Graphene would be a great material to build a rocket out of. Graphene is extremely thin. One single atom thin layer of graphene can withstand 15 000 000 pascal . A square meter of this material only weighs 0.77 milligrams. So why doesn’t NASA use it for their rockets? I would think that if they had such a light material that can withstand so much, it would be used for spaceflight.
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The Technology Readiness Level (TRL) of graphene is at 2 or 3 as far as I can tell. And that is TRL as related to making very tiny stuff.
Anything used to build a structure for aircraft or spacecraft must be at TRL 8/9 and highly characterized in that specific application (a large database of statistical evidence of performance over a large range of operational parameters).
Graphene as a structural material is still a pipe dream. This says nothing about the insane costs graphene would entail.
Besides the fact BradV pointed out that we don't have the technology to do this yet, the fact is that graphene on a macroscopic scale would not perform as well as the numbers you cited suggest. All the results indicating insane strengths for graphene are done on a tiny, tiny scale. Bulk materials are always much weaker, because the slightest imperfection is a place where force is concentrated by huge factors.
Graphene will eventually be valuable as a bulk material, I believe, but it won't be as magical as the quoted numbers indicate.
Many processes being used to make graphene only yield small and irregularly shaped flakes with a lot of defects. The size of those flakes is barely enough to render them visible for the naked eye, and that is nowhere near what would be required to build a whole vehicle.
Graphene as a miracle construction material is overhyped in a similar manner to supercapacitors being overhyped as a miracle alternative for batteries. However, we haven't seen any supposed miracles outside of the sketching boards yet.
While pure, large, monolithic graphene sheets remain in the realm of fantasy for now, small and irregularly shaped graphene flakes have their own utility in the context of engineering of materials, as they could be used as a strengthening additive in composite materials. For example, one kilogram of cement can be made 30% stronger by mixing in just one gram of flash graphene flakes (here is the source, which in general talks about the method of obtaining flash graphene from organic materials, like coffee grounds).
In this thread one can find information about how composite materials in general (but not specifically graphene-based ones) are currently being used in rocket construction, and why in most scenarios they are inferior to metals. In short, structures constituting a rocket have to withstand extreme thermal stress, extreme temperature swings, and be resistant to radiation. Composites aren't fulfilling those requirements.
Specific strength, the ratio of strength to weight, is a very important figure of merit for a material since it determines a number of theoretical limits. However, this totally fails to take into account other neccessary properties for a material suitable for more general use. A material should typically also be rigid, be practical to apply loads to evenly, not be subject to fatigue or hidden cracks, not be brittle or subject to catastrophic failure, and should have strength in the directions desired which depends on the application, but always some strength in each direction is desired for rigid components.
A material also needs to be affordable both to produce and to form or machine to the desired shape, and be resistant to the environmental conditions it will experience. While space programs expect and operate with large budgets, they do want to choose a less expensive option even if it is technically not the "best" one.
Ductile metals (steel, aluminum, titanium, etc) have many of the above desired features as well as fairly good specific strength, so they are extremely popular. Historically, the useful advance has often been fabrication techniques rather than fundamental materials, such as careful machining or 3D printing that allows a large, complex part to be made of a single piece of material with skins, structural members, and mounting brackets all combined and no wasted weight.
Simply put, the world of aerospace engineering does not react to trends in technology very quickly at all, and especially not at the rate of hype and rumor. Aerospace projects often take years from beginning of design to completion, and safety concerns and the need for very expensive projects to succeed the first time mean that if an old technology works, a new technology will only be used if it provides a huge benefit or makes something possible or economically feasible that wasn't before.
While SpaceX has been less conservative in a number of respects than the "old guard" aerospace contractors and space agency staff who have historically designed rockets, that doesn't mean they aren't bound by the same conservatism for the same reasons, and moreover SpaceX has often followed the principle of using inexpensive, simple technology where possible to save costs -- for example, making Starship out of ordinary high strength stainless steel.
Graphene refers to a single sheet of the graphite crystal. Carbon fiber, such as is used in carbon fiber composites, is made of carbon. Depending on the type of carbon fiber, a significant portion of the carbon is indeed in the form of graphene! While carbon fiber composites are a pretty good material and are used in a number of aerospace applications, they have limitations that mean they aren't particularly heavily used in space. Notably, Rocket Lab's Electron uses carbon fiber composite for the main body of the rocket.
Looking at Wikipedia's list of materials by specific strength, the highest specific strength materials (higher than ductile metals) listed include mostly:
Graphene has recently been developed to a form where lots of experimental applications can be tested, but anticipation of what it might do is well ahead of what has been practically achieved. This is accompanied with a great deal of hype. This is far from new: I remember when carbon nanotubes were the big thing, and occupied roughly the same position in hype. Hopes of space elevators and similar super-engineering were much depressed over time as nobody really succeeded in bonding individual carbon nanotubes into a rope or yarn with strength matching that of the individual tubes.
Because it is carbon with high reactivity. It burns at fairly low temperatures in the presence of oxygen: ["Graphene combusts at 620 K."] (https://chemistry.stackexchange.com/questions/76647/is-graphene-a-combustible-material) Supersonic skin would burn up. Descending through its own exhaust plume would incinerate a returning stage, etc.
Graphene lacks density, compared to graphite and the cost would be more expensive than using Pure Silver if using pure Graphene. Now if you were to melt Titanium, Stainless Steel, and Graphene powder, than you would have the strongest, densest, corrosive-resistant, thermal conductive, radiation-resistant, alloy that could withstand decades of use and would be worth the high cost.
