10 ms·

Like all engineering decisions, hydrogen is a choice with tradeoffs. It's specific impulse ('fuel efficiency') is unbeatable. That buys you a lot more cargo ca

by mabbo 4y ago

Like all engineering decisions, hydrogen is a choice with tradeoffs. It's specific impulse ('fuel efficiency') is unbeatable. That buys you a lot more cargo capacity for the same sized rocket. And since this program is putting large items into lunar orbit, you need that badly. But as you said, it's... complex to work with. Hey it worked for the space shuttle.

xcskier56 4y ago
Another trade off: Liquid hydrogen is very low density, and requires extremely low temperatures. This means that you need a much bigger tank for a given weight of fuel, and that tank needs to be insulated bc liquid hydrogen must be kept below 33 kelvin. Both of these downsides can come close to negating the upside of increased ISP. But the upside for SLS is that they already had engines that can run on hydrogen. Like you said, trade offs all around
adastra22 4y ago
> That buys you a lot more cargo capacity for the same sized rocket. I think you mean the same mass rocket, because hydrogen is not dense, which factors into the tradeoffs (more structural mass to contain it).
- mabbo 4y ago
  An excellent point. Fortunately bigger tanks get better and better the bigger they are. The mass of the tanks is mostly a function of it's surface area, growing with n^2, while the fuel stored grows by n^3. And the SLS tank is pretty darned big indeed, with a very large diameter.
  - wumpus 4y ago
    Big tanks are a big part of why SLS is financially inefficient.
  - adastra22 4y ago
    That is only a first order approximation. Second order effects scale badly. Larger tanks for the same amount of propellant have more weight, which means they need to be built sturdier, which means even more weight, etc. LH2 can’t have a common bulkhead with LOX like Falcon 9 has, so that’s more weight, and sitting on top of the tank so the tank needs to be studier still. And LH2 is way lower temperature so you need foam insulation, which adds weight. And you don’t want that foam peeling off and hitting the SRB on the way up, so you need a layer of sealant, which (all together now!) adds weight. Etc. etc.

by mabbo 4y ago

xcskier56 4y ago

Another trade off: Liquid hydrogen is very low density, and requires extremely low temperatures. This means that you need a much bigger tank for a given weight of fuel, and that tank needs to be insulated bc liquid hydrogen must be kept below 33 kelvin. Both of these downsides can come close to negating the upside of increased ISP. But the upside for SLS is that they already had engines that can run on hydrogen. Like you said, trade offs all around

adastra22 4y ago

> That buys you a lot more cargo capacity for the same sized rocket. I think you mean the same mass rocket, because hydrogen is not dense, which factors into the tradeoffs (more structural mass to contain it).

mabbo 4y ago
An excellent point. Fortunately bigger tanks get better and better the bigger they are. The mass of the tanks is mostly a function of it's surface area, growing with n^2, while the fuel stored grows by n^3. And the SLS tank is pretty darned big indeed, with a very large diameter.
- wumpus 4y ago
  Big tanks are a big part of why SLS is financially inefficient.
- adastra22 4y ago
  That is only a first order approximation. Second order effects scale badly. Larger tanks for the same amount of propellant have more weight, which means they need to be built sturdier, which means even more weight, etc. LH2 can’t have a common bulkhead with LOX like Falcon 9 has, so that’s more weight, and sitting on top of the tank so the tank needs to be studier still. And LH2 is way lower temperature so you need foam insulation, which adds weight. And you don’t want that foam peeling off and hitting the SRB on the way up, so you need a layer of sealant, which (all together now!) adds weight. Etc. etc.