Author Topic: Any rule of thumb for costs of a wide, squat rocket?  (Read 9057 times)

Offline gospacex

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Re: Any rule of thumb for costs of a wide, squat rocket?
« Reply #20 on: 05/27/2017 08:23 PM »
Aerodynamic losses are greater if you're wider. So you want to go taller if you can

Don't forget mass fraction. Flying needles are not particularly good at it.
I think "squat ITS with fewer engines" is not a bad idea. Saturn V's 1st stage had fineness ratio of about 1:4.

Offline Robotbeat

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Re: Any rule of thumb for costs of a wide, squat rocket?
« Reply #21 on: 05/27/2017 08:40 PM »
Counterpoint: Falcon 9 is a flying needle if ANYTHING is, and it gets industry-leading mass fraction.
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Offline Kaputnik

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Re: Any rule of thumb for costs of a wide, squat rocket?
« Reply #22 on: 05/27/2017 09:20 PM »
In theory, a sphere is the most efficient shape. But is this an example of where theory and practice fail to meet?
Is a sphere actually a complete PITA to build, and all the supposed advantages get swallowed up by the time you've factored in all the additional welds, and the difficulty of transferring loads through it?
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Offline AncientU

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Re: Any rule of thumb for costs of a wide, squat rocket?
« Reply #23 on: 05/27/2017 10:23 PM »
Counterpoint: Falcon 9 is a flying needle if ANYTHING is, and it gets industry-leading mass fraction.

But if you put same set of M1-Ds under a more optimum shape (theoretically -- please don't touch anything), it would easily beat... itself.  :D
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Offline Semmel

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Re: Any rule of thumb for costs of a wide, squat rocket?
« Reply #24 on: 05/28/2017 07:40 PM »
In theory, a sphere is the most efficient shape. But is this an example of where theory and practice fail to meet?
Is a sphere actually a complete PITA to build, and all the supposed advantages get swallowed up by the time you've factored in all the additional welds, and the difficulty of transferring loads through it?

"In theory, practice and theory are identical. In practice, they are not."

Offline hkultala

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Re: Any rule of thumb for costs of a wide, squat rocket?
« Reply #25 on: 05/29/2017 03:32 AM »
Aerodynamic losses are greater if you're wider. So you want to go taller if you can

Don't forget mass fraction. Flying needles are not particularly good at it.
I think "squat ITS with fewer engines" is not a bad idea. Saturn V's 1st stage had fineness ratio of about 1:4.

Saturn V is never flying without the second stage (except to the bottom of the ocean after staging).

Together the stags have fineness ratio of much more, and that is what matters for drag.



Offline AncientU

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Re: Any rule of thumb for costs of a wide, squat rocket?
« Reply #26 on: 05/29/2017 10:10 AM »
Aerodynamic losses are greater if you're wider. So you want to go taller if you can

Don't forget mass fraction. Flying needles are not particularly good at it.
I think "squat ITS with fewer engines" is not a bad idea. Saturn V's 1st stage had fineness ratio of about 1:4.

One advantage of a squat ITS (10m) is that there is plenty of room to add engines and go taller.  Also accommodates engine performance growth.
Similar in some ways to F9 v1.0 situation to current max'd out fineness/length due to M1D performance improvements.
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Offline Wolfram66

Aerodynamic losses are greater if you're wider. So you want to go taller if you can

Don't forget mass fraction. Flying needles are not particularly good at it.
I think "squat ITS with fewer engines" is not a bad idea. Saturn V's 1st stage had fineness ratio of about 1:4.

One advantage of a squat ITS (10m) is that there is plenty of room to add engines and go taller.  Also accommodates engine performance growth.
Similar in some ways to F9 v1.0 situation to current max'd out fineness/length due to M1D performance improvements.

In lower atmosphere limiting factor is cross-sectional area x length. just like airplanes. flying wing is extreme lifting body. f-9 flying needle. ITS appears to be partial lifting body so gravity losses might be affected. just musing here.

Offline Nilof

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Re: Any rule of thumb for costs of a wide, squat rocket?
« Reply #28 on: 06/30/2017 12:59 AM »
There is certainly a trade between reducing aero losses by lengthening your cylinder, and increased dry mass needed to handle bending loads, and narrowing the launch commit parameters to deal with increased bending loads, and simply cost of retooling/transporting a rocket of larger diameter and how large/many engines can fit on the rocket, etc etc.

It's a long and complex trade that will likely be dominated by cost considerations (which includes transportation)..

You can get both by building a huge rocket though. IIRC, the Saturn V held the record for best payload fraction (not just total payload) for quite a while, with almost twice that of Ariane V.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

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