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Viewer Spaceflight Questions 4.2

1: SpaceFlight Questions and Answers

https://www.reddit.com/r/askscience/comments/ukzfir/why_do_ion_engines_prefer_high_molecular_weight/

Welcome to spaceflight questions and answers, edition 4.2

2: Thank you for your questions...

Thanks to everyone who asked questions.

Many of the answers are in videos that I've done in the past. You can find past videos by going to eagerspace.net and searching around.

Onto the questions

3: How do you see China's private space sector in the coming years? They seem to be making fairly rapid progress given their lack of foundational knowledge.

I get asked that often and I don't know enough about Chinese commercial space to have a meaningful opinion.

I linked to a few people who know a lot more near the end of this video.

4: Do you think blue origin will beat SpaceX to the moon?

Two years ago I would have said "definitely not".

Today I would say "unlikely".

Starship is definitely in a bad state but SpaceX has a lot of money, a lot of experience, and a big team.

Blue Origin is still working hard to fly New Glenn more than once.

I'll also note that the lunar lander contracts aren't like crew or cargo resupply for ISS, where there were two companies competing to see who was first. SpaceX *has* the sole contract for Artemis 3 and 4 right now, and it's not clear if NASA can change that even if Blue Moon is ready earlier.

5: Rocket Lab has mentioned the possibility of a crew capsule, but with the hungry hippo fairing and enclosed upper stage of Neutron, how do you expect them to design for crewed flights vs regular launches?

Crew on Neutron is definitely in the "if there was market we might do it" category, and right now the market is tiny and served by dragon.

Here's something that I found pretty surprising...

You can fit a crew dragon inside a Neutron second stage if you can fix the small issue of the fins on the trunk, so I think that there's no reason you can't put a capsule inside the current design.

For abort you just blow the fairings off and then trigger the abort engines in the capsule.

6: How did they make the Soyuz spacecraft so lightweight compared to other crewed spacecraft?

Interesting question. Let's look at some capsules.

The workhouse Russian Soyuz consists of 3 modules. The crew rides in the descent module on both launch and descent, the orbital module gives them more space when they are in orbit, and the service module at the bottom provides power, engines, etc. The descent module is the only part that returns intact.

The Chinese Shenzhou (shen-joe) capsule uses the same layout as the Soyuz - it's almost like the Chinese adopted the Russian design - but it is slightly upsized, with about a 25% larger diameter.

The US and European Orion capsule skips the orbital module but is much larger than the others.

And finally, the Crew Dragon looks like it has a service module, but the rear section is the hollow trunk. The trunk changes the aerodynamics so the capsule flies forward during abort, provides a location for the solar panels and radiators, and can carry unpressurized cargo inside.

As for size, the Soyuz launch mass is 7.1 tons and 3 tons of that returns.

The Shenzhou launch mass is 7.8 tons and 3.2 tons of that returns.

The Orion is a beast, with the capsule massing 10.4 tons by itself and the service module massing 15.5 tons, for a total of 25.9 tons. It's intended to be a capsule that astronauts can stay in for the long lunar missions and the service module has enough propellant to get into and out of lunar orbit. It's still very portly.

Crew Dragon has a total mass of 12.5 tons but returns 9.6 tons to the ground.

7: Soyuz

Now let's look inside.

The Soyuz carries a crew of 3 but has only 3.5 cubic meters of interior volume.

Quite a few astronauts have flown on Soyuz; here is what two of them said...

Astronaut Don Pettit said (read)

Astronaut Doug Wheelock said (read)

It's bad enough that the crew sometimes takes strong pain relievers before the flight to help deal with the muscle cramps that result from the position.

The reason that Soyuz is so small and cramped is that the Soyuz rocket can only carry about 8.2 tons into low earth orbit, so the spacecraft is pretty much as big as it could be.

8: Shenzhou

The Shenzhou capsule also carries three, but it's much larger at 6 cubic meters. You can see that there is considerably more space than Soyuz.

9: Orion

Orion is designed to carry up to 6 crew but is currently only planned to fly with 4. The two-row arrangement means some crew members have feet over the heads, and the capsule has a palatial 8.9 cubic meters of volume.

10: How did they make the Soyuz spacecraft so lightweight compared to others crewed spacecraft?

And finally, Crew Dragon is sized to carry up to 7 but normally carries only 4, and it has 9.3 cubic meters of habitable volume.

Discounting Orion - which is really doing a very different job - crew dragon is bigger and heavier than the Russian and Chinese capsules for two reasons.

The first is that it is designed for a larger crew, and the second is that it is reusable and brings the bulk of the launch mass back to the ground. More mass coming back requires a heavier and more robust heat shield.

11: I've wondered for a while now if Stirling engines would be great at generating electricity in space. Do you know if that sort of power solution has been studied?

The small NASA kilopower reactors for lunar surface use are based on stirling engines. They need large radiators to get rid of the excess heat.

12: Why won't spacex develop a disposable upper stage for super heavy?

They have.

Just like there's a disposable Falcon 9 without grid fins or landing legs, take starship and get rid of the fins, all the heat shield tiles, and the landing fuel tanks, and you have a disposable Starship.

13: My biggest gripe and question is why we still use Specific Impulse. Like I get the whole "easy to translate between units" but it is horribly unintuitive for me.

Wikipedia says the following about specific impulse in reference to rockets.

This definition has little utility - the rocket equation is based on exhaust velocity, which is far more intuitive in my opinion.

But we're stuck with specific impulse because it's what has always been used. Just use it as if it were dimensionless.

14: Do you think starlink and similar systems provide a genuine benefit to humanity that extends beyond providing competition to particularly greedy internet companies in rich countries, or giving scientists in remote places opportunity to scroll social media?

Yes.

Estimates for how many people do not have internet access range from 2.5 billion to 2.9 billion, or about 35% of the world's population.

15: In the long run, which is better, a reusable launch vehicle that only flies occasionally or a mass-produced expendable rocket that is launched frequently

Too many variables to answer.

There are cases where reuse is hugely useful in terms of overall company aims - Falcon 9 is the obvious example here.

There are other cases where reuse makes little sense - Vulcan is an example of this right now.

16: Do you think SpaceX would have failed if not for some lucky timing and appallingly bad decisions/performance by established competitors?

Absolutely. Here's a video for you.

17: Any interest is exploring EU or Russian space industry? I know you are not interested in the Chinese launch industry but is that the same with all non American industries

I've talked a little about the EU space program. Russia is hard to talk about because none of their official information is trustworthy and you therefore need insider sources, which I don't have.

I've talked a little about India's ISRO, and will probably talk about it more in the future.

18: Do you think congress will create a new launcher for NASA after they are unable to justify SLS? And what do you believe they could justify?

Congress has never felt the need to justify SLS and I'm not sure they would change that post SLS.

The space act of 2010 that create SLS pretty much said the following (read)

They talk about deep space but they never define a specific mission.

19: Do you think starliner still has any chance of actually flying astronauts to the ISS with the deorbit date quickly approaching?

After the early issues I thought that starliner still had a good chance, and then after the one way crew flight and Boeing's other issues I felt strongly that there wasn't a good business case for it.

Boeing has since then sold a bunch of new stock and they've been working on a thruster redesign so I'm not sure anybody should care about my opinion now...

The longer they wait, the less money there is in the ISS contract and it's not clear what post-ISS business there might be from the "I hate dragon" contingent.

20: What are some of the technical complexities of orbital refueling? And has anyone achieved it in the past, before starship and blue origin?

I talk about propellant depots in this video.

From a technical standpoint, orbital refueling isn't a big challenge. It hasn't been done in the past because nobody wanted a mission architecture that required a propellant depot.

As for why NASA hasn't done it, Eric Berger shared this quote about Alabama senator Richard Shelby, "Senator Shelby called NASA and said if he hears one more word about propellant depots he's going to cancel the space technology program".

NASA's Marshall space flight center is in Alabama and it owns - among other things - management of the SLS rocket.

Berger wrote an article for Ars Technica titled, "so long, Richard shelby, and thanks for all the pork" that goes into more detail.

21: Any thoughts on Chemical Cislunar Tugs, refueled from NEOs and/or processed Lunar Regolith

Two questions come to mind.

The first is "what sort of market is there for cislunar tugs?" That seems very hard to predict.

The second is "how much it will cost to research, design, develop, transport, and operate facilities that can do that and can you build a business around it?"

There are certainly technical challenges, but the big question is "who is going to pay to do it and why?"

22: Outside of internet/observation what commercial markets are promising in the satellite industry?

NASA has tried to answer this question with ISS for years and hasn't really come up with any real answers.

I talk about it a bit in my free fliers video. My short answer is that there might be something related to manufacturing but it's too soon to tell. I don't think there's a meaningful market for tourism even if transportation is much cheaper.

23: In the future if nuclear fusion ever becomes practical and modular would it open up new possibilities for engines?

There are many fusion rocket engine designs floating around, including the sunbird from Pulsar Fusion.

It is possible that they can work, and if they do, you can just let the products of fusion - or at least some of them - escape out the back. That gives you really high specific impulse. Whether it gives you enough thrust to be interesting isn't clear right now.

The big problem I see is that you need a *lot* of power input to get a fusion reaction started. If you use inertial confinement, you need a ton of power to drive your lasers, and if you use magnetic confinement, a ton of power to heat the plasma and perhaps to get your magnets up and running.

That's not a problem for a terrestrial power plant, but it seems to be a significant barrier for a spacecraft.

24: Could Auriga Space succeed?

Auriga is trying to use electric propulsion to do hypersonic testing, suborbital launches, and ultimately orbital launches.

I talk about electromagnetic launchers in my "guns to space" video.

It's theoretically possible, but you have many of the same issues that SpinLaunch had.

You need to design payloads to be very resistant to high G loads, deal with heating and noise as they travel through the atmosphere at high speed, and you are going to invest a ton of money into development before you get any customer money back.

They can probably get to hypersonic testing for some values of "hypersonic". The suborbital market doesn't have a lot of money in it except for space tourism and this launcher can't carry people.

I think it's unlikely they get to orbit.

25: Why is Starship V2 such a disaster?

Here's a video for you....

26: Do you think Starship will ever carry humans? I'm concerned about its vulnerability to debris impacts and its complete lack of redundancy.

This video is a bit out of date but my thoughts are mostly the same.

27: Can you do a video on rotovators and maybe the ones tethers unlimited might do. I dream of 100s of short 20km rotovators flinging starships in stacked shelled orbits to go beyond the tyranny of the rocket equation.

A video on non-rocket spacelaunch is somewhere on my topic list, but it's honestly not very high on the list, as I struggle to see how they might ever be practical.

They generally are:

Hyper expensive

Time consuming to build

The effect of atomic oxygen on them may be an issue

Simple to attack

And if anything happens to them they may deorbit directly on top of a populated area.

28: Do you think rotating detonation engines would be feasible (in terms of manufacturing, economy, and performance) for a mass produced rocket, like Falcon 9?

There's a short answer in this video.

I'm skeptical and there isn't much real data out there. I put them in the same class as aerospike engines, though I'm pretty

sure that aerospikes aren't practical and not sure about RDE engines.

29: What are your thoughts on the possibility of spacex hosting customer payloads on starlink satellites?

It's going to depend on the scenario - what a customer wants to do, how that would impact the satellite design, and how much they are willing pay for the capability. And whether their business model works in the long run.

There's a pretty high barrier - Starlink needs to be very reliable and they need to be spending the vast majority of their mass budget on serving their actual customers.

Probably not.

30: How will starship come back from mars? Would it need to refuel in low mars orbit? How many radiators will it need to stop cryogenic fuels from boiling off?

It's much easier to get from the surface of Mars into orbit than from earth because of the lower gravity and lack of atmosphere. A fully-fueled starship can do it, assuming that the design works out the way I expect it to work out.

If you are not making fuel on the surface, it would be better to refuel in orbit - you don't want to drop fuel to the surface only to bring it back up into space.

As for radiators, see my propellant depot video. I don't think keeping the propellants cold is a major issue, and they have to do some of that going to mars anyway.

31: Which approach for full reusability is more feasible, realistic, easy to realize - Starship or Stoke Space's Nova?

That's easy.

We don't know. Both approaches are under development and they are both trying to do things that have never been done. SpaceX has clearly run into some issue along the way and it's likely that Stoke will also run into issues along the way.

And it's not clear that full reusability is going to be the winner as it imposes a significant payload penalty. Maybe the "stupid cheap second stage" approach that Neutron is taking will turn out to be better.

32: In your opinion, as of pre launch of Starship flight 10, has Spacex learned enough from the past few launches (attempts) to have a better or worse track record with block 3 starship compared to block 2 starship?

It's post flight 10, which was successful, but my opinion is still the same.

33: How do you think we could make human spaceflight actually cheaper and have a good business case? I know that many people will default to "robots better" but I've always felt that to be an oversimplification that assumes human spaceflight will always remain as expensive and limited as it is now.

It's all about the markets. If you have a compelling reason for people to do useful things in space, that is useful-sized market, and somebody will serve that market.

Right now - with Dragon costs - there's a market for about 4 people a year to go into orbit, and it's costing on the order of $50 million per person.

34: How expensive do you expect to be one Starship launch when the system is fully reusable and a few years in service. Some math to support that please? Fuel, people, materials...

Interesting question.

You will find various people making assertions around how much starship will cost and how much payload it will carry. I made some predictions early on and I do still have a Starship model but I don't think anybody outside SpaceX has data that is good enough for estimates. I'm not even sure that SpaceX has good enough data for estimates .

But that's unsatisfying, so let's see if we can put in some bounds...

At the low end, commercial airlines operate at 3-4 times fuel cost. Fuel is supposedly roughly $1 million, so let's say the lower bound is $5 million. Starship probably doesn't have passengers to deal with, which makes it simpler, but many airliners spend half their lifetime in the air and that spreads out the fixed costs and the plane costs over many many flights. So I think $5 million is in the right ballpark.

On the high end, the heaviest Falcon is a decent benchmark, and that probably costs SpaceX around $60 million.

I'd say somewhere between the two. I'm confident on the low end, not so confident at the high end.

And note these are costs, not prices.

35: Could heat pumps be used to increase the temperature of waste heat to make radiators more efficient?

Here's a paper for you to look at. It says:

To summarize, you can reduce the radiator surface area by about 40%, but the system is more complex and you will pay to launch the heat pump and power it.

And if it goes out, you have a satellite that no longer works.

36: Why weren't Methalox engines as popular in the past as they are today?

There's a video for that...

And that's all the questions and answers for this episode.

37: If you enjoyed this video, listen to this...

If you enjoyed this video, listen to this...