If the lightest metal around were iron, would aerospace even be possible?

[swampwiz]

I was reading about an old project I was involved in (X-33), and it got me thinking. It seems that without aluminum (or something as strong per unit mass), modern aerospace vehicles would not be possible. As everyone here knows, the most important design criteria is weight, and iron is much heavier, and it seems that there would be no such vehicles as the B-52, Boeing 747 or Saturn V if iron were the only choice of metal. I suppose that there would still be vehicles like the Spruce Goose, but that is painfully quaint, and in any case, there is absolutely no way that there could be a wooden launch vehicle.

 

[anorlunda]

Sure. Next you could wonder about the existence of the electric industry and the Internet if we had no conductors or semiconductors. Possible musings are endless, but what's the point?

 

[BWV]

We would have never existed for want of Sodium, so the point is moot

 

[Vanadium 50]

We would have never existed for want of Sodium, so the point is moot

But I sure wouldn't want to fly an aircraft made out of sodium.

 

[jrmichler]

most important design criteria is weight

No. The important design criteria include, but are not limited to:

1) Strength to weight ratio. Wood, aluminum, titanium, and steel all have similar strength to weight ratios.

2) Stiffness to weight ratio. Stiffness to weight and stiffness to density are important where a primary design criteria is buckling, as in aerospace vehicles.

3) Cost. Wood is cheap, while aircraft grade wood is not so cheap. Add in problems with moisture and high labor costs, and wood gets expensive. Titanium is expensive.

4) Corrosion resistance. Steel is bad, aluminum better, stainless steel better, and titanium the best. But titanium is expensive.

The MIG-25 fighter was made with stainless steel. From the Wikipedia article: https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-25:
Because of the thermal stresses incurred in flight above Mach 2, the Mikoyan-Gurevich OKB had difficulties choosing what materials to use for the aircraft. They had to use E-2 heat-resistant Plexiglas for the canopy and high-strength stainless steel for the wings and fuselage. Using titanium rather than steel would have been ideal, but it was expensive and difficult to work with. The problem of cracks in welded titanium structures with thin walls could not be solved, so the heavier nickel steel was used instead. It cost far less than titanium and allowed for welding, along with heat-resistant seals.[11] The MiG-25 was constructed from 80% nickel-steel alloy, 11% aluminium, and 9% titanium.

Space X is using 304L stainless steel for their Starship. From the Wikipedia article: https://en.wikipedia.org/wiki/SpaceX_Starship:
In January 2019, Musk announced that the Starship would no longer be constructed out of carbon fiber, and that stainless steel would be used instead, citing several reasons including cost, strength, and ease of production.

304L is one of the most common, and lowest cost, stainless steel alloys.

 

[DaveE]

What about composite structures. Fiberglass, carbon fiber, kevlar, epoxies, plywood...

 

[sysprog]

@jrmichler very nice post and good information on materials and on aerocraft $\dots$

 

[Averagesupernova]

But I sure wouldn't want to fly an aircraft made out of sodium.

Some parts of a plane likely are. Exhaust valves are likely sodium filled. ;-)
Sorry, couldn't resist.

 

[DaveE]

Some parts of a plane likely are. Exhaust valves are likely sodium filled.

Plus those little salt packets you get with your in flight meal. Oh, wait, not a metal, is it? Sorry, I take it back...

 

[256bits]

What about composite structures. Fiberglass, carbon fiber, kevlar, epoxies, plywood...

Already done - see some of the WWII plane construction - maybe not the newer materials, but at least wood composites.
https://www.historynet.com/the-miraculous-mosquito.htm

 

[sysprog]

Plus those little salt packets you get with your in flight meal. Oh, wait, not a metal, is it? Sorry, I take it back...

Well, yes, sodium is a metal, usually not free $-$ as you already indicated, we most often encounter it combined with the non-metal (halogen) chlorine $\dots$

 

[boneh3ad]

If my grandmother had wheels she would be a wagon.

 

[swampwiz]

OK, with the mass properties of an aerocraft not quite as important, what about a launch vehicle?

 

[swampwiz]

Sure. Next you could wonder about the existence of the electric industry and the Internet if we had no conductors or semiconductors. Possible musings are endless, but what's the point?

I don't think we'd have desktop-sized computers and the internet if we had to rely on relay switching for a computer. Heck, we'd still be in bad shape if we had semiconductor material but not the IC.

 

[mfb]

OK, with the mass properties of an aerocraft not quite as important, what about a launch vehicle?

See Starship, mentioned in post 5.

 

[jackwhirl]

See Starship, mentioned in post 5.

Or the Sea Dragon. It's one of my favorite rocket concepts. It was designed to use 8mm steel sheeting, which would give it a really high dry mass, but the 23m diameter and the square-cube law come into save the day. It's got ridiculous lifting capacity (550Mg) and was totally feasible with then-current tech.

 

[glappkaeft]

Or the early Atlas rockets

Or the Sea Dragon.

Sea Dragon is a bit too much of a paper design for my taste but even in the late 50s you could build and fly a rocket (Atlas) built using steel with near SSTO performance.

 

[cmb]

If you are asking if it's possible to make an aircraft from metals with densities of >7g/cc, I'd say yes.

What would happen is that we'd get very clever at maximising the strength versus form of the metal.

The metal could be foamed, or honeycombed, or hollowed struts with cables, which are things already done, and I am sure we'd just get even more creative!

Myth Busters made a lighter-than-air machine using lead foil!

So, potentially, you are asking the wrong sort of question. Not 'could we' but simply 'what would they be like'. However, even that question might be 'wrong'.

We work with the materials around us, if there are some good, cheap reliable options found, we tend to stick with those for a good while and try not using lesser performance materials.

So the answer to the question 'what would they be like' becomes 'who cares?! We have a cheap reliable solution already!'.

 

[Astronuc]

I was reading about an old project I was involved in (X-33), and it got me thinking. It seems that without aluminum (or something as strong per unit mass), modern aerospace vehicles would not be possible. As everyone here knows, the most important design criteria is weight, and iron is much heavier, and it seems that there would be no such vehicles as the B-52, Boeing 747 or Saturn V if iron were the only choice of metal. I suppose that there would still be vehicles like the Spruce Goose, but that is painfully quaint, and in any case, there is absolutely no way that there could be a wooden launch vehicle.

One cites several examples with different missions.

jrmichler has cited several materials considerations. Other considerations include: Melting point, particularly with respect to high stress areas, where the structure cannot exceed a stress limit at a certain homologous temperature; Fatigue resistance; Fracture toughness; Thermal expansion and Creep resistance.

In the Mig-25 example, the leading edges, which experience high temperature during supersonic flight, were Ti-alloy, but they were not critical load-bearing structures.

256bits cited the DeHaviland Mosquito, which was mostly wood.

Carbon composites are also finding limited applications in modern aircraft.

For NASA's Space Shuttle, the solid rocket booster (SRB) rockets are made of high strength maraging steel. The sole purpose of the SRBs are to lift the Shuttle's Main Propellant Tank, which contains the liquid H and O for the Shuttle's main engines. Once the Shuttle is high enough in the atmosphere, the SRBs detach.
https://ntrs.nasa.gov/citations/19740012287

Supersonic and hypersonic aircraft require different materials. The SR-71 and predecessors where made of Ti-alloy and some composite materials which had to withstand very high temperatures.

The XB-70 wings were largely of stainless steel, sandwiched honeycomb (composite) panels, and titanium.
https://www.nationalmuseum.af.mil/V...heets/Display/Article/195731/xb-70-honeycomb/

The X-15 aircraft used various Ni-based alloys, e.g., Inconels, since the structure experience very high temperatures. Note the limited range and launch from a carrier aircraft. Altitude and speed records were set in separate runs.
https://en.wikipedia.org/wiki/Inconel#Uses

https://en.wikipedia.org/wiki/7075_aluminium_alloy
Aluminum alloys are used for relatively low speed aircraft as well as some military aircraft, e.g., F16.

The F-16 was designed to be relatively inexpensive to build and simpler to maintain than earlier-generation fighters. The airframe is built with about 80% aviation-grade aluminum alloys, 8% steel, 3% composites, and 1.5% titanium.

https://en.wikipedia.org/wiki/General_Dynamics_F-16_Fighting_Falcon#Overview

Aluminum alloys played a significant role in the Space Shuttle airframe. Weight was a significant consideration.
https://www.nasa.gov/centers/johnson/pdf/584733main_Wings-ch4g-pgs270-285.pdf
The Shuttle had a unique thermal protection system
https://www.azom.com/article.aspx?ArticleID=11443
Some early considerations on the Shuttle design - https://history.nasa.gov/SP-4221/ch8.htm
Failure of the thermal protection system lead to loss of the Space Shuttle Columbia.

Today, Al-Li alloys play important roles in aircraft frames
https://en.wikipedia.org/wiki/Aluminium–lithium_alloy#Third-generation_alloys_(1990s–2010s)
http://www.icas.org/ICAS_ARCHIVE/ICAS2006/PAPERS/195.PDF
http://www.metallurgie.rwth-aachen.de/new/images/pages/publikationen/3_908451_26_4_5_id_5266.pdf
https://www.nasa.gov/centers/marshall/pdf/113020main_shuttle_lightweight.pdf

Composites play a more substantial role in more modern aircraft. Ti-alloys are used in various systems.

the F-35 drew upon lessons from the F-22; composites comprise 35% of airframe weight, with the majority being bismaleimide and composite epoxy materials as well as some carbon nanotube-reinforced epoxy in newer production lots.

https://en.wikipedia.org/wiki/Lockheed_Martin_F-35_Lightning_II#Overview
https://www.businesswire.com/news/h...Lockheed-Martin-for-F-35-Joint-Strike-Fighter

There are more advanced Al alloys now available, but they are used for structures not subject to high temperature. They are also quite expensive.
https://aluminiuminsider.com/aluminium-scandium-alloys-future/
http://www.dunand.northwestern.edu/refs/files/JOM-0302-35.pdf
https://ntrs.nasa.gov/citations/20050158693

 

[jrmichler]

The 1936 Fleetwings Sea Bird was made from stainless steel. It's currently flying:

Photo scanned from page 49 of the February 2021 Sport Aviation magazine. Some more photos online - search N16793 to find them.

 

[BigDon]

As I understand from past classes, pure titanium has to be alloyed before it can be used in near vacuum. Or it begins to degrade through an unremembered mechanism.

 

[jack action]

I might be hijacking this thread, but it reminded me of what one of my teacher told us in class: Without mercury (only metal in the liquid phase at room temperature), science would have probably been stuck for a long time as it is very difficult to quantify temperature without it. Of course, with the advance of science, we have other ways now, but those methods only exist because we first made a thermometer made with mercury to help us understand.

 

[jrmichler]

This is a good time to use a search engine. Search terms titanium in high vacuum found numerous hits. From the very first hit:

Titanium is a new candidate material for use in UHV and extremely high vacuum (XHV). It is advantageous in that it is lightweight, nonmagnetic, and shows no deterioration in the welded joints.

 

[BigDon]

Which alloy?

 

[jrmichler]

pure titanium has to be alloyed before it can be used in near vacuum. Or it begins to degrade through an unremembered mechanism.

PF is about solid science, science that is based on peer reviewed research. If titanium degraded in vacuum, there would be published research on the effect. We expect you to do make an honest effort to find out for yourself before posting. Unsubstantiated rumors are not acceptable here.

 

[hmmm27]

https://outgassing.nasa.gov/

 

[swampwiz]

No. The important design criteria include, but are not limited to:

1) Strength to weight ratio. Wood, aluminum, titanium, and steel all have similar strength to weight ratios.

2) Stiffness to weight ratio. Stiffness to weight and stiffness to density are important where a primary design criteria is buckling, as in aerospace vehicles.

3) Cost. Wood is cheap, while aircraft grade wood is not so cheap. Add in problems with moisture and high labor costs, and wood gets expensive. Titanium is expensive.

4) Corrosion resistance. Steel is bad, aluminum better, stainless steel better, and titanium the best. But titanium is expensive.

The MIG-25 fighter was made with stainless steel. From the Wikipedia article: https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-25:
Because of the thermal stresses incurred in flight above Mach 2, the Mikoyan-Gurevich OKB had difficulties choosing what materials to use for the aircraft. They had to use E-2 heat-resistant Plexiglas for the canopy and high-strength stainless steel for the wings and fuselage. Using titanium rather than steel would have been ideal, but it was expensive and difficult to work with. The problem of cracks in welded titanium structures with thin walls could not be solved, so the heavier nickel steel was used instead. It cost far less than titanium and allowed for welding, along with heat-resistant seals.[11] The MiG-25 was constructed from 80% nickel-steel alloy, 11% aluminium, and 9% titanium.

Space X is using 304L stainless steel for their Starship. From the Wikipedia article: https://en.wikipedia.org/wiki/SpaceX_Starship:
In January 2019, Musk announced that the Starship would no longer be constructed out of carbon fiber, and that stainless steel would be used instead, citing several reasons including cost, strength, and ease of production.

304L is one of the most common, and lowest cost, stainless steel alloys.

As someone who had worked on the External Tank, liasing with the Composites team, I can understand why Musk abandoned carbon fiber; as I recall the only major part that composites was used was for the nose cone, and that was a PITA to get manufactured at the proper tolerance.

 

[swampwiz]

One cites several examples with different missions.

jrmichler has cited several materials considerations. Other considerations include: Melting point, particularly with respect to high stress areas, where the structure cannot exceed a stress limit at a certain homologous temperature; Fatigue resistance; Fracture toughness; Thermal expansion and Creep resistance.

In the Mig-25 example, the leading edges, which experience high temperature during supersonic flight, were Ti-alloy, but they were not critical load-bearing structures.

256bits cited the DeHaviland Mosquito, which was mostly wood.

Carbon composites are also finding limited applications in modern aircraft.

For NASA's Space Shuttle, the solid rocket booster (SRB) rockets are made of high strength maraging steel. The sole purpose of the SRBs are to lift the Shuttle's Main Propellant Tank, which contains the liquid H and O for the Shuttle's main engines. Once the Shuttle is high enough in the atmosphere, the SRBs detach.
https://ntrs.nasa.gov/citations/19740012287

Supersonic and hypersonic aircraft require different materials. The SR-71 and predecessors where made of Ti-alloy and some composite materials which had to withstand very high temperatures.

The XB-70 wings were largely of stainless steel, sandwiched honeycomb (composite) panels, and titanium.
https://www.nationalmuseum.af.mil/V...heets/Display/Article/195731/xb-70-honeycomb/

The X-15 aircraft used various Ni-based alloys, e.g., Inconels, since the structure experience very high temperatures. Note the limited range and launch from a carrier aircraft. Altitude and speed records were set in separate runs.
https://en.wikipedia.org/wiki/Inconel#Uses

https://en.wikipedia.org/wiki/7075_aluminium_alloy
Aluminum alloys are used for relatively low speed aircraft as well as some military aircraft, e.g., F16.
https://en.wikipedia.org/wiki/General_Dynamics_F-16_Fighting_Falcon#Overview

Aluminum alloys played a significant role in the Space Shuttle airframe. Weight was a significant consideration.
https://www.nasa.gov/centers/johnson/pdf/584733main_Wings-ch4g-pgs270-285.pdf
The Shuttle had a unique thermal protection system
https://www.azom.com/article.aspx?ArticleID=11443
Some early considerations on the Shuttle design - https://history.nasa.gov/SP-4221/ch8.htm
Failure of the thermal protection system lead to loss of the Space Shuttle Columbia.

Today, Al-Li alloys play important roles in aircraft frames
https://en.wikipedia.org/wiki/Aluminium–lithium_alloy#Third-generation_alloys_(1990s–2010s)
http://www.icas.org/ICAS_ARCHIVE/ICAS2006/PAPERS/195.PDF
http://www.metallurgie.rwth-aachen.de/new/images/pages/publikationen/3_908451_26_4_5_id_5266.pdf
https://www.nasa.gov/centers/marshall/pdf/113020main_shuttle_lightweight.pdf

Composites play a more substantial role in more modern aircraft. Ti-alloys are used in various systems.
https://en.wikipedia.org/wiki/Lockheed_Martin_F-35_Lightning_II#Overview
https://www.businesswire.com/news/h...Lockheed-Martin-for-F-35-Joint-Strike-Fighter

There are more advanced Al alloys now available, but they are used for structures not subject to high temperature. They are also quite expensive.
https://aluminiuminsider.com/aluminium-scandium-alloys-future/
http://www.dunand.northwestern.edu/refs/files/JOM-0302-35.pdf
https://ntrs.nasa.gov/citations/20050158693

Fe is better for fatigue, which is an issue for SpaceX but not for NASA components other than then SSSRMs.

 

[Astronuc]

As I understand from past classes, pure titanium has to be alloyed before it can be used in near vacuum. Or it begins to degrade through an unremembered mechanism.

Used in a terrestrial vacuum chamber or in space. Ti (an alloys) often contain residual hydrogen from manufacturing. If one wants an ultrahigh vacuum in a Ti chamber, one will find that Ti will outgas hydrogen in a vacuum, which will take longer to get to a very low vacuum, but would also potentially contaminate what is placed in the vacuum. In space, one would not be concerned about outgassing of hydrogen into the space environment. Besides, there are lighter Al alloys for spacecraft .

Alloying Ti is done for strengthening, creep resistance, thermal performance, and/or chemical/phase stability, i.e., suppress the formation of brittle phases. The particular alloy depends on the intended environment and mission/performance goal.

issue for SpaceX but not for NASA components other than then SSSRMs.

One should distinguish between liquid-propellant rockets vs solid rocket motors - two different structural requirements for the propellant containment and thermal performance. In liquid fueled motors, the high pressure and temperature are in the power head, combustion chamber and nozzle. In contrast, the solid rocket body is subject to high pressure and temperature of the combustion of the propellant, toward the end of the burn.

The function of the solid rocket boosters in the space shuttle is simply to the the LOX/LH₂ tank high enough in the atmosphere with the space shuttle, in about 2 minutes (127 s), when the space shuttle can continue without them.

So different missions/environments - different materials.