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Would an airship-launched rocket be practical?

Space Exploration Asked on September 29, 2021

This is essentially a mix of the rockoon (https://en.wikipedia.org/wiki/Rockoon) and air launch to orbit (https://en.wikipedia.org/wiki/Air_launch_to_orbit) concepts. A rockoon is a rocket that’s carried to a high altitude and then launched, but it has the disadvantage of balloons not being steerable. Air launch is when a rocket is carried up by an airplane and then launched, but this is limited by the size of the plane.

Could an airship (and especially a hybrid airship: https://en.wikipedia.org/wiki/Hybrid_airship) be more practical than either of these options? An airship can be steered, unlike a balloon. At the same time, an airship is more fuel-efficient than an airplane, and can potentially be bigger.

I was able to find one scientific study looking into this: https://pdfs.semanticscholar.org/2429/f5f3df3ee0d4b992d681c8d40630bf82a4dd.pdf. Other than that, this idea doesn’t seem to have received much attention.

One Answer

A blimp does have some potential use cases, but making it sensible cost wise takes some very specific needs.

Air launch in general always comes up against the problem that going to orbit does not involve going high, it involves going fast.

Specifically it involves getting to seven kilometers a second a couple of hundred kilometers up. Starting from a couple of kms higher does have advantages in allowing a better nozzle design and shaving off a couple of km of loft but adds a number of complications:

Rocket size is limited by the lifting vehicle (km sized balloons if doing a serious launch)

The rocket needs to be structurally strong enough to handle being mounted to the lifting vehicle and flown (potentially hanging sideways while fully fueled)

Actual launch is tricky, either you start the engines while secured in place (high risk to launch vehicle) or drop the rocket and hope it starts first try.

For a blimp launch you also need to avoid destroying the envelope with engine exhaust, especially if you are using hydrogen as lifting gas (Helium is in global short supply when you start talking hundred ton lifts)

Following from above, all support equipment for check out and startup needs to be vehicle mounted if being drop started, adding weight (and makes it non re-usable unlike a ground launch)

For cryogenic fuels the launch vehicle then needs to carry the support equipment to top off tanks.

In return you gain a reusable first stage, a slight payload to orbit advantage (math varies but seems less than 5%), and the ability to launch into any desired inclination on a safe ground track, and avoid weather at a fixed launch site.

For most cases the economics comes out better for accepting the performance penalty and make the rocket slightly bigger.

Air launch from heavier than air vehicles have a potential advantage if high launch cadence is required (more than one per week) but a blimp probably cannot cycle and return around in that time frame.

Some use cases where a blimp might make sense is where the blimp already exists for some other service and therefore does not need to recoup entire build costs, the launching country is after a sane payload (less than one ton) and the orbit is unreachable from their own territory (Eg Israel). Especially if the potential target orbit needs to be flexible both in time/inclination (extended loiter time). This would suggest the most likely use case would be military in some form - for example a launch on demand Anti Satellite system or a once around surveillance system (or a pure ICBM for that matter).

Correct answer by GremlinWranger on September 29, 2021

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