Author Topic: Should Super Heavy (BFR/ITS) have a smaller prototype to ease development?  (Read 70203 times)

Offline Bob Shaw

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Re: Should ITS have a smaller prototype to ease development?
« Reply #120 on: 02/24/2017 11:43 pm »
Yes, people don't seem to be grasping the size difference going from F9/FH to ITS. Putting a mini-ITS on a scale that most propose on FH would be like putting a F9 upper stage on a F1 first stage. It is just too big.

A version that is 1/6 or 1/8 the mass would work fine on FH. It would be only slightly larger in diameter than the current fairing, but would have much better performance than the current stage.

But when you scale something *that* small, it is no longer super helpful beyond verifying the most basic stuff. Scale changes things, it changes how air flows around the vehicle, and it changes how much heat needs to be dissipated, and so on.

The area of a sphere goes goes up as the square of the radius, the volume rises by the cube. Sub-scale models may be of value in terms of technology development, but in areas where basic physics define your spacecraft (like hypersonic retro-propulsion) then the gain is an illusion. A smaller ITS - call it the ITS-SP after the 747-SP - is only of value if it fills a market niche, eg for LEO or Lunar/HEO missions, and it may well be that simply flying a standard ITS half-empty would also fill that niche, and without any unique development or manufacturing costs.

Offline OneSpeed

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Re: Should ITS have a smaller prototype to ease development?
« Reply #121 on: 02/25/2017 12:00 am »
But when you scale something *that* small, it is no longer super helpful beyond verifying the most basic stuff. Scale changes things, it changes how air flows around the vehicle, and it changes how much heat needs to be dissipated, and so on.

That's why engineers use dimensional analysis. There is no reason to suggest that a half scale model is not useful. Wind tunnel tests at much greater differences of scale can be extremely accurate and revealing, especially if the models have dynamic similitude.

Offline Lars-J

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Re: Should ITS have a smaller prototype to ease development?
« Reply #122 on: 02/25/2017 12:13 am »
But when you scale something *that* small, it is no longer super helpful beyond verifying the most basic stuff. Scale changes things, it changes how air flows around the vehicle, and it changes how much heat needs to be dissipated, and so on.

That's why engineers use dimensional analysis. There is no reason to suggest that a half scale model is not useful. Wind tunnel tests at much greater differences of scale can be extremely accurate and revealing, especially if the models have dynamic similitude.

Of course it is useful. You *will* learn things, but the question is if what you learn from it is worth the added cost, or if it could be learned some other way in a more cost-effective way.
« Last Edit: 02/25/2017 12:14 am by Lars-J »

Offline corneliussulla

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Re: Should ITS have a smaller prototype to ease development?
« Reply #123 on: 02/25/2017 07:41 am »
I think there may be no other option for him than to develop a mini ITS architecture. Currently all his revenue comes from being the most cost effective way of getting into LEO and other earth orbits. However Bezos is building what could evolve into a fully reusable system to get into similar orbits based on a raptor like engine. To stop Bezos eating his lunch I would think he needs to look at development of a raptor powered F9 replacement with a reusable upper stage. He could use the falcon heavy with reusable upper stage but hard to see how that would be competitive against Bezos single stick approach

Something similar in shape to ITS would make sense. It could have a cargo and manned version and would be capable of moon missions etc if it's used semi permanent ITS tankers in orbit. Cargo version would be first to meet commercial requirements. But down the track a crewed version would also have much greater crew capacity than the dragon2 if such a requirement was to evolve.

Online envy887

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Re: Should ITS have a smaller prototype to ease development?
« Reply #124 on: 02/25/2017 04:33 pm »
Yes, people don't seem to be grasping the size difference going from F9/FH to ITS. Putting a mini-ITS on a scale that most propose on FH would be like putting a F9 upper stage on a F1 first stage. It is just too big.

A version that is 1/6 or 1/8 the mass would work fine on FH. It would be only slightly larger in diameter than the current fairing, but would have much better performance than the current stage.

But when you scale something *that* small, it is no longer super helpful beyond verifying the most basic stuff. Scale changes things, it changes how air flows around the vehicle, and it changes how much heat needs to be dissipated, and so on.

Basic stuff like methalox storage and transfer on orbit? Raptor Vac coast and restart?

But the main advantage would be putting similar payload to a better GTO orbit than Ariane 5, with a fully reusable system. And beating DIVH payload to direct GEO insertion if fully expended. Falcon Heavy doesn't have either of those capabilities now, and both could be worthwhile in the current market.

Offline hkultala

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Re: Should ITS have a smaller prototype to ease development?
« Reply #125 on: 02/25/2017 04:51 pm »
I think there may be no other option for him than to develop a mini ITS architecture. Currently all his revenue comes from being the most cost effective way of getting into LEO and other earth orbits. However Bezos is building what could evolve into a fully reusable system to get into similar orbits based on a raptor like engine. To stop Bezos eating his lunch I would think he needs to look at development of a raptor powered F9 replacement with a reusable upper stage. He could use the falcon heavy with reusable upper stage but hard to see how that would be competitive against Bezos single stick approach

New glenn will have only first stage reusable, both second and third stages are expendable.

And it cannot easily and quickly evolve into "fully reusable system".

For example, they do not have practical engines that could land those stages. Second stage is too light to land with one BE-4, and it would be impractical to have multiple fuel types for some smaller engine. And third stage is too light to land with on BE-3.

Offline GWH

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Re: Should ITS have a smaller prototype to ease development?
« Reply #126 on: 02/25/2017 04:56 pm »
Basic stuff like methalox storage and transfer on orbit? Raptor Vac coast and restart?

But the main advantage would be putting similar payload to a better GTO orbit than Ariane 5, with a fully reusable system. And beating DIVH payload to direct GEO insertion if fully expended. Falcon Heavy doesn't have either of those capabilities now, and both could be worthwhile in the current market.

This, also how is a composite construction stage protected by PICA-X going to handle MMOD and re-entry from LEO multiple times with minimal refurbishment?  How will the Raptor perform in the space and re-entry environment repeatedly?  A lot more procedural knowledge to be gained in a complete reusable architecture besides just constructing a prototype, big or small. 


Offline philw1776

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Re: Should ITS have a smaller prototype to ease development?
« Reply #127 on: 02/25/2017 05:21 pm »
An alternative would be to have a full size (dimensions) ITS prototype, but at lower performance.

So lower numbers of lower thrust Raptor on the booster. Only part fill the tanks. Aim for 50 tonnes of payload, rather than 300 tonnes.

Agreed.
Earlier in this thread I had proposed simply flying a fully mfg tooled, partially fueled ITS with initially just 3 seal level Raptors sub-orbitally. Maybe move to an interim all SL Raptor test vehicle or maybe not.

Followed by a fully mfg tooling sized booster stage but partially fueled with just 21 Raptors.  Fly and recover, then next it's enough to launch a SL & vac ITS with close to zero payload into LEO to test re-entry.

I ran some rocket equation #s and this seems to work. 

And so on...

As someone who has developed hardware/software systems with 5 9s reliability goals my view (yes, I know I'm not aerospace experienced) is that sub scaled vehicles will only increase your already expensive tooling costs, delay final schedules and won't give the exact test results that flying your actual base hardware configuration does.  Too many unknown unknowns in scale models.
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Offline philw1776

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Re: Should ITS have a smaller prototype to ease development?
« Reply #128 on: 02/25/2017 05:28 pm »
An alternative would be to have a full size (dimensions) ITS prototype, but at lower performance.

So lower numbers of lower thrust Raptor on the booster. Only part fill the tanks. Aim for 50 tonnes of payload, rather than 300 tonnes.

Agreed.
Earlier in this thread I had proposed simply flying a fully mfg tooled, partially fueled ITS with initially just 3 seal level Raptors sub-orbitally. Maybe move to an interim all SL Raptor test vehicle or maybe not.

Followed by a fully mfg tooling sized booster stage but partially fueled with just 21 Raptors.  Fly and recover, then next it's enough to launch a SL & vac ITS with close to zero payload into LEO to test re-entry.

I ran some rocket equation #s and this seems to work. 

And so on...

As someone who has developed hardware/software systems with 5 9s reliability goals my view (yes, I know I'm not aerospace experienced) is that sub scaled vehicles will only increase your already expensive tooling costs, delay final schedules and won't give the exact test results that flying your actual base hardware configuration does.  Too many unknown unknowns in scale models.

..., the ITS will be tested as Musk outlined in his Powerpoint show.  He can't afford the dollars and more importantly to him personally the TIME wasted in mini-ITS whatevers that do not lead directly to his goal. Per his presentation, instead of building & testing the 42 engine booster first, he moves right to the 2nd stage.  Here's one scalable, incremental, direct pathway SX engineers could follow.

Build the simpler to outfit 90 tonne ITS stage 2 Tanker version first.
Equip with just the 3 SL Raptors to flight test them and the stage 2 airframe, TPS & avionics
~700 tonnes of propellant and the 90 tonne craft can get up to ~5 Km/sec (rocket equation)
Fly & recover, testing heat shield and airframe at lower stress and qualifying the avionics

Next build initial booster stage with ~ half the engines, but full sized airframe & tanks (no extra tooling costs)
Maybe equip 2nd stage with some Rvacs, or do so later after 1st flights
Then
Execute orbital tests & re-entry with minimal "payload" to LEO
Proves out landing technique for booster stage & more Raptor engine flight experience

and so on...

I'm convinced SpaceX's engineers will come up with a more clever, lower cost approach than this, an approach that does not divert them from their goal and require non-essential R&D time and money for spacecraft not utilized in the Mars settlement core mission.
« Last Edit: 02/25/2017 05:29 pm by philw1776 »
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Offline spacenut

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Re: Should ITS have a smaller prototype to ease development?
« Reply #129 on: 02/25/2017 06:31 pm »
I still think that because pad 39A is designed for 12 million lbs thrust, this should be the maximum for the BFR, and then scale ITS accordingly.  Also, 12 m or 40' is the widest a river/coastal waterway barge can carry.  Again scale BFR accordingly. 

Otherwise, a new launch pad and facility would have to be built somewhere, and the manufacturing for both BFR and ITS will have to be built at the launch facility.  This is expensive in and of itself. 

There are probably any number of vacant buildings that could be used to manufacture the BFR/ITS along the waterways.  No new manufacturing facility, only new tooling.  Even McCloud, while only manufacturing one SLS per year, could make 6 Saturn V's.  So if BFR/ITS is fully reusable, 3-4 per year could be made there, while not making SLS.  This would keep the work force fully employed and as the BFR/ITS come off the assembly line fully reusable, a fleet of these can be built over several years.  Again taking advantage of existing facilities with a slightly smaller ITS. 

Offline RotoSequence

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Re: Should ITS have a smaller prototype to ease development?
« Reply #130 on: 02/25/2017 07:14 pm »
I think there may be no other option for him than to develop a mini ITS architecture. Currently all his revenue comes from being the most cost effective way of getting into LEO and other earth orbits. However Bezos is building what could evolve into a fully reusable system to get into similar orbits based on a raptor like engine. To stop Bezos eating his lunch I would think he needs to look at development of a raptor powered F9 replacement with a reusable upper stage. He could use the falcon heavy with reusable upper stage but hard to see how that would be competitive against Bezos single stick approach

Something similar in shape to ITS would make sense. It could have a cargo and manned version and would be capable of moon missions etc if it's used semi permanent ITS tankers in orbit. Cargo version would be first to meet commercial requirements. But down the track a crewed version would also have much greater crew capacity than the dragon2 if such a requirement was to evolve.

Or Elon skips this step entirely and uses the absurd mass fraction of the Spaceship as the basis for a SSTO launcher that can service all foreseeable LEO payloads. If you use twelve sea-level raptors, it should be possible to push 60 tons to LEO, including landing fuel.

Offline OneSpeed

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Re: Should ITS have a smaller prototype to ease development?
« Reply #131 on: 02/27/2017 10:17 am »
Yes, people don't seem to be grasping the size difference going from F9/FH to ITS. Putting a mini-ITS on a scale that most propose on FH would be like putting a F9 upper stage on a F1 first stage. It is just too big.

Agreed, here's how a full scale ITS ship would look on top of a FH launcher. It would not perform well.

Offline dror

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Re: Should ITS have a smaller prototype to ease development?
« Reply #132 on: 02/27/2017 11:46 am »
yes

i had surgery today and i came out of the haze to say this.

There have been a lot of threads about this. the conclusion always is that it is a waste of time and money, and actually slows things down. OP should do their homework and link to all of them.  Why is this time different?
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Offline Rei

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Re: Should ITS have a smaller prototype to ease development?
« Reply #133 on: 03/02/2017 10:17 am »
The area of a sphere goes goes up as the square of the radius, the volume rises by the cube.

Yes, but the thickness of a pressure-bearing spherical shell increases linearly with the radius of the sphere when at constant pressure and stress.

No, things don't scale perfectly, but it's not a squared/cubed difference.  If you scale size as well as CF mass per unit area, you're roughly scaling all properties evenly.
« Last Edit: 03/02/2017 02:50 pm by Rei »

Online envy887

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Re: Should ITS have a smaller prototype to ease development?
« Reply #134 on: 03/02/2017 01:58 pm »
Yes, people don't seem to be grasping the size difference going from F9/FH to ITS. Putting a mini-ITS on a scale that most propose on FH would be like putting a F9 upper stage on a F1 first stage. It is just too big.

A version that is 1/6 or 1/8 the mass would work fine on FH. It would be only slightly larger in diameter than the current fairing, but would have much better performance than the current stage.

But when you scale something *that* small, it is no longer super helpful beyond verifying the most basic stuff. Scale changes things, it changes how air flows around the vehicle, and it changes how much heat needs to be dissipated, and so on.

The area of a sphere goes goes up as the square of the radius, the volume rises by the cube. Sub-scale models may be of value in terms of technology development, but in areas where basic physics define your spacecraft (like hypersonic retro-propulsion) then the gain is an illusion. A smaller ITS - call it the ITS-SP after the 747-SP - is only of value if it fills a market niche, eg for LEO or Lunar/HEO missions, and it may well be that simply flying a standard ITS half-empty would also fill that niche, and without any unique development or manufacturing costs.

Actually, subscale makes entry easier, because more area and less mass means lower heating rates. It does make launch less efficient, because drag is a function of area, but delta-v a function of mass. As Rei pointed out, pressure vessel mass scales linearly, so the same PMF should be achievable.

The "unique development or manufacturing costs" for ITS are the gigantic booster and 12m infrastructure. A 1/6 volume mini-ITS at ~375 tonnes GLOM for the upper stage + payload and ~6 meter diameter could fly on Falcon Heavy. That means all the existing infrastructure can be used: build in Hawthorne, truck overland, test in McGregor, process in the 39A HIF, launch on 39A with minimal modifications to the TEL and pad.

And the "niche" filled would be 15 tonnes to GTO fully reuseable. Plus demonstrating orbital refueling, high-energy lifting VTOL EDL, and the chance to land 50 tonnes on Mars or the Moon. Basically the entire ITS paradigm without most of the massive infrastructure investment.

Offline Lars-J

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Re: Should ITS have a smaller prototype to ease development?
« Reply #135 on: 03/02/2017 05:34 pm »
The "unique development or manufacturing costs" for ITS are the gigantic booster and 12m infrastructure. A 1/6 volume mini-ITS at ~375 tonnes GLOM for the upper stage + payload and ~6 meter diameter could fly on Falcon Heavy. That means all the existing infrastructure can be used: build in Hawthorne, truck overland, test in McGregor, process in the 39A HIF, launch on 39A with minimal modifications to the TEL and pad.

No, A 6 meter diameter payload is NOT easily truckable. SpaceX settled on a maximum practical diameter of 3.7m for F5/F9 because of highway transport limitations.

Bigger things can be transported, but the roads you can use dwindle fast as the size goes up.
« Last Edit: 03/02/2017 05:35 pm by Lars-J »

Offline JasonAW3

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Re: Should ITS have a smaller prototype to ease development?
« Reply #136 on: 03/02/2017 06:30 pm »
The "unique development or manufacturing costs" for ITS are the gigantic booster and 12m infrastructure. A 1/6 volume mini-ITS at ~375 tonnes GLOM for the upper stage + payload and ~6 meter diameter could fly on Falcon Heavy. That means all the existing infrastructure can be used: build in Hawthorne, truck overland, test in McGregor, process in the 39A HIF, launch on 39A with minimal modifications to the TEL and pad.

No, A 6 meter diameter payload is NOT easily truckable. SpaceX settled on a maximum practical diameter of 3.7m for F5/F9 because of highway transport limitations.

Bigger things can be transported, but the roads you can use dwindle fast as the size goes up.

Sounds like a "Build on site" type of project.

      This wouldn't exactly be such a bad idea, as it would be good training and practice for construction of the final product.
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Online punder

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Re: Should ITS have a smaller prototype to ease development?
« Reply #137 on: 03/02/2017 06:41 pm »
And the "niche" filled would be 15 tonnes to GTO fully reuseable. Plus demonstrating orbital refueling, high-energy lifting VTOL EDL, and the chance to land 50 tonnes on Mars or the Moon. Basically the entire ITS paradigm without most of the massive infrastructure investment.

I'm not a rocket scientist, so it's not clear to me why the vehicle could only put 15 tonnes in GTO but land 50 tonnes on Mars or the Moon. Could you explain that? Seriously, no snark intended.

Online envy887

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Re: Should ITS have a smaller prototype to ease development?
« Reply #138 on: 03/02/2017 07:24 pm »
The "unique development or manufacturing costs" for ITS are the gigantic booster and 12m infrastructure. A 1/6 volume mini-ITS at ~375 tonnes GLOM for the upper stage + payload and ~6 meter diameter could fly on Falcon Heavy. That means all the existing infrastructure can be used: build in Hawthorne, truck overland, test in McGregor, process in the 39A HIF, launch on 39A with minimal modifications to the TEL and pad.

No, A 6 meter diameter payload is NOT easily truckable. SpaceX settled on a maximum practical diameter of 3.7m for F5/F9 because of highway transport limitations.

Bigger things can be transported, but the roads you can use dwindle fast as the size goes up.

At 12m they NEED to have facilities on a barge waterway or at the test/launch site, while at 6m trucking, barging, or flying (in a Super Guppy) a upper stage are all options. I didn't say they were easy, but much more so than a 12m beast. It doesn't have to be as quick as moving a F9 because they would only need 2 or 3 per year, with the stages flying back to CCAFS or Vandy after missions.

Online envy887

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Re: Should ITS have a smaller prototype to ease development?
« Reply #139 on: 03/02/2017 07:25 pm »
And the "niche" filled would be 15 tonnes to GTO fully reuseable. Plus demonstrating orbital refueling, high-energy lifting VTOL EDL, and the chance to land 50 tonnes on Mars or the Moon. Basically the entire ITS paradigm without most of the massive infrastructure investment.

I'm not a rocket scientist, so it's not clear to me why the vehicle could only put 15 tonnes in GTO but land 50 tonnes on Mars or the Moon. Could you explain that? Seriously, no snark intended.

15t to GTO would be a single launch+return, while 50t would take ~6 refueling launches and be one way (unless refueled on Mars)

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