Falcon Heavy - General Discussion

[lklundin]

The Falcon Heavy SpaceX page was updated yesterday. No more talk of a cross-feed.

Right. The text that disappeared is:
"For missions involving exceptionally heavy payloads—greater than 45,000 kilograms or 100,000 pounds—Falcon Heavy offers a unique cross-feed propellant system. Propellant feeds from the side boosters to the center core so that the center core retains a significant amount of fuel after the boosters separate", see
Falcon Heavy

- and at the same time there has been a slight increase in the payload capability (for LEO, GTO and Mars).

So I wonder how exactly FH will burn its 27 engines. Expelling mass at the maximum possible rate right from the start is optimal, so if they want a lift-off with all 27 engines burning at full rate and then separating the two side boosters before the core stage, then they will want to consider the cross-feed.

But to take one step at a time, the may initially launch FH without the cross-feed. Then, when they can do that with an acceptable reliability, then they can consider optimizations, such as the cross-feed.

I guess time will tell.

 

[ggr]

Right. The text that disappeared is:
"For missions involving exceptionally heavy payloads—greater than 45,000 kilograms or 100,000 pounds—Falcon Heavy offers a unique cross-feed propellant system. Propellant feeds from the side boosters to the center core so that the center core retains a significant amount of fuel after the boosters separate", see
Falcon Heavy

- and at the same time there has been a slight increase in the payload capability (for LEO, GTO and Mars).

So I wonder how exactly FH will burn its 27 engines. Expelling mass at the maximum possible rate right from the start is optimal, so if they want a lift-off with all 27 engines burning at full rate and then separating the two side boosters before the core stage, then they will want to consider the cross-feed.

But to take one step at a time, the may initially launch FH without the cross-feed. Then, when they can do that with an acceptable reliability, then they can consider optimizations, such as the cross-feed.

I guess time will tell.

Crossfeed is not completely necessary. There comes a point in the launch where they reach "Max Q", maximum allowable aerodynamic pressure, and have to throttle back. The more powerful the launch, the sooner this will happen. So they can arrange that all the throttling is done by the center booster, while leaving the side ones on full power, and leaving the center one with some extra fuel to be burned later, after separation.

Someone over at NasaSpaceFlight forum did some complicated calculations that seemed to show that this was almost as good as full-blown crossfeed, if you make some assumptions about the cost and weight of plumbing, not to mention the complexity/reliability. It fits with SpaceX to "do it in software" like this.

 

[lklundin]

Crossfeed is not completely necessary. There comes a point in the launch where they reach "Max Q", maximum allowable aerodynamic pressure, and have to throttle back. The more powerful the launch, the sooner this will happen. So they can arrange that all the throttling is done by the center booster, while leaving the side ones on full power, and leaving the center one with some extra fuel to be burned later, after separation.

Someone over at NasaSpaceFlight forum did some complicated calculations that seemed to show that this was almost as good as full-blown crossfeed, if you make some assumptions about the cost and weight of plumbing, not to mention the complexity/reliability. It fits with SpaceX to "do it in software" like this.

Actually, to be strict "Max Q" is defined not as an allowable pressure, but as the maximum aerodynamic pressure experienced by the rocket while in flight. It is a function of increasing velocity due to the thrust (and thus a function also of the actual payload mass) and the decreasing atmospheric friction due to the atmosphere getting thinner with increased altitude (and therefore also of the type of orbit it aims for). So while it is correct that a given rocket can be designed for a given maximum stress due to acceleration and atmospheric friction, this design parameter is not called "Max Q". But it is naturally advisable to design a rocket so its thrust does not cause it to reach a Max Q that exceeds its strength.

That is why one hears the mission control report "supersonic" and "Max Q", as these are two characteristics of each launch (unless, of course that they are preceded by a RUD, rapid unscheduled disassembly...)

 

[ecarfan]

I know next to nothing about rocket design, but assume that a cross feed system would significantly complicate the stage jettisoning process that has to occur before the second stage ignites. It also surely adds cost and complexity. If it could be avoided that would seem desirable.

 

[Ben W]

If crossfeed were in place to transfer fuel from the side tanks to the center tank, perhaps the center tank could be fueled this way on the launch pad. (I.e. not need a separate external port for fueling.) In fact, if the crossfeed were reversible, only the center core would need an external fueling port, and could fuel the side boosters through the crossfeed. This might compensate for some of the added complexity/weight.

Also, I don't understand why crossfeed is important only for "exceptionally large payloads". The SpaceX website implies that FH is capable of 8mT to GEO with all cores recoverable, but 22mT to GEO with no cores recoverable. There must be a significant payload range (e.g. 8mT - 12mT) where crossfeed would enable the recovery of all cores, but no crossfeed would require at least one to be expended. Similarly, there must be a meaningful range (e.g. 14mT - 18mT) where crossfeed would allow the recovery of the side cores, but no crossfeed would lose them both. (I can't imagine a scenario where it would make sense to expend the side cores and recover the center core.) For these payload ranges, it would seem that crossfeed would be worth quite a bit of cost/complexity, if it saves at least one $60m core per launch.

Another consideration in these scenarios: even if the crossfeed pumps fail in flight, the primary mission could probably be completed by expending the cores. Even if the separation mechanism fails and both side cores fail to detach, it's possible that running the center core til empty (carrying the empty side cores) could still complete the primary mission. But if one side core detaches and the other doesn't, that's quite a different story

 

[Ben W]

Someone over at NasaSpaceFlight forum did some complicated calculations that seemed to show that this was almost as good as full-blown crossfeed, if you make some assumptions about the cost and weight of plumbing, not to mention the complexity/reliability. It fits with SpaceX to "do it in software" like this.

Do you have a link to this? Since the center core can only throttle down to 70%, then without crossfeed it will have at most 30% of its fuel remaining when the side cores detach. (Probably more like 25%, since it lifts off at full throttle.) With crossfeed it could theoretically still be near 100% full at side core detachment, and also not have to throttle back until closer to Max-Q. (And it could throttle up again after Max-Q as well.)

Also, as I've noted, for some range of payloads it's likely that crossfeed pump failure would still allow the completion of the primary mission, at the cost of some of the cores (that would be expended anyway without crossfeed). So that would balance the extra cost/complexity somewhat I think.

 

[chickensevil]

So, their new costs they show, this *must* be including a cost reduction due to recovering and reusing the 1st stage. Someone feel free to check my math:

Let X = 1st stage
Let Y = 2nd stage/top of rocket/other costs not directly tied to the cost of each booster
F9 Cost: 62M = X + Y
FH Cost: 90M = 3X + Y

Therefore:
X = 62 - Y
90 = 3(62 - Y) + Y
90 = 186 - 2Y
-96 = -2Y
Y = 48
62 = X + 48
X = 14

Therefore:
The associated cost per 1st stage is being calculated at 14M per booster. All other costs are at 48M per launch. Surely it is not just me, but that 14M per core seems really low and feels like they must be expecting some amount of cost savings due to reuse on their 2018 launch costs.

Cause I seem to recall that they had originally stated the FH cost would be like 150M or some such and the F9 was like 80-90M, right?

 

[sandpiper]

So, their new costs they show, this *must* be including a cost reduction due to recovering and reusing the 1st stage. Someone feel free to check my math:

Let X = 1st stage
Let Y = 2nd stage/top of rocket/other costs not directly tied to the cost of each booster
F9 Cost: 62M = X + Y
FH Cost: 90M = 3X + Y

Therefore:
X = 62 - Y
90 = 3(62 - Y) + Y
90 = 186 - 2Y
-96 = -2Y
Y = 48
62 = X + 48
X = 14

Therefore:
The associated cost per 1st stage is being calculated at 14M per booster. All other costs are at 48M per launch. Surely it is not just me, but that 14M per core seems really low and feels like they must be expecting some amount of cost savings due to reuse on their 2018 launch costs.

Cause I seem to recall that they had originally stated the FH cost would be like 150M or some such and the F9 was like 80-90M, right?

Unfortunately your first two equations are not quite correct. I've corrected them below

62M = X + Y + P1
90M = 3X + Y + P2

P1, P2 are the expected profit, and they are determined by SpaceX marketing / Elon / random number generator when setting the selling price. There's not enough information to determine X and/or Y. For all we know, P2 may well even be negative for the first few launches.

 

[JRod0802]

Unfortunately your first two equations are not quite correct. I've corrected them below

62M = X + Y + P1
90M = 3X + Y + P2

P1, P2 are the expected profit, and they are determined by Tesla marketing / Elon / random number generator when setting the selling price. There's not enough information to determine X and/or Y. For all we know, P2 may well even be negative for the first few launches.

Unverified estimate of P1: 24.5M
Unverified estimate of P2: 36M

Source: SpaceX's reusable Falcon 9: What are the real cost savings for customers? - SpaceNews.com

Relevant table:

 

[chickensevil]

Unfortunately your first two equations are not quite correct. I've corrected them below

62M = X + Y + P1
90M = 3X + Y + P2

P1, P2 are the expected profit, and they are determined by SpaceX marketing / Elon / random number generator when setting the selling price. There's not enough information to determine X and/or Y. For all we know, P2 may well even be negative for the first few launches.

Actually you may be on to something with the profit side and intentionally running negative. Apparently the 62M$ number is the highest the F6 has cost on their list price since they first started putting numbers out their. Back in 2010 the price was listed between 49.9 to 56M$

Also, according to SpaceX data, they are saying that ~75% of the cost is in the 1st stage. So 14M feels too low and I like the Jefferies estimate above a bit better. Must be why they get paid the big bucks to be an analyst and I am just a fan on a website, lol. That said, I don't agree with their numbers on the FH. I wouldn't think that the 1st stage cost would continue to be 75% of the rocket. The only explanation for the odd price discrepancy is that their profit number for the FH is intentionally low if not potentially negative. If the 1st stage does cost around 27.5M then that would put the FH 1st stage cost at 82.5M, right? Then add on the 9.2M$ in "other costs" since the 2nd stage and such should be relatively the same and you are over 90M.

Maybe I am missing something here and there is an extra cost on the single booster that you don't have when you make 3 of them such that you get some amount of cost savings... But I just can't put my finger on what that would be.

 

[sandpiper]

The only explanation for the odd price discrepancy is that their profit number for the FH is intentionally low if not potentially negative. If the 1st stage does cost around 27.5M then that would put the FH 1st stage cost at 82.5M, right? Then add on the 9.2M$ in "other costs" since the 2nd stage and such should be relatively the same and you are over 90M.

Maybe I am missing something here and there is an extra cost on the single booster that you don't have when you make 3 of them such that you get some amount of cost savings... But I just can't put my finger on what that would be.

I agree. It doesn't quite make sense. I suspect that they're place a substantial "risk discount" on the FH launches right now. Customers are going to be very careful about putting a half B$ piece of hardware on a complex and unproven rocket and so SpaceX needs to make it attractive.

 

[Grendal]

I agree. It doesn't quite make sense. I suspect that they're place a substantial "risk discount" on the FH launches right now. Customers are going to be very careful about putting a half B$ piece of hardware on a complex and unproven rocket and so SpaceX needs to make it attractive.

You make an interesting point but SpaceX has a backlog of FH launches. So why would they need to make something that will be proven soon attractive?

 

[sandpiper]

You make an interesting point but SpaceX has a backlog of FH launches. So why would they need to make something that will be proven soon attractive?

Fair question.... obviously it's a SpaceX marketing decision and we people up in the cheap seats can only speculate. I'm going to guess that they're looking to establish themselves as quickly as possible and to take a full leadership position. They have such an enormous cost advantage that, once their heavy lift capability is proven, they could literally dominate the entire launch industry.

Off topic, but I find this morbidly fascinating to watch. I imagine that a lot of folks in the traditional aerospace firms are very very worried. You have very large industries that are carrying 50 years of bloat, bureaucracy and barnacles, and personally I really doubt that those firms can be fixed by anybody. Who would have predicted, a decade ago, that these big space contractors would be facing an existential threat?

 

[djplong]

Indeed. SpaceX doesn't need a *billion* dollar per year retainer just to "keep engineers around, just in case".

 

[GBleck]

Your cost estimates are treating launch cost like a car. A fairly fixed amount per launch goes to personnel and range facilities. Shouldn't vary greatly from a f9 vs fh.

 

[RDoc]

Do you have a link to this? Since the center core can only throttle down to 70%, then without crossfeed it will have at most 30% of its fuel remaining when the side cores detach. (Probably more like 25%, since it lifts off at full throttle.) With crossfeed it could theoretically still be near 100% full at side core detachment, and also not have to throttle back until closer to Max-Q. (And it could throttle up again after Max-Q as well.)<snip>

Disclaimer: I've not done the math and have no idea if this is SpaceX's plan.

However, as the fuel is burned, the thrust limit becomes maximum acceleration the payload can handle. Perhaps the idea is to not only throttle the central core engines, but to turn some of them off while running the side boosters at full throttle. When the side boosters run out of fuel, they separate, at which point the stack is a lot lighter. The main core has more fuel left, so they can ramp up the main core's running engines to full, and possibly restart some (although I doubt this) to maintain max acceleration.

 

[HVM]

There's now enough boosters to make one Falcon Heavy First Stage at hangar A39 ; )

but not engines ;(

 

[ecarfan]

So cool. A sight never seen before in the history of spaceflight. Thanks for posting.

 

[Ben W]

Disclaimer: I've not done the math and have no idea if this is SpaceX's plan.

However, as the fuel is burned, the thrust limit becomes maximum acceleration the payload can handle. Perhaps the idea is to not only throttle the central core engines, but to turn some of them off while running the side boosters at full throttle. When the side boosters run out of fuel, they separate, at which point the stack is a lot lighter. The main core has more fuel left, so they can ramp up the main core's running engines to full, and possibly restart some (although I doubt this) to maintain max acceleration.

As I understand it, three of the nine engines per core currently have relight capability. Obviously SpaceX could change this for FH if there were a reason. But with optimal crossfeed (center core stays full), I'm pretty sure all three boosters could run at 100% up until side-booster separation. From Wikipedia, the fully fueled FH masses about 1420 MT, and develops 24,681kN maximum thrust in vacuum, which would work out to about 1.77G's when pushing the full weight of the stack. At side-booster separation, assuming optimal crossfeed, the total mass would have decreased to about 620 MT, so the acceleration would be a hair over 4G's. This should be within the tolerances of most payloads. Of course, if some fuel is reserved for landing the side boosters, the max acceleration will be less.

No doubt SpaceX has sophisticated mathematical models for all the various permutations, and it certainly makes sense for them to keep it "simple" for v1 of FH. But as the payloads get larger, especially as Mars becomes a serious goal, I would not be at all surprised to see crossfeed make a reappearance. Then again, once BFR rears its beautiful head, all bets are off.

 

[bxr140]

So cool. A sight never seen before in the history of spaceflight.

Cool? Absolutely.

'A sight never seen...'? I dunno about that, other than the self evident aspect of spacex never having done this before. The beauty of the company is that they're not really doing anything revolutionary--they're just making sure everything they're doing is as evolutionary as possible.