Questions, Falcon Heavy

In article ,
says...

On Monday, March 7, 2016 at 9:03:15 AM UTC+13, Jeff Findley wrote:
Last I heard, SpaceX is dropping cross-feeding,

cite? reference? I looked for one and couldn't find it.

They're a bit cagey about it on their site, but here is the public
facing statement on their Falcon Heavy site:

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.

From what I gather from other sources, initial (development) Falcon
Heavy flights will not feature cross-feeding. But they are clearly open
to developing it in the future given customer(s) with heavy enough
payloads. It won't fly with cross-feeding unless the flight needs it.
Eliminating that extra complexity for lower payload flights will
initially lower development costs and later operational costs.

Jeff
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All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.

In article om,
says...

On 2016-03-07 22:56, Jeff Findley wrote:

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.

This makes sense. Allows you to ditch the boosters earlier, meaning
main stage is lighter for rest of trip. So the boosters become
self-propelled auxiliary tanks and once the fuel is transfered to the
main tank, the booster is no longer needed.

Yes, but that option comes with added complexity (higher chance of
failure) and higher cost. So, initial flights will quite likely not
incorporate this feature.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.

I agree that the elimination of the cost and complexity of cross feeding for payloads less than 40 tons is a rational thing to do since they can throttle back the core engine and achieve enough savings to get to orbit anyway.

They're offering payloads up to 53 tons and more because they have cross feed available to handle that. It is something they have something they offer for clients who need it but have yet to schedule a launch using it. So in your mind it doesn't exist and they may abandon it and statements are cagey.. Lol

Look, Musk is committed to opening the inner solar system to development. This will require heavy payloads in the coming period as it as always has. This will create a demand for heavy lift and that means Cross fed stages and large boosters that are reused. As it always has.

These developments will take place. I'm happy Musk chose America and not China or Russia to develop SpaceX.

Over the next decade humanity will return to the moon. Mine the asteroids. Settle Mars. Because of the booster innovations SpaceX is developing today..

Jeff Findley wrote:
Last I heard, SpaceX is dropping cross-feeding, at least in the
short term. Part of the reason for this is densifying propellant
will give them some of the performance that cross-feeding would
have. But the complexity of sub-cooling propellant is less than
adding cross-feeding to the stages.

I'd not heard of the dropping of the cross-feeding. I wonder how much
the attempt to get cross-feed going contributed to the delays in the
Heavy. http://www.spacex.com/falcon-heavy still seems to mention it.
There is a little qualification about "exceptionally heavy payloads."

rick jones
--
The glass is neither half-empty nor half-full. The glass has a leak.
The real question is "Can it be patched?"
these opinions are mine, all mine; HPE might not want them anyway...
feel free to post, OR email to rick.jones2 in hpe.com but NOT BOTH...

On Wednesday, March 9, 2016 at 8:26:47 AM UTC+13, Rick Jones wrote:
Jeff Findley wrote:
Last I heard, SpaceX is dropping cross-feeding, at least in the
short term. Part of the reason for this is densifying propellant
will give them some of the performance that cross-feeding would
have. But the complexity of sub-cooling propellant is less than
adding cross-feeding to the stages.

I'd not heard of the dropping of the cross-feeding. I wonder how much
the attempt to get cross-feed going contributed to the delays in the
Heavy. http://www.spacex.com/falcon-heavy still seems to mention it.
There is a little qualification about "exceptionally heavy payloads."

rick jones
--
The glass is neither half-empty nor half-full. The glass has a leak.
The real question is "Can it be patched?"
these opinions are mine, all mine; HPE might not want them anyway...
feel free to post, OR email to rick.jones2 in hpe.com but NOT BOTH...

Engineers rarely write ad copy, though they are consulted on occasion.

For anyone who knows how to solve the Tsiolkovsky equation its clear that for the Falcon Heavy propellants and the Falcon Heavy propellant volume and the falcon Heavy propellant densities, cross feeding for payloads less than 40 tons is efficient since it saves propellant certainly, but for these lighter payloads (about 75% of the fully loaded Falcon Heavy) cross feeding is not strictly needed since you can save core stage propellant by throttling back thrust after reaching Max Q. You're not as efficient, and lack surplus delta vee. However, you can get to orbit with light payloads.

That cannot be denied.

But why do it? If you can lift 53 tons, why not use 53 tons? Why pay for something you don't use? Yet, if you realise that a lightly loaded Falcon Heavy can put up a Delta IV payload more cheaply than the Delta IV, you can operate the Heavy like a Delta IV if you must. I guess that was the point of the other post I was responding to. I agree wholeheartedly with the notion that Musk has committed to developing and using cross feeding to maximise performance of his launchers. Customers will make use of this capacity, notwithstanding the theoretical capacity to operate the Falcon less efficiently without cross feeding.

To see how that works let's consider a three element launcher capable of putting up 1 ton of payload. It's 1/53rd the size of the Falcon Heavy.

Assume it has a 4.35% structure fraction like the Falcon. It uses LOX/RP1 like the Falcon. It has 2.7 km/sec exhaust speed through the first stage and 3.1 km/sec exhaust speed through the second stage, similar to the Falcon..

This theoretical rocket does not have an upper stage like the Falcon, so the whole beast is heavier at take off (1974.8 tons TOW for 53 ton payload compared to 1,394.0 tons for the Falcon Heavy).

So, the three booster elements and core element are as follows;

12.42 tons - total booster/core mass
0.54 tons - structure mass
11.88 tons - propellant

The first stage burns out at 2.575 km/sec and the second stage burns out at 9.200 km/sec not counting for air drag and gravity losses which total 1.3 km/sec throughout the burn.

S1: Ve=2.700 km/sec Vf=2.575 km/sec u=0.61469 s=0.0435 p=0.341811 TOW=39.2713 ton
S2: Ve=3.100 km/sec Vf=6.625 km/sec u=0.88200 s=0.0435 p=0.074600 TOW=13.4234 ton
P: Payload: 1.000 ton Vf=9.200 km/sec - 1.300 km/sec loss = 7.900 km/sec orbital speed.

Now, to limit air drag and gravity loss to 1.3 km/sec we must have sufficient acceleration at lift off to rise quickly and modulate that thrust so that we don't move too fast too soon and induce more air drag than necessary or hover too long fighting gravity, and once we gain sufficient altitude, we pitch over.

The ideal thrust depends on the gee force and lapse rate of where you're taking off. Generally speaking for Earth the best acceleration is 1.63x Earth normal gravity, propellants and materials.

So, with a 39.3 ton Take Off Weight we have a requirement of 64 tons of force. Divided across three stages equally this is 21.3 tons of force.

By shutting off the core engine, instead of cross feeding propellant to it, doesn't provide sufficient thrust to lift off the 39.3 ton rocket (1.08 gees). So running the core engine at full thrust is a requirement to maintain 1.63 gees.

Rising vertically until Max Q, which occurs at 0.3 km/sec, at 6 km altitude in less than a minute. and then with the core engine cut down thrust, burns 10.5% of the take off weight across the three elements; 1.38 tons each booster with no cross feeding, leaving 10.5 tons propellant in each element.

Of course with cross feeding this would leave 11.88 tons in the core and reduce the propellant in the strap on to 9.81 tons each, preserving the core element for later use, even though we get the benefit of the core engine at full thrust.

Now, without cross feeding the remaining 35.16 ton vehicle must have 57.3 tons of force applied to it, to maintain 1.63 gees - a 6.7 ton force reduction of thrust. Taken off the core engine, this is a 31.5% reduction in thrust of the core, and propellant use.

By the time 0.6 km/sec is reached, another 3.7 tons of propellant is burned and 1.38 tons comes from each strap on and 0.94 tons comes from the core. Total weight is reduced to 31.46 and thrust is further reduced to 51.3 tons of force. This allows the core booster to reduce thrust to 8.7 tons. 40.8% of its full thrust.

With cross feeding 1.85 tons of propellant is drawn from the strap on tanks and 11.88 tons remain in the core tank.

By the time 0.9 km/sec is reached, another 3.31 tons of propellant is burned. 0.57 tons comes from the core, and 1.37 tons come from each of the strap ons.

With cross feeding 1.655 tons come from the strap ons and 11.88 tons remain in the core tank.

WITHOUT CROSS FEEDING WITH CROSS FEEDING
Velocity Strap on Core Strap on Core

0.0 km/sec 11.88 tons 11.88 tons 11.88 tons 11.88 tons
0.3 km/sec 10.05 tons 10.05 tons 9.81 tons 11.88 tons
0.6 km/sec 8.67 tons 9.11 tons 7.96 tons 11.88 tons
0.9 km/sec 7.30 tons 8.54 tons 6.31 tons 11.88 tons

Ultimately we throttle back to a very low figure, and the strap ons do most of the work, while the core idles along until its throttled up prior to strap on separation.

The end result is that we have a second stage separation with 25% or more of the core stage propellant gone, which reduces the payload on orbit by that amount. So, this is why 0.75 ton in this case, could make it to orbit with the same cost at 1.00 ton with cross feeding.

Why someone who had a vehicle capable of 1 ton would choose to design a satellite massing only 0.75 tons is an almost rhetorical question. You would obviously design your payload to make use of the capacity available.

I accepted the other poster's comment at face value, assuming that maybe payloads designed for the Delta Heavy might be switched to the Falcon Heavy since it provides so much value for money.

Under these highly specialised circumstances, the Falcon Heavy can lift a Delta IV Heavy 28.8 tons with or without cross feeding.

Yet the fact remains that the Falcon Heavy can lift two satellites of 26.5 tons each WITH cross feeding. So, for $90 million SpaceX could launch two NRO-L32 satellites to GSO, compared to a single launch on the Delta IV for $375 million!

Since the Delta IV costs $375 million per launch, whilst the Falcon Heavy costs $90 million per launch, with a 53.0 ton capability to LEO, Falcon Heavy is clearly the better value, putting up satellites of comparable size for 12.5% the cost per unit weight.

Sure, for $90 million even a 28.8 ton payload on the Falcon Heavy is a bargain compared to its competitors, but why throw away 25% of the vehicle's capacity by excluding cross feeding? Especially if you can get good experience with the technology when its not mission critical? That's why I don't find the previous poster's comments compelling, without some real citations that can be tracked down.