Nuclear Engines - friznit/Unofficial-BDB-Wiki GitHub Wiki

  • Los Alamos Small Nuclear Engine (BDB Aphid) uses a reactor derived from the Pewee from Project Rover and features an unusual sideways folding nozzle to maximise room inside spaceplane payload bays.

  • NERVA XE (BDB Baobab) is an experimental nuclear engine running a hot bleed cycle and features built-in roll control utilising the bleed exhaust.

  • NERVA Full Flow Flight Engine (BDB Banyan) uses the same KIWI B reactor as the NERVA XE with improved performance. Unique features include an aerospike nozzle variant.

  • NERVA II (BDB Redwood) is built around the incredibly powerful Phoebus II reactor and emphasises relatively higher thrust for demanding interplanetary missions.

  • SNTP 75 TIMBER WIND (BDB LUMBER GUST II) is a nuclear thermal rocket powered by a high temperature particle bed reactor (PBR). PBR reactors allow for higher efficiency and higher thrust to weight ratio than traditional fuel rod based designs. SNTP 75 is a dual mode engine where it can either run on pure LH2 like most nuclear engines or in an Oxidiser augmented mode for a significant increase in thrust at the cost of efficiency. In the Ox augmented mode SNTP 75 is a very viable sea level sustainer engine and can be switched in flight to LH2 only at higher altitudes. Finding the perfect switchover point can be an interesting challenge.

  • SNTP 45 is a smaller, vacuum optimised version of the TIMBER WIND with lower thrust but very high efficiency. This engine does not feature an Ox augmented mode unlike its larger sibling.

Nuclear Stages

Nuclear engines enjoy very high ISP when using pure Liquid Hydrogen (LH2) as fuel. However, LH2 is a low density fuel compared to LFO so an equivalent size of fuel tank produces lower dV. To get equivalent dV with LH2, you'd need a lot more volume (a bigger fuel tank). Of course the dry mass of tankage weighs more so you get diminishing returns and the large size can make rocket designs awkward. Very large nuclear tugs for interplanetary missions are therefore typically assembled in orbit. Alternatively, considering that an LH2 stage is much lighter than a similar size LFO stage, nuclear upper stages can be useful since the main booster stage of the rocket has less overall mass to lift.

Example Uses

Nuclear Upper Stage - heavy lift vehicle for high orbit insertion requiring very high dV but low thrust in the upper stage, such as launching a large satellite directly into geostationary orbit

Interplanetary Tug - extremely high dV requirements for long interplanetary ejection burns. The massive tanks typically require assembly in orbit.

Cryofuel Boil Off

LH2 is subject to boiloff: cryogenically cooled fuel dissipates due to heating from external sources (engines, radiation, friction, the sun etc), making these stages less efficient to use for long duration missions. Cryo stages are generally best used for early phases, such as ejection burns from the home planet. However, boiloff can be mitigated by insulating the tanks or using radiators.

BDB includes an (optional) integration with System Heat to support boiloff mitigation.

TL/DR:

  1. In VAB "Enable Cooling" in PAW on all tanks that require it (i.e. ones that have fuel like LH2 that boils off).
  2. Spam radiators until the loop temperature stablises in the PAW.

Better approach:

  1. Open SH UI and click on the red temp warning indicator.
  2. Check the Net Flux value. This is essentially how much cooling the tank needs.
  3. Add radiators to bring the Net Flux value to 0 (or negative to give yourself a little margin).
  4. Heat Loops can be used to create separate groups of parts & radiators. Useful for ringfencing high operating temp parts such as reactors from low operating temp parts like fuel tanks.