SpaceX Fires Mars-Bound Raptor Engine (extremetech.com)

Elon Musk took to Twitter Sunday evening to announce the "first firing of Starship Raptor flight engine." While SpaceX has fired individual components before and experimented with various designs, this is the first time the now-completed design has been assembled and fired in its intended spaceflight configuration. ExtremeTech reports: Raptor has gone through a number of design changes -- originally, SpaceX planned to mount it to the ITS launch vehicle back in 2016 (powered by 42 Raptor engines), before changing gears and unveiling its BFR rocket concept (officially known as "Super Heavy" for the first stage, and Starship for the second). The Super Heavy mounts 31 Raptor engines, while the Starship has seven. The engine has been designed with a priority on lowering overall wear and tear and removing failure points that could limit its reusability or increase long-term operating costs. Unlike SpaceX's Merlin engine, which runs on a mixture of RP-1 and LOX, the Raptor engine is fueled by cryogenic liquid methane and LOX. The Raptor uses subcooled methane (subcooling refers to keeping the temperature of the liquid well below its boiling point). Subcooling the methane allows SpaceX to increase the amount of propellant stored in the rocket. It increases specific impulse and reduces cavitation.

The actual test burn only goes on for a few seconds, but yields tremendously valuable information about the actual performance of the rocket and its ability to ignite in a controlled fashion. The green glow in the exhaust near the end of the firing indicates the copper liner in the engine chamber burned by accident. While this should not have happened, it's precisely to find these pain points that engineers conduct test firings in the first place. There is no substitute for this kind of test-firing and, as Ars Technica notes, "any 'first' test firing of a new, full-scale rocket engine that doesn't end in an uncontrolled explosion is a good thing." Ars also states that this specific engine may be deployed for "hopper" flights this year when SpaceX attempts to fly the Starship roughly 5km high, then land it again.

Comments

Green

[religionofpeas]

The green glow in the exhaust near the end of the firing indicates the copper liner in the engine chamber burned by accident

It's burning an engine-rich flame.

Re:Green

[jfdavis668]

It's not easy being green...

Re: Green

[InfiniteBlaze]

Given that it burns a condensed greenhouse gas and burning methane only produces carbon dioxide and water...yeah, you might be right.

Moon-Bound at Least

[mentil]

The new plan is to send the BFR on trips to the moon, early on. Later, they'll send it to Mars; this may only happen after they build their next-gen Raptor engines.
There is some sense to this. If they can cram 100 space tourists into Starship, then send it to orbit the Moon for a day (7 day space vacation), they could make LOTS of money. $10 million per ticket x 100 seats = $1beeelion per launch (prior launch of fuel into orbit required, however), and they'd only pay the cost of fuel, probably less than $1million. After a few years, once they get more super heavies/starships built, they could bring the price down to $1 million for a ticket, still make massive profits, and it'd hugely increase the number of people who would pay to go to the moon. Only a few dedicated people would be willing to spend a few years of their life to go to Mars, wait a while for a launch window to arrive, then come back. Of course, they could also offer moon landings, maybe build a moon hotel. Maybe put some Starlink satellites in lunar orbit for lunar internet connectivity, although the 1,250ms latency would be killer.

Internet access on Mars would suck (due to the half-hour latency). Bet there's a business opportunity for an orbiting data center that'd host mirrors of various sites.

Re:Moon-Bound at Least

[Big Hairy Ian]

Nobody is going to the moon or Mars on a tourist trip in your lifetime, try to stop being an idiot before you die.

Funny though Virgin Galactic's whole business model is based around Space Tourism and they are only planning Sub Orbital. I think Anonymous Cowards should try having dreams rather than crushing them.

Re:Moon-Bound at Least

[Kjella]

Nobody is going to the moon or Mars on a tourist trip in your lifetime, try to stop being an idiot before you die.

You underestimate how much money some people have.
Jeff Bezos: Net worth $142 billion (before divorce)
Apollo program: $125 billion in today's dollars

Jeff Bezos could single-handedly fund the Apollo program. In fact, that should now be the new unit when describing how just how rich they are. Is there a market for $10 million joyrides to the moon? Well we know 7 tourists went to the ISS on the Soyuz when the Russians were selling seats for $20-40 million and that was a very limited opportunity. The moon sounds grander and cheaper.

Of course to normal people spending millions of dollars on this is crazy talk. But I remember passing by a TV show that was selling crazy stuff to the super rich, the store had made a gold plated $200k bicycle that was actually just intended for show. A Sheik's buyer came in, thought that was cool and that was it. That's $0.2 million for a bicycle, that sounds like the type of guy who could hire an entire flight just to throw a destination wedding.

Basically, don't underestimate what can happen when billionaire wants something.

Re:Moon-Bound at Least

[AmiMoJo]

I doubt they could get 100 people on a spacecraft for 7 days, just due to the logistics. Food, water, waste management, and of course providing some entertainment because much of that time is just uneventful travel through the void. Even just the need for people to exercise and move around presents a challenge you wouldn't get on an airliner - can't expect people to stay mostly seated for a week.

There would also be issues with regulators interested in passenger safety. It's not like if someone gets sick or they lose an engine they can just divert to the nearest airport.

Re:Moon-Bound at Least

[Kjella]

There is some sense to this. If they can cram 100 space tourists into Starship, then send it to orbit the Moon for a day (7 day space vacation), they could make LOTS of money.

The budget version would be a free return trajectory where you're never in orbit, you do one flyby around the moon at a fairly long distance because you have so high relative velocity and that's it. If you first do lunar orbit, that'd probably be the two week option where you spend a week orbiting the Moon. Then neat thing is that with no atmosphere you can get real close, the LM was normally at a 110 km circular orbit but went down to 15 km for Descent Orbit Insertion, like you could get an airplane-like closeup view. That'd take at least one more tanker mission though, maybe more.

P.S. Even with some degree of re-usability it's silly to assume just the cost of fuel. There will be wear and tear parts, there will be parts that must be deprecated over 10 or 100 flights, there will be launch range/mission control operation costs and they'll never come free. Fuel is just a lower bound on how cheap it could get.

Mistake in the summary

[Brett Buck]

In an unthinakbly rare mistake, particularly given the laser-accurate reporting on Musk's antics in the space buff community, there is a mistake in the summary.

Supercooling the methane don't increase the ISP, it increases the density, making the tanks slightly smaller for a given volume, slightly reducing the weight of the tanks. Technically, it increases the mass ratio (ratio of fueled to dry mass) very slightly. That's still good, as good as if it had increased the ISP, but the effect (which, as all things Musk "invents", has been used for about 60 years or more) is not to change the ISP.

Once you start burning it, you need exactly the same amount per unit oxygen and use the same mass of it per unit impulse you would have had anyway.

      The rest of the concept is marginal, Methane is only slightly better than RP-1 (refined kerosene) in terms of ISP in an ideal situation for both, and even supercooled methane is still less dense than RP-1. So the effect is that the effect you are creating with supercooling is more effective if you went to RP-1 - still smaller tanks for a given amount of energy.

  The same effect is why hydrogen makes a bad fuel, particularly for a first stage (or even worse, an airplane). Giant tanks for a given energy and the difficulty of handling outweigh the very large increase in ISP aside from special cases like upper stages.

Re:Mistake in the summary

[Anonymous Coward]

The advantages of methane aren't really in its ISP, which as you noted are marginal. The advantages are it is much nicer to your engines (no buildup or metal fatigue) and you can make it on Mars/other places without too much difficulty. Both of which will be very helpful in our initial forays into truly reusable spacecraft and interplanetary travel.

Re:Mistake in the summary

[lgw]

The main attraction of methane is that you can make it on Mars, and only have to send up hydrogen. Secondarily, pumping fuel is a big part of rocket engine deign, and that's just easier with cryogenic fuels than fuels that are liquid at room temperature. It also has slightly better ISP, and you get less coking, than RP-1, but I'm not sure those differences are compelling.

Re:Mistake in the summary

[cjameshuff]

"Practical" involves more than specific impulse. Liquid hydrogen has not proven to be an economical booster propellant. Its low density makes tanks huge and hurts mass ratio while making it difficult to achieve sufficient thrust, typically requiring the cost and complexity of additional boosters to get off the ground. It works better as an upper stage propellant, but handling multiple propellants is also costly, and it's difficult to store in liquid form for any length of time. SpaceX has achieved major reductions in cost with a simple gas-generator engine using kerosene fuel.

All else being equal, methane's specific impulse advantage is canceled by its density disadvantage, but all else is not equal: methane is less prone to coking at high temperatures and is a closer match to LOX in temperature, enabling use of a full-flow staged combustion cycle that provides a specific impulse advantage far exceeding that from the different propellant chemistry, and greatly simplifying storage of propellants in orbit and during transit. And it provides these advantages while not being nearly as costly and difficult to handle as hydrogen and being dense enough that vehicles don't struggle to produce enough thrust to leave the ground, making it a far more practical choice.

Re:Great picture

[lgw]

the shape of the flame is really interesting.

Mach diamonds. It's what happens when the exhaust is at a slightly lower pressure than the atmosphere. (What happens when the exhaust is at a much lower pressure than the atmosphere is far more exciting, briefly.) Atmosphere-optimized engines are usually optimized for a higher altitude, since the engine will spend more time there.

When the exhaust is at higher pressure than the atmosphere you can still get Mach diamonds (assuming enough atmosphere to matter) but the flame will expand larger than the nozzle before coming back.