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How much higher ISP, and how much more engine weight/complexity? If it’s 15-20% higher but requires heavier engines that are harder and more expensive to mainta
by valuearb 7y ago
How much higher ISP, and how much more engine weight/complexity? If it’s 15-20% higher but requires heavier engines that are harder and more expensive to maintain or use, it’s not going to be that useful or revolutionary. If it causes excess vibrations, it’s unusable. If it doesn’t scale up over a hundred thousand pounds of thrust, it’s a not very useful toy.
The biggest cost in space travel isn’t fuel, fuel is so cheap it’s a rounding error in today’s launch costs. The big costs are the manufacturing cost of making expensive hardware.
We already know the solution to dramatically lowering the cost of space access, it’s reusability. Burning up your entire stack every launch is insanely expensive. SpaceX ‘s reuse of Falcon 9 first stages saves as much as $30M per launch on the already cheapest stack due to mass manufacturing techniques.
The Shuttle cost was around $50k per pound of payload to orbit. Before SpaceX commercial launchers were $5k to $10k per pound of payload. A expendable Falcon 9 sells for around $1,500 per payload pound, a reusable Falcon Heavy launch sells for near $1,000 lb.
A fully reusable launch system would be dramatically cheaper. Each Starship launch is expected to put over 200,000 lbs of payload into orbit for about $500,000 in fuel. Keep refurbishment costs to $10M per launch and amortize the build cost over dozens of launches, and your cost per pound of payload is down to roughly $100. And it would be the largest, most capable lift vehicle ever made.
And it will do it with engines 20% less efficient than the Shuttle. Efficiency is only useful if it’s application cost is low.
- wahern 7y agoAccording to Wikipedia it can increase efficiency by 25%. By the time this method becomes practical (if ever), that 25% might provide a competitive advantage. In fact, we should hope it would, as it would mean that other launch companies met or exceeded the efficiency of SpaceX.
- valuearb 7y agoA 25% more efficient engine sounds like a lot, but doesn’t necessarily mean squat for increasing total system efficiency. The RS-25 for the Shuttle and SLS has an ISP of 366 at sea level and 451 in Space. The first stage F1 for the Saturn V had an ISP of 261. SpaceX Merlin has an ISP of 282 at sea level, the vacuum Merlin achieves 311 in space, but require a different bell. That makes the RS-25 more than 30% more efficient than Merlin or F1. Yet this is the result. Payload fractions: SLS: 3.4%, Saturn V: 4.3%, Falcon 9: 4.1%, Falcon Heavy: 4.5% Thrust to weight ratio: RS-25: 54 F-1: 82 Merlin: 194 The big difference is the SLS choice of hydrogen as fuel. That provides the higher ISP, but also requires significantly larger and heavier tanks. It’s efficiency requires more dead weight in engines and structure that more than negates the ISP advantage. So much weight that the SLS and the Shuttle required the help of massively expensive solid rocket boosters just to reach orbit. If a detonation engine is heavier, that undercuts its efficiency. If it cokes up, it’s not going to be usable in deep space, if not easily reusable it useless. It requires hydrogen, which means it needs a sea level ISP of 450 to be worth the heavier structure in a launch system. But if it can do 550 in vacuum, it will make a great deep space engine.
- Animats 7y agoPayload fractions: SLS: 3.4% That money drain needs to be cancelled. Building RS-25 engines like the ones from the Space Shuttle to use once and throw away is a pure pork program. And they get a crap payload fraction on top of that.
- cryptonector 7y agoWhat, the Senate Launch System?
- deepnotderp 7y agoThe RDE's efficiency comes from detonation combustion, not use of hydrogen. Whether or not an RDE uses hydrogen is an orthogonal question. But claiming that a 25% increase in Isp is nothing is simply wrong.
- valuearb 7y agoHydrogen is the most efficient fuel and the one they claim to work with. We are already within 25% of the maximum theoretical ISP rocket fuels can provide, so their 25% claim is highly suspect. https://thephysicsofspacex.files.wordpress.com/2016/07/isp-upper-limits.pdf https://thephysicsofspacex.files.wordpress.com/2016/07/isp-u...
- deepnotderp 7y agoI'd encourage you to read the wikipedia page on rotating detonation engines as a start, they're not the only ones who've worked on this concept.
- cryptonector 7y agoEfficiency isn't what you need. High thrust and reusability are much more important. For applications like BFR and Starship you need high thrust to surface area -- a rating you never see. Otherwise you need high thrust to weight. Efficiency does matter, but the other things are first. Finally, a more efficient burn cycle will help more if you can also make the fuel pumps more efficient. (Classic engines have pre-burners. A detonation ring engine might need a plain burner turbopump.
- WalterBright 7y ago> The Shuttle cost was around $50k per pound of payload to orbit. Wings, rudder and landing gear was a terrifying addition of weight.
- cryptonector 7y agoTerrifying and rather pointless too. Really, the space shuttle was a terrifying public expenditure vehicle, and as such it can safely be said that it succeeded.
- himinlomax 7y agoThe article talks about them targeting upper stage engines, which are typically not reusable.
- valuearb 7y agoThey will have to be reusable to be economically competitive in the future.
- tigershark 7y agoAnother comment says that the paper cites 1000-1200 ISP that is simply HUGE.