It sounds like you're preparing for a tough challenge! Restarting on HARD for the Caveman challenge can definitely be tricky, especially with limited contract slots and challenging engine setups like the Thud. I'd recommend avoiding engine contracts that are known to have issues or aren't versatile in your early designs. Focusing on smaller, more manageable contracts can help keep your science progression steady.
Are you planning to do anything differently this time around to avoid the pitfalls you experienced with the Thud engine contract? Best of luck on this next attempt—stay patient, and hopefully, the science won’t bottom out!