Why is it desirable to land a used rocket (Bezos/Musk)?

I don’t understand why it is preferable and more economical to land a used rocket upright instead of having it parachute down to water or land. (Bezos accomplished it yesterday.)
I know zilch about rocket engineering, but it seems obvious that when compared to a parachute drop and recovery, landing a rocket is more expensive, more difficult, and requires far more technology, which increases the chance of a system failure. (Not to mention the cost of developing this system in the first place. We already know how parachutes work.) I know the space shuttle method was not without cost or complexity, but I don’t see how the new approach offers benefits that outweigh the negatives. So what are the big benefits?

(I noticed that in Bezos’ flight yesterday, the occupant capsule parachuted down to earth. Why not land that too?)

If I understand correctly, landing a rocket like this results in much less damage to the rocket so it can be reused. I don’t think a parachute-landed rocket can avoid significant damage, and a damaged rocket is not something you want to launch.

Yes, less damage. It has to be thoroughly tested afterward, fixed and the tested again which ends up being pricey (Ref.: The “money saving” Space Shuttle).

I don’t quite understand why it has to land upright, however. Seems you could deploy wings like a Small Diameter Bomb and wheels and have it land like a glider. But maybe the physics make it impossible at that scale.

Ever owned a boat that you used in salt water?

Enough said :slight_smile:


I believe the general idea is to get the velocity of the rocket as close to zero as possible before it touches down, so that damage to the engines (not to mention the big honkin’ fuel tank inside the rocket that has remaining explodey stuff inside) is eliminated. Parachutes are great at putting things in slow gear, not so great at putting things in “very nearly stopped” gear.

As for landing in the ocean, I saw one quote about a year or so ago – it was either from Elon Musk or Gwyenne Shotwell, the top executives at SpaceX – which literally went, “Rocket engines do not like seawater.” Unfortunately I can’t find the quote right now, but there is this statement from someone who claims to be a former engineer on the Falcon 9 rocket program. "Water logging : For me, this is the one of the worst problems. If the water seeps into (salt water) the engine parts, you can as well throw the rocket away or sell it for scrap. There are so many intricate fluid passages and holes in the engine. If they become water logged, it is impossible to purge them and get a clean engine. The reliability of such an engine is very poor. "

The vertically landing parts of both Blue Origin and SpaceX’s rockets are the first stage boosters. The whole point of soft landing these is to be able to re-use them. Boosters are actually very fragile things. Any sort of impact would probably cause them to collapse. Some boosters are not even capable of holding up their own weight if not pressurised. Most need some sort of strongback support if laid on their side. They are designed to take the loads in exactly the only manner needed for flight, and any extra strength means mass, mass you are not able to have in your payload going into space. The only chance of getting one back in useful form is to soft land one. The extra cost and complexity isn’t all that great. SpaceX’s Falcon-9 simply leaves one motor running, and the same guidance system that took it into flight, takes it back. There is additional complexity in the software, and some additional mechanical complexity in being able to keep the motor running longer. It does get legs, but these are not exactly the hight of technical difficulty. The additional cost is in the additional fuel capacity. This is essentially an additional capability that SpaceX don’t list as part of a standard Falcon-9 - although they will sell you that extra fuel capability if you want to use it for a heavier payload or higher orbit. It will mean that the booster won’t be recovered. There is some suggestion that boosters that have flown a number of missions and reached their designed number of launches could be used in this mode on their last flight.

Landing the crew capsule is a different problem. It isn’t a huge disaster if the booster has a hard landing (or worse) and is written off. Just money. But one tries to avoid writing off the crew. So they have the simplest, most reliable, systems possible. Parachutes, and something to take the final bump. Which can be water or retro-rockets. And even then, a capability to land hard without retro-rockets - although the capsule may be wrecked, the crew should still survive.

SpaceX is developing rocket powered landing for their Dragon spacecraft. However, it uses toxic hypergolic fuels, dinotrogen tetroxide and monomethylhydrazine (MMH). These propellants allow simple, lightweight, and reliable thrusters, so SpaceX must have decided that the hazard was an acceptable tradeoff. The astronauts that are exposed to this hazard will be trained and equipped to deal with it.

OTOH, the New Shepard is being developed as a space tourism launcher. They probably don’t want to expose their customers to any additional hazards (beyond the whole riding on top of a controlled bomb thing).

There was some hysteresis during the landing. Does the engine nozzle move, or to they have attitude thrusters?

I didn’t realize that the space shuttle boosters were damaged so much by saltwater. It makes sense that saltwater in such a complex device would ruin it, but we used that method for so long that I thought NASA had overcome the issue.

Why not parachute the boosters into the desert? I’m thinking one reason is that there’s a lot more ocean to choose from than uninhabited desert, but if an area is clear for vertical landing then shouldn’t you be able to make a parachute landing in the same area? No pinpoint accuracy, but seems you could get close enough.

Ah, you need to distinguish between the Shuttle’s SRBs (Solid Rocket Boosters) and a liquid fueled booster. The SRBs are seriously strong and heavy. They are made of steel, and despite this, and despite landing on parachutes, the impact with the water could take the sections out of round - requiring serious effort to rectify. The SRBs are very powerful but short acting boosters, and the higher dead weight matters less. They do however have some significant capability. The nozzle is directional, and is driven by small APUs, and there is thus some complexity in control. But there are no delicate liquid fueled rocket motor bits, nor a fragile and very salt water averse aluminium-lithium structure. Even then, the effort required in refurbishing the SRBs was huge and expensive. Like everything else on the shuttle.

The soft landing boosters are not SRBs. Nor could they be. Once an SRB has burnt out, that is it. You would need an entire separate recovery system, rather than using the existing motors and remaining fuel.

A liquid fuelled booster dropping into the ocean on a parachute would have simply broken up, and there would be grave misgivings about using much out of the actual motors. This has never been tried, nor is it every likely to be.

They wouldn’t slow down the rocket enough to avoid damage to the engine and structure. As said above, liquid boosters are dainty ladies and parachutes only slow you from a crash to a thump.

Still curious about turning the empty booster into a glider though.

The shuttle SRB parachutes are also freaking huge, with each of the main chutes weighing a ton. Even with those chutes, some of the biggest ever made, the empty 83-ton SRB hit the water at 50 miles per hour (23 m/s).

If you wanted to parachute this SRB over land, for example, you’d either need to add legs capable of absorbing huge quantities of energy in the impact, even more humongous parachutes to bring down the terminal velocity, or solid rocket motors to bring the final impact velocity down. I’m guessing any of those things would add many more tons to an already heavy booster.

Given that Musk really, really wants manned missions to other worlds to be a thing, the idea might be to get experience with softly landing rockets in a manner that does not rely on a thick atmosphere. Gliding and parachutes are fairly effective on Earth, but they need to develop vertical powered landing capability anyway if they want to go to Mars someday.

I have no doubt that is exactly what Musk is thinking/doing. The fact that if doable it is probably the best way to do it anyway is just a bonus.

I have a couple of slight corrections to Francis Vaughn’s otherwise excellent summary: Liquid propellant systems are referred to as engines; only solid propulsion systems are called motors. The other is the claim that the addition of deployable legs do not add to the technical complexity; in fact, developing landing legs which can be reliability deployed in such a way as to securely land the vehicle but not contribute an egregious amount of extra weight turns out to be a very complicated problem, and one that I’m not at all sure SpaceX will make work reliably. The legs have to be secure enough to hold the vehicle against wind loads and structural oscillation but compliant enough to allow some degree of off-axis landing without inducing excessive offset load.

The rationale for powered landing versus the use of aerodynamic decelerators (parachutes, ballutes, et cetera) has three components; one is that the engines are an existing system while adding a deployable decelerator adds another system with attendant dead mass. The second is that decelerators require repacking, and unlike personnel chutes, these are difficult and complex to pack, requiring complex folding and a heavy press. The third is that parachutes and ballutes are single shot textile devices that cannot be functionally verified before use with no redundancy or requiring significant excess mass to have redundant systems.

This gets back to the question of the essential value of reuse; e.g. is there a cost or flight frequency rationale for trying to reuse flight hardware? From a naive standpoint reuse seems like an essential component of reducing launch costs, with advocates comparing current single use systems to flying an airliner once and then throwing it away. However, this ignores the fact that the usable lifetime of airliner engines is hundreds of hours between major servicing and thousands of hours between rebuild or replacement, and airframes have seen tens of thousands of hours of service between repair and on the order of a hundred of thousand hours of lifetime service. The most robust rocket engines have demonstrated a few thousands of seconds of use between major rebuilds (RS-25 SSME) and only after significant development. Other pressurized components such as valves, composite overwrapped pressure vessels, and feed systems also have limited lifetimes. Other structural items on rockets see significant structural fatigue during flight, experiencing vibration levels that are orders of magnitude beyond what is seen in any normal terrestrial environment, and because the philosophy behind most rocket launch systems is to try to get the highest possible performance per mass, a key metric is reducing ‘dead’ weight (e.g. anything that is not propellant or payload) as much as possible, which emphasizes performance over robustness.

The fact of the matter is that the actual cost of building hardware, while significant, is not the major driver in total launch cost. It’s difficult to get a good handle on hardware costs because of the way costs are tracked, i.e. production costs are not clearly separated from processing and non-recurring engineering (NRE), but the studies that I’ve worked on indicate that the actual build cost of a two or three stage liquid propulsion rocket averages somewhat less than 10% of the total launch cost and never exceeds 20%. This means that, all other costs being equal and assuming no remanufacture or rebuild outside of normal processing, you’d have to fly between five and ten flights just to break even on hardware cost. And all other costs are not equal; reusability demands substantially more NRE and inspection of post-flight items, and to date nobody has built a reusable launch system of significant capability which can just land, refuel, and fly again without substantial refurbishment. The study that NASA performed in the 'Seventies indicated that a reusable multistage vehicle would be cheaper than an expendable system only at a flight rate of 50-60 vehicles per annum. Orbital Sciences redid that study in the 'Nineties and reached the exact same conclusion, which is unsurprising because engine performance and vehicle mass ratios have only improved in tiny increments, being limited by the basic physics of combustion and materials.

There may be other benefits to reusability, such as increasing the number of flights without increasing the throughput on your production line, but fabrication cost is actually not a significant driver or a major opportunity to reduce launch costs by the order-of-magnitude necessary to substantially increase access to space. The real opportunities are reduction of complexity of launch vehicles, e.g. trading performance for simplicity and robustness (which actually gains more in practical reliability over redundancy) and reducing processing and integration effort by automation and simplicity. The Sea Dragon/SEALAR concept is the prime example of this; gaining partial reusability by trading performance for a low tolerance, highly robust design that requires minimal launch infrastructure and allows for launching payloads that would be almost impossible to manage with land-based logistics. However, it is really only advantageous for large payloads that can either be securely encapsulated or tolerant against marine conditions.

The other tact–and the one I personally favor–is to abandon the traditional high L/D cylindrical profile for a launch vehicle (which is inherently structurally weak and is has poor mass utilization) to a squat profile. While this would require a very different manufacturing flow than conventional launch vehicles and is logistically complex for transportation to a manufacturing site to launch facility, it offers a better trade in mass utilization, performance, robustness, and the potential for genuine reusability with a base entry high drag reentry profile with only a modest aerodynamic penalty for an orbital ascent profile. The Chrysler Aerospace SERV proposal for the STS (which was rejected out of hand, ostensibly for not meeting the adjusted crossrange requirement but really because it was just too far away from the winged spaceplane concepts that the ‘Huntsville group’ had been pursuing since before Apollo) is the epitome of this, most most plausible RSSTO concepts and the ‘Big Dumb Booster’ type two stage proposals have relied on this kind vehicle layout.

Trying to land boosters with a horizontal glide mode requires the addition of inert mass of deployable wings (or wings that protrude during flight, increasing drag during ascent), landing systems, and reinforcing structure. Every time a proposal for liquid flyback boosters came out to replace the Shuttle SRBs, the resulting concept was nearly as large as the External Tank in order to achieve the required thrust and flight time. Powered vertical landing, if it can be done successfully, requires only a very modest amount of propellant (somewhere around 5% of the total load out), although that last 5% of propellant is a loss of somewhere around 20%-30% of ascent payload capability, so there is a significant tradeoff in terms of capability limits versus reusability, which again, is not likely to be a significant cost reduction in overall launch cost.


Every time I read one of your outstanding posts I feel compelled to accuse you of just making everything up. Nobody can know so much about these subjects and also communicate it so clearly.

While our resident aerospace engineering experts are in the thread:

How much of an advance is the BE-3 engine? If used as a second-stage engine, does it add any useful capabilities that the RL-10 doesn’t have?

At first glance it seems pretty remarkable:

  1. It’s a cryogenic hydrogen/oxygen powered engine (developed in less than 5 years!)
  2. It can throttle down to 20% of max thrust.
  3. It can restart multiple times.
  4. It’s intended to be economically reusable.

As far as I know there isn’t any other engine with quite the same capabilities. However, I don’t know whether reusability or deep throttling makes any difference for a second stage engine.

I don’t have any particular insight or knowledge about the BE-3 other than what little has been publicly released by Blue Origin so I don’t know how far they’ve gotten in development or how well they’ve achieved design goals, and I’m wary of taking advertised capability at face value. However, a cryogenic engine design is always challenging just by virtue of the relative temperatures of the propellants alone. The current RL10B-2 used on the Delta Cryogenic Upper Stage (DCSS) is widely regarded as the pinnacle of upper stage engine expander cycle technology in terms of performance, but it is and old design that is very expensive to build and has very little down-throttling capability. Several efforts have been made to replace the design with a more simple engine but none have come to fruition.

The BE-3 is an ascent stage engine (although I see that they also have a proposed upper stage version which is being considered for the Advanced Cryogenic Evolved Stage) which claims to have deep throttle capability. The value of throttling depends on capability; for certain types of maneuvers or variable payload capacity (e.g. multiple payload deployments) it can be advantageous, but down-throttling during ascent phase is more valuable in shaping the trajectory for optimal propellant usage to conserve propellant for a longer, more efficient thrust profile. Restart is the most critical function for an upper stage engine, especially for multiple or complex deployments, and this ends up being a surprisingly challenging capability to exercise.

From a cost standpoint, since it is a given that the upper stage will not be recovered and reused, minimizing complexity is the most crucial factor but there is a certain degree of unavoidable complexity with deep throttling and restarting as well as all of the handling and processing problems associated with loading cryogenic propellants and especially hydrogen (one of the many significant cost drivers for the STS). It also limits launch availability; propellants have to be loaded within a certain time to assure availability through the launch window, and if they plan to use densified propellants (supercooled LO[SUB]x[/SUB] and slush LH[SUB]2[/SUB]) then that window is even more critical. I’m frankly kind of surprised that Blue Origin is planning to use a hydrogen burner for main ascent but more power to them if they’ve figured out logistics that make it workable. The major advantage for an upper stage engine aside from the high performance is the low contamination potential of LO[SUB]x[/SUB]/LH[SUB]2[/SUB], which ends up being basically water and some free diatomic hydrogen. This is much better than hydrocarbon fuel which poses a significant contamination challenge even from just free molecular flow of vented fuel, much less the carbon-rich combustion products that tend to stick to everything.


I have a question(s) about reusability.

  1. How much reduction in reliability can we expect over multiple launches. If its rated for 10 launches, then won’t the risks of a loss increase with every flight?

  2. What about performance? Carry less payload every flight?

It seems that the problem is that you require refurbishment of such an amount to avoid such degradation that it is not more economical to simply have a new rocket every flight.