate, production rate, services provided, and negotiated discounts.

[envy887]

It's not clear to me what was different about this center core's flight post MECO.  3 engine burns have been done before.  Why did they run out of ignition fluid on this flight, but not earlier ones?

The center core did a 3 engine boostback, a 3 engine entry, and a 3 engine landing. That's 9 engine relights, no previous flight did more than 7 engine relights (3 engine boostback, 3 engine entry, single engine landing).

[aero]

Didn't Gov-Sat-1 do three 3 engine burns? Of course, they expended that booster so they couldn't measure the residual starter fluids.

[envy887]

Didn't Gov-Sat-1 do three 3 engine burns? Of course, they expended that booster so they couldn't measure the residual starter fluids.

I don't think any GTO launches have done a boostback.

[Davp99]

After all is Said and Done, My Hat is Still Off to Elon Musks SpaceX Company   ....Just to See something New & Exciting must burn the Old Guard ...Oh Well, Congratulations Anyway !!

[Prettz]

Didn't Gov-Sat-1 do three 3 engine burns? Of course, they expended that booster so they couldn't measure the residual starter fluids.

I don't think any GTO launches have done a boostback.

I could've sworn I remembered some GTO launches doing a boostback burn, but I went back and checked and none did. Only LEO missions landing on the barge.

Has there been a discussion of what they gain by doing a boostback burn rather than a longer, possibly earlier, entry burn? There must be a reason beyond just landing closer to shore, right?

[Hominans Kosmos]

I submit for consideration an alternate theory on why they used more fluid than they needed.  Its based on Lou's theory but mine happens on the way up rather than the way down.  Perhaps on the way up rather than just throttling down the center core (which is visible) they also switched off a few engines and restarted them.

Nice theory.

During the reentry burn 3-engine portion, you can see an "Eye of Sauron" (got that from Lars Blackmore's talk) effect, in which the longer axis of the plume cross section is orthogonal to the line of the three lit engines.  Lars claims that SpaceX doesn't really know why that happens.  I didn't see any equivalent effect on the way up.  I wouldn't expect it near sea level with the exhaust overexpanded.  But once the plume bloomed out, I thought I'd see the effect again, and didn't.  Maybe that's because there was a different pressure pattern.

While not easily visible as such the same billowing effect as happens during the multi-engine deceleration burns also happens on every Falcon 9 ascent phase. It also happens during other multi-engine flights, highlighted most clearly in the medium-high altitudes of the ascent. Easier examples are the twin-engine Atlas 5 launches, as well as those of Antares:



As you can see at that time point (2m42s) in the flight the engines have rolled to fly downrange one on top of the other, but the expanding plumes interacting causes the impinged sheet to billow out orthogonally to the sides.

With the following Atlas video you can see the billowing sheet start forming right after 1:55 into the video

Seeing these you can already understand what causes the flower petal figure to appear during Falcon 9 launches, the prominences of the plume align with the gaps between the engines, which is sometimes clearly caught from the ground during good atmospheric seeing. During the Zuma mission the cameras pick up two points of reference for you to see this, first (T+0:55s) is the alignment of inter-engine gaps relative to the landing leg centers and gaps, and secondly (T+1:50) the actual glowing engine bells glowing through the transparent exhaust, the same sheets that form the mysterious parallel lines aligns between the engines to form radial lines: 


I don't claim to know why this eye of Sauron happens either, but it appears to be at least remotely related to phenomena such as these

[whitelancer64]

It's an interference pattern. Like drops of water on a pond. I thought that everyone knew that already.

[speedevil]

It's an interference pattern. Like drops of water on a pond. I thought that everyone knew that already.

Classical interference patterns like drops on a pond are made from non-interacting ripples.

More formally, you can add or subtract ripples, and they have no effect on each other - the ripple pattern made from two different drops are entirely isolated, and you can compute the height at any point by summing the heights at each point of the isolated ripple.

These shocks are interacting, and very, very nonlinear.
There is significant chaotic behaviour as they mix.

When water gets non-linear, the surface breaks up.
This chaotic behaviour can be observed at the impact sites of the drops, as you get away from the immediate drop impact, it gets very predictable, which isn't the case with impinging jets.

[whitelancer64]

Forgive the crappy hand drawing, as oddly I couldn't find any pictures of 3 point interference patterns with a quick google search.

3 point interference produces two distinct "bars" of intense interaction, which is exactly what we see.

Naturally, it's not perfect, there's other aerodynamic interactions going on.

[Hominans Kosmos]

Forgive the crappy hand drawing, as oddly I couldn't find any pictures of 3 point interference patterns with a quick google search.

3 point interference produces two distinct "bars" of intense interaction, which is exactly what we see.

Naturally, it's not perfect, there's other aerodynamic interactions going on.

The problem is the relevant physics are different. We are talking about fluid flows here, not sound waves.

[vanoord]

What are the odds on all 3 cores landing intact?

21/23* landing attempts have been successful for F9 v1.2 (ignoring Govsat-1 because I've no idea whether to consider it a landing attempt or not) - so that's 91% reliability.

In reality, the probability of landing any given core now should be higher due to experience - although that's tempered slightly by these not being Block 4 cores (which have a 5/5 success rate).

Say 95% for any given core - which nominally gives around 83% for getting all three back (under normal circumstances).

Then less a bit because this is experimental and there has to be a consideration about separation, control authority for the side cores and the effect of the hardware that links the cores together.

So perhaps 70% to 75%?



* they lost 2 v1.2 cores on landing - SES9 and Eutelsat 117W

66.67% recovery rate for this flight - so I wasn't far off

[StuffOfInterest]

I wonder if/when we'll get a video of the center core smacking into the ocean at 300 mph.  Elon mentioned that they may have captured video of it.  Should be quite interesting.  Hopefully we don't have to wait two years for another blooper film.  Actually, hopefully they don't have enough bloopers in the next two years to make a film.