# How practical would a blimp like spacecraft be?

I have always wondered if a kind of blimp-like spacecraft would not be our answer for some kind of futuristic spacecraft design.

Imagine this: a blimp-like spacecraft is loaded with cargo on the ground, then starts to rise into the sky. As it goes up, it begins to collect and compress oxygen from the atmosphere, which it will use it on the second stage. As is rises, the helium inside will expand, so it will need to recapture it and store it compressed.

When it reaches the limit of our atmosphere, it would burn a mixture of fuel and the capturated oxygen. Shape would only be a minor problem now, since the atmosphere is so thin. The oxygen would only be used when it reaches the no-oxygen area, and only for orientation.

Once the spacecraft finishes its mission and has to go back to the ground, it would undock from the mothership and target Earth. When it re-enters the atmosphere, the helium should be restored in its interior, which would help reduce the re-entry speed. Once the atmosphere is thick enough, it would need some heat shield for sure, but once it's fully inside the Earth's atmosphere, the helium would slow down the fall. The spacecraft could return safely by controlling the amount of helium inside, again using compressors and reducing the amount of helium inside the hull.

I am interested in the math. Is this possible with current technology?

• I think you are asking 2 different questions here. Do you want to know if this would be possible at all or if this would actually be a practical way one could design a spacecraft? – Raditz_35 Sep 18 '17 at 13:43
• This isn't a great way to get into orbit. Getting into orbit is hard because to stay in orbit, you have to go really really fast. Would an aerostat that flew (instead of orbiting) in the very top of the atmosphere count? – MissMonicaE Sep 18 '17 at 13:44
• I am quite sure it is possible, but not sure if it could be a practical way to do it – Bernardo Nicoletti Heller Sep 18 '17 at 13:54
• Going up is the easy part; even amateur associations can make rocket models (or balloons) able to climb to the edge of space. What's difficult (and where you need NASA or Arianespace or Roscomos or an eccentric billionaire) is accelerating to cosmic speeds, and, if you want the spacecraft to return, decelerating from cosmic speeds to mundane aircraft speeds. Short version: going up is easy and cheap; staying up is difficult and expensive. – AlexP Sep 18 '17 at 13:56
• In case you're wondering what people are talking about. Being in orbit and being very high up are very different.. – Samuel Sep 18 '17 at 18:14

This is an OK way to get to the 'edge of space'.

This is a terrible way to get to orbit or slow down on re-entry.

Firstly: getting to the edge of space. Weather balloons use this trick all the time. You fill a suitably light balloon with helium and let the difference in ship density to air density lift you out to the edge of the atmosphere. The trick here is minimising weight, because every bit of weight is a little more density, which equates to a long way away from getting out of the atmosphere. If, however, you have a suitably large, suitably light and suitably strong balloon you can in theory make your balloon big enough to nearly escape

By the way, your balloon will probably have to be tissue paper thin, so any strong winds and you're dead.

Now we hit the first really big problem, namely that space is not orbit. To be orbiting you have to be going sideways very, very fast, and to do that you need fuel. Problem being that fuel is heavy, which will keep your balloon deep enough in the atmosphere to make any acceleration attempt tear your balloon.

You might think that retracting the balloon will help, but in the time that you're retracting the balloon you'll have to burn fuel to keep aloft, and if you're going to do that you're better off just loading the extra fuel while you're on the ground so you can be going sideways at the same time as up. If you don't burn fuel station keeping then you'll fall back into the atmosphere with your rocket booster attached, which is at best inefficient. At worst you're dead.

But let's assume that we're re-entering instead. That means trying to slow down really quite fast, even after the initial scorchingly hot re-entry. A light and thin helium balloon won't do the trick. A heavy parachute would, but by the time you've deployed the parachute what's the point in the helium balloon? Perhaps a more controlled landing?

Well, unless you can finely control your trajectory on the way down then you're likely to get picked up by nearby weather systems, which could push your balloon into any number of hazardous situations.

I'll let you fill in the worst case scenario there.

ADDENDUM: this might work as the first part of a two stage system, if a large airship were used to get a rocket most of the way up. The problem still remains that getting to orbit is a lot harder than getting to space, but perhaps you could take some of the aerodynamic considerations off the rocket.

Trying to land an orbiter on an airship is probably a terrible idea though.

• Plus, the blimp would probably have lost most of its Helium by the time it got to the upper atmosphere, so it would need to be refilled before heading down – nzaman Sep 18 '17 at 16:39
• It probably wouldn't work as a two-stage system - it'd only reduce the fuel needed by a few percent, and the rocket would still be hugely heavy. Bear in mind that the payload for the Hindenberg (including fuel, crew,etc) was around 100 tons - the mass of a Saturn V rocket was around 3,000 tons. And that's using hydrogen - with helium it's probably more like 60 tons. – Matt Bowyer Sep 18 '17 at 23:14
• @MattBowyer: it would have to be a hybridised airship where the envelope is designed to provide further lift. I seem to remember Skycat had designs for some heavy lift hybridised airships that could potentially be scaled up. – Joe Bloggs Sep 19 '17 at 8:31
• Trying to land an orbiter on an airship is probably a terrible idea though. I'd have said the same thing about a barge in the ocean up until recently. – Shane Sep 19 '17 at 15:36
• @Shane: ... Very good point. Someone suggest this to Elon Musk! – Joe Bloggs Sep 19 '17 at 15:43

"Airship to orbit" is a current private project not so well funded by the US military. wiki link. They do it with several stages, but the idea is the same. The low funding actually reflects the high probability that the idea will not work. But if the idea works it will most likely be cheaper than current rockets. And US military actually supports the idea to have near space cheap surveillance and not to reach low earth orbit. But the founder really believes in the idea.

The main challenge of the project as stated by the critics is acceleration from low speed to orbit speed. The atmosphere resistance should be too high. But taken into account that no object flies at this height and therefore there is little data to say how objects behave in this environment the question is a current field of research by the team. Height is not a problem. The team already holds records for the highest balloon flight. And with a two stage system the second airship will operate in an environment where no winds exist and it will have much lower pressure allowing it to go much higher. And higher means less resistance with air.

So in the end no one knows if the idea works, but low funding suggests low probability of success.

And it would take days to reach orbit, so the tech is less relevant to humans but more suitable for cargo.

I think something like this may work with a few minor changes. The first change I would make is to use hydrogen instead of helium since it is much cheaper, renewable, slightly more efficient, and a few more reasons I'll get to later in the answer Second I doubt a blimp can achieve orbital speeds, to make up for this we include a skyhook (https://en.wikipedia.org/wiki/Skyhook_%28structure%29 or https://www.youtube.com/watch?v=TlpFzn_Y-F0) third I would want to make the blimp have a skeleton so it isn't just a pressurized balloon but is more like a zeppelin and fourth we need to make sure that what you are thinking of a blimp is much larger than any blimp you've ever seen before since this is one of the very few questions where the square cube law is in our favour

So for simplicity I'm going to assume that the "blimp" is a cylinder 1km long and has a radius of 100m, this gives you an internal volume of 31,400,000 m^3 which can hold 3.140000e+9L of Hydrogen (according to this calculator http://www.metric-conversions.org/volume/cubic-meters-to-liters.htm) which means your blimp contains 282600000kg of hydrogen (assuming this is roughly what I want https://answers.yahoo.com/question/index?qid=20110201223956AAg5mcP)

I'm going to assume that you design your blimp to behave not unlike a weather balloon (but with an envelope that doesn't pop since you have a lot more lifting gas compared to the surface area of the envelope) and can reach an altitude of 20-40 km while doing nothing but floating. Now this is the other reason I wanted to switch to hydrogen, because you have 282.6 Mg of hydrogen which can be used as rocket fuel and assuming you can pump this fuel out of your blimps envelope and can either get oxygen out of the atmosphere or has oxidizer on board as payload you can use that hydrogen for thrust. Now you probably won't get the same amount of thrust a rocket would since you are using gaseous and not liquid hydrogen but I'm going to assume you can get to several times the speed of sound at an altitude higher than weather balloons usually go. Though it is worth mentioning that the front of your blimp will need to withstand the air resistance at those speeds so you would likely have a rigid layer on the front to force your way through the air, which would lower the mass budget for the cargo you could carry since that material on the front is going to be heavy

Now once your up to speed and at a high altitude you get hooked by the sky hook which leaves you with two options, Hook your cargo and let that be lifted into space while your blimp envelop returns to the ground to pick up a new batch of cargo or the sky hook lifts the entire blimp and you use all that hydrogen in space for something like cold gas jets.

Congratulations you've gotten a blimp (or something like it) into space

As for re-entry, you would have a large amount of surface area acting like a parachute, assuming you can keep the rigid nose of your blimp in front of you you could probably re enter and slow down relatively rapidly acting as a giant parachute, though if you de-orbit going sideways or backwards I doubt the envelope of your airship could take the strain and would rip apart, though as long as they all stayed attached they might still act like parachutes

side notes:

In this design the gas envelope doesn't expand much at higher altitudes but instead burns the hydrogen to propel itself up

Assuming it has a payload to volume ratio similar to the biggest balloon available here https://www.scientificsonline.com/product/professional-weather-balloon by my math it would carry a payload of 70 000kg or so, a lot of that would likely be eaten by adding structure, gas pumps, rockets, and other necessary equipment

And as a disclaimer I am not experienced in any of these fields which I hope is quite evident from my copious links, if I have made a mistake please let me know.